Whereas tunnel single ionization is an attosecond process taking place within a fraction of an optical cycle, multiple ionization occurs within at least one half-cycle in the case of non sequential multiple ionization or within several cycles. In molecules, multiple ionization leads to the fragmentation into multicharged atomic fragments and is usually labelled Coulomb explosion. In principle Coulomb explosion allows one to image the positions of the atoms within the molecule provided that multiple ionization times remain small in comparison with the timescales of nuclear motions. In these experiments, the time resolution is a major issue in order to compete with accelerator-based techniques such as foil-induced and ion-induced multiple ionizations which occur during a few tens of attoseconds. The advantage of the laser excitation lies in the compactness of the experimental set-ups and more importantly in the possibility to perform pump-probe excitation schemes for imaging excited molecules. Using few-cycle laser pulses to multiply ionize molecules, we study the simultaneous electronic and nuclear relaxations as a powerful probe of the ultrafast ionization sequence at the femtosecond time scale. The molecular systems range from the hydrogen molecule to simple polyatomic molecules. The molecular response is analysed using the multicharged atomic fragments time-of-flight detection, ions correlations, energy analysis, and fluorescence detection.

Time-resolved photoion and photoelectron velocity mapped images from NO₂ excited close to its first dissociation limit [to NO(X²P) + O(³P₂)] have been recorded in a two colour pump–probe experiment, using the frequency-doubled and frequency-tripled output of a regeneratively amplified titanium–sapphire laser. At least three processes are responsible for the observed transient signals; a negative pump–probe signal (corresponding to a 266 nm pump), a very shortlived transient close to the cross-correlation of the pump and probe pulses but on the 400 nm pump side, and a longer-lived positive pump–probe signal that exhibits a signature of wavepacket motion (oscillations). These transients have two main origins; multiphoton excitation of the Rydberg states of NO₂ by both 266 and 400 nm light, and electronic relaxation in the ¹²B₂ state of NO₂, which leads to a quasi-dissociated NO2 high in the ¹²A₁ electronic ground state and just below the dissociation threshold.

Collaborations : M.-E. Couprie et al., SOLEIL Synchrotron (St-Aubin, France)
T. Hara et al., SPring8 Compact SASE Source (SCSS), XFEL Project/RIKEN (Hyogo, Japan)
L. Giannessi et al., ENEA & INFN/LNF (Frascati, Italy)

For producing intense ultra-short pulses in the XUV, the combination of laser- and accelerator-based sources will obviously play a major role in the design of the fourth-generation FEL. In the collaboration with Soleil-Synchrotron and SPring-8 Compact SASE Source (SCSS, SPring-8, Japan, SASE is for Self-Amplified Spontaneous Emission), we have recently coupled a seed harmonic source at 160 nm (5th harmonic of IR laser) to the LINAC FEL amplifier [G. Lambert, T. Hara et al., Nature Physics 4, 296 (2008)].

The time evolution of electronically excited vitamin B₁₂ (cyanocobalamin) has been observed for the first time in the gas phase. It reveals an ultrafast decay to a state corresponding to metal excitation. This decay is interpreted as resulting from a ring to metal electron transfer. This opens the observation of the excited state of other complex biomimetic systems in the gas phase, the key to the characterisation of their complex evolution through excited electronic states.

Chem. Phys. (2007)

Atoms in a strong laser field: an electron wave packet is launched and driven by the field over one optical cycle. The EWP can return to its parent ion and be scattered as an outgoing electron wave or an attosecond burst of XUV light. The EWP recollision has therefore a double interest: it can be exploited either as a probe of the system with an extreme resolution, or as an ultra-short source of XUV light.

The dynamics of atomic and molecular electrons in a strong laser field is particularly rich and has been a central research topic at LIDYL for more than thirty years. This program is experimentally and theoretically continued in the Attophysics group.

Basically, the ultra-fast electron dynamics in a strong laser field can be described from both quantum and semi-classical concepts, as for instance electron wave packets and electron trajectories, respectively. The semi-classical picture has popularized the elementary dynamical process under the so-called “three-step” model [P. Corkum, Phys. Rev. Lett. 71, 1994 (1993)].

Its profound physical content is illustrated in the figure. When the atom (molecule) is submitted to a laser field – in the intensity range 10¹³-10¹⁶ W/cm², that is strong enough to distort significantly the core potential -, an electron initially in a valence orbital can escape the core (step 1). This electron subsequently "rides" the laser field and may return back to its parent core within one optical cycle (of duration 2.7 fs = 2.7 10^-15 s with the infra-red lasers we use), after it has gained kinetic energy in the field up to a few tens or even hundreds of electronvolts (step 2). In the recollision with the core, the electron can be quasi-elastically scattered (electron diffraction) or inelastically but coherently scattered (step 3). In the latter inelastic recollision, the electron can either further ionize the core or recombine radiatively with it, releasing its energy as an attosecond burst of extreme–UV light. The above three steps including the attosecond emission constitute the elementary sequence in High Harmonic Generation or HHG, first observed in 1987 simultaneously in Chicago and Saclay [A. Mc Pherson et al., J. Opt. Soc. Am. B 4, 595 (1987), M. Ferray et al., J. Phys. B 21, L31 (1988)]. Each optical cycle drives two recollisions so that a train of attosecond pulses in produced in HHG; their temporal characterisation was first achieved in Saclay in 2001 [P.-M. Paul et al., Science 292, 1689 (2001)].

The atomic/molecular electron dynamics in the strong field encompass basic processes, such as ionization and EWP scattering in the different channels, which are studied for themselves along by two research lines. They are detailed in the Multiple ionization & Molecular Imaging and High Harmonic Generation and Attosecond physics pages.

Now, speaking quantum mechanics, the electron is better described as an electron wave packet (EWP) that dynamically splits into two parts, respectively bound and quasi-free, in the laser field, where the quasi-free component undergoes the recollision and scattering onto the core. The free EWP has a de Broglie wavelength in the Angstrom range, which makes it a very appropriate local probe of the system which extends over a comparable scale. Since the EWP probe has attosecond temporal resolution, it can in principle image ultra-fast motion of electrons and nuclei in molecules. Two research lines, described in the Ultra-fast imaging of molecules from electron diffraction and High Harmonic Generation and Attosecond physics pages, build on this "self-probing" paradigm.

Recolliding EWP in a strong laser field. The two coherent scattering channels, EWP diffraction and EWP radiative recombination keep an imprint of the nuclear structure and the electronic orbital in the molecule.

Besides the fundamental studies of the electron dynamics in strong field, and its use as a probe of transient systems, HHG provides with a source of ultra-short coherent pulses in the XUV (from 100 nm down to a few nm). The source's brightness, which reflects the high instantaneous flux and coherence in both the "narrowband" femtosecond and "broadband" attosecond ranges and its natural synchronization with a driving laser, make it very attractive for a number of applications. Among the Examples of applications we have performed multi-color Photoionization in the gas phase, and studies of XUV/solid interaction in the solid state. The coherence properties and partial tunability of the HHG source make it attractive for Seeding a Free Electron Laser, which constitutes another research line. A promising new application concerns the Coherent diffraction imaging of nanometric objects. Most of the applications are developed in collaboration with expert groups, either in France or in Europe, USA, Canada, Japan,…

Eventually, we pursue a theoretical activity to support the several experimental programs. It focuses on microscopic aspects of the gas phase-strong field interaction, i.e., the electron dynamics in atoms and molecules, including Strong Field Approximation (SFA) models in HHG. It also deals with the macroscopic aspects of the interaction, with the development of 3D propagation codes for the laser and XUV fields.

The laser-driven coherent processes in atoms and molecules can lead to applications in two distinct ways. On the one hand, the EWP diffraction or radiative recombination can probe the system from which it is derived, in a "self-probing" scheme. On the other hand, the XUV pulses produced can subsequently be focused to excite/probe a separated target system. Of the latter type, Photoionization in the diluted phase and XUV pulse/solid interaction are two recent applications that we performed, which exploit the high XUV intensity on target. As a rule, applications to time-resolved dynamical studies (in a pump-probe scheme), non linear studies, or using coherence, requires that a high coherent flux is delivered by the source. In the perspective of developing high flux and coherent XUV source, we contribute to the research on the Seeding of a Free Electron Laser by an external seed, namely the harmonic source.

We report fluorescence measurements of DNA components performed on the femtosecond time-scale using the fluorescence upconversion technique. Aqueous solutions of thymine (T), thymidine (dT) and thymidine 5′-monophosphate (TMP) were studied in room temperature by excitation at 267 nm and detection at wavelengths between 310 and 380 nm. The fluorescence decays are complex and cannot be described by single exponentials. About 25% of the fluorescence disappears within 150 fs and the remaining part decays more slowly when going from the base through the nucleoside to the nucleotide. The initial fluorescence anisotropy was found to be 0.35±0.03 and did not show any drastic change on the examined time interval.

Aqueous solutions of adenine (A), deoxyadenosine (dA) and deoxyadenosine 5′-monophosphate (dAMP) were studied in room temperature by femtosecond fluorescence upconversion. The fluorescence decays cannot be described by single exponentials. They consist of an ultrafast component (230 fs for A, <100 fs for dA and dAMP) and a slower one (8 ps for A, 0.5 ps for dA and dAMP). The slow component constitutes 95% of the total fluorescence (time-integrated) for the base while only 24% for the nucleoside or the nucleotide. The initial fluorescence anisotropy is 0.30±0.03 for A, 0.25±0.05 for dA and dAMP. The anisotropy of the A fluorescence partially decays during its lifetime due to rotational diffusion.

Understanding the primary photophysical processes in molecules is essential for interpreting their photochemistry, because molecules rarely react from the initially excited electronic state. In this study the ultrafast excited-state dynamics of chlorophenylcarbene (CPC) and trifluoromethylphenylcarbene (TFPC), two species that are considered as models for carbene dynamics, were investigated by femtosecond time-resolved pump probe spectroscopy in the gas phase. Their dynamics was followed in real time by time-resolved photoionization and photoelectron imaging. CPC was excited at 265 nm into the 3 1A′ state, corresponding to excitation from a π-orbital of the aromatic ring into the LUMO. The LUMO contains a contribution of the p-orbital at the carbene center. Three time constants are apparent in the photoelectron images: A fast decay process with τ1 ≈ 40 fs, a second time constant of τ2 ≈ 350 fs, and an additional time constant of τ3 ≈ 1 ps. The third time constant is only visible in the time-dependence of low kinetic energy electrons. Due to the dense manifold of excited states between 3.9 and 5 eV, known from ab initio calculations, the recorded time-resolved electron images show broad and unstructured bands. A clear population transfer between the states thus can not directly be observed. The fast deactivation process is linked to either a population transfer between the strongly coupled excited states between 3.9 and 5.0 eV or the movement of the produced wave packet out of the Franck−Condon region. Since the third long time constant is only visible for photoelectrons at low kinetic energy, evidence is given that this time constant corresponds to the lifetime of the lowest excited A 1A′ state. The remaining time constant reflects a deactivation of the manifold of states in the range 3.9−5.0 eV down to the A 1A′ state.

J. Am. Chem. Soc., 2008, 130 (45), 14908-14909

« Back to the Group page « Back to the Oxides page

A multi-ferroïc material possesses simultaneously two or more ferroïc orders: ferroelectric, ferromagnetic and ferroelastic. Particularly fascinating is the coupling between these orders. For example magneto-electric coupling allows electrical control of the magnetization or, inversely, magnetic control of the polarization. The coupling coefficient α is defined by M=αP. Multi-ferroicity provides additional handles to control oxide properties. To these can be added collective orbital, spin and charge phenomena.

α

« Back to the Group page « Back to the Oxides page

The new physics emerging from two-dimensional films in the limit of a few unit cells has a host of exciting applications. However, understanding the ferroelectric properties of such engineered thin film systems requires taking into account not only the material but also its interfaces with electrodes, substrates or atmosphere; in other words, the electrical boundary conditions. In the case of a thin film these can even determine the ferroelectric polarization stability. The depolarizing field can place a lower limit on the film thickness capable of supporting a stable polarization.

« Back to the Group page

Electronic and chemical structure of functional oxides

We study oxide band structure, surface and interface chemistry, ferroelectric and multiferroic films.

The defining property of a Ferroelectric (FE) material is a spontaneous macroscopic polarization which can be reversed under an applied electric field. The polarization as a function of applied electric field exhibits a hysteresis loop, analogous to ferromagnetic materials, hence the name ferroelectricity.

It was discovered by Valasek from the University of Minnesota who presented his results at the Washington meeting of the American Physical Society in April 1920.

Perovskite oxides, of general formula ABO3 with a pseudocubic structure, where A and B are two different cations, furnish many interesting ferroelectrics. The B-type cation is octahedrally coordinated with oxygen. In the example shown, BaTiO3, it is the relative symmetry breaking displacement of the Ti atoms with respect to the O atoms which is responsible for the spontaneous polarization. BaTiO3 has three ferroelectric phases: tetragonal, orthorhombic and rhombohedral.

• Epitaxial thin films
• Electrical boundary conditions
• Multi-ferroïcs
• Interface chemistry
• Screening
• Switching the polarization
• Response to applied bias
• Domain Walls
• Chemisorption
• Our experimental approach

« Back to the Group page « Back to the Oxides page

The question of the interface is a key issue in multi-ferroïc heterostructures. Hybridization between filled d orbitals responsible for magnetization and empty d orbitals in the ferroelectric oxide may be one path to such coupling. Several coupling mechanisms have been identified and these can be quite complex. For example, the charge ordering of a magnetic layer can be modulated by the polarization state of an adjacent ferroelectric.

« Back to the LENSIS Group page « Back to the Oxides page

The advent of high quality epitaxial film growth, decisive theoretical advances and experimental tools capable of characterizing in detail ferroelectric materials has led to a resurgence of interest, in particular the perspective of engineering films for ferroelectric-based electronics. Substrate-imposed strain, for example, can increase the Curie temperature, stabilizing ferroelectricity in BaTiO₃ up to 600°C.

We report for the first time the direct measurement of spin polarization in epitaxial CoFe2O4 tunnel barriers using the Meservey-Tedrow technique. We further analyze the effect of oxidation conditions during film growth on the polarization of the tunneling current (PSF), revealing an important role played by oxygen vacancies in the spin-filter efficiency of this material.

These spin filtering measurements have been performed in collaboration with the team of J. S. Moodera from the Francis Bitter Magnet Lab, MIT.

Head: Cécile Reynaud

This topic is in the field in full rise of nanoscience and nanotechnology, which involves the study of structures or materials which have at least one characteristic dimension below 100 nm and have, therefore, specific properties. We develop our own nano-objects synthesis methods with a bottom-up approach. The aim is the formation of new entities with controlled physical and chemical properties. We study their growth process and their specific properties, particularly those induced by size effects. Taking advantage of the diversity of nanostructured objects that we are able to synthesize (nanoparticles, carbon nanotubes, membranes, electrodes...), we develop original nanomaterials or nanocomposites. Dispersion and suspension of these nano-objects, with or without prior functionalization, are studied in order to enable their convenient and safe manipulation. Many characterization methods are implemented, in particular optical spectroscopy and electron microscopy, often through collaborations. The concerned applications are in the fields of optics, optoelectronics, catalysis, sensors and markers (fluorescent or magnetic), new technologies for energy, new technologies for health, next generation nuclear materials, etc. This activity is in close relation with the national and international network of basic and applied research through many national and international contracts.

Spectrally and temporally resolved reflectivity measurement for studying the temporal evolution of a dense plasma of interest for the Warm Dense Matter problematic...

The properties of Warm Dense Matter ( WDM - solid density and temperature of several eV's ) are a subject of strong interest among a wide scientist community ranging from astrophysicists to solid-state physicists. A major difficulty to characterize the ultra-fast dynamics of such dense plasmas results from their opacity to visible light. Thus, only a few sets of experimental data are available making the validation of theories and numerical simulations difficult.
 We have suggested to study the temporal variation of a XUV probe beam reflected by a plasma created on metallic target. The IR pump-XUV probe beam experiment has been set up on LUCA laser facility (CEA-Saclay) lien vers SLIC. An IR has been focused on Au target at intensity ranging from 7x10¹³W/cm² to 10¹⁵W/cm².  XUV beam is HHG from argon gas jet. As the XUV spectrum is made of odd harmonics of the fundamental laser frequency, we record the wavelength dependant reflected signal in one single shot.

Internal conversion frequency imaging interferometer at 32nm

Interferometry allows for getting electronic density information in 2D. For this purpose, we have set up, in collaboration with Attophysic group and LCF-IO, an innovative instrument based on the mutual coherence properties of two High order Harmonic (HHG) from gas jet in the XUV domain. Interferometry, in this wavelength domain, exhibits strong difficulties, due to handling of the beams.
To overcome part of these problems, we have proposed a novating scheme in which a significant part of the optics is in the IR domain. The principle of the interferometer is reported on the following figure.

.....................

SLIC facilities typically provide 900 experimental laser days per year. LUCA annually offers 540 days whereas UHI and PLFA provide 180 days each. On average, more than 85 scientists access SLIC facilities every year. Most of the available beamtime is used internally by CEA scientists (so-called “internal access”), often in collaboration with external researchers. SLIC facilities are also open to external scientists through the allocation of “national access” to French research groups outside CEA and “European access” which, since 2004, is offered within the LASERLAB-EUROPE consortium. Companies are also regularly received on SLIC facilities. Statistics on these different kinds of access to SLIC facilities are summarised in Table 2

Besides operating the lasers at their best performances, the SLIC laser group also supports users with many aspects of their experiments. The quality of this support is often emphasized by our users. For instance, all our users who filled in the LASERLAB user assessment form have given the highest mark for the overall appreciation of the services provided at SLIC and of the facility operation.

SLIC has become a "European Major Research Infrastructure" in 2003 within the FP5 European program. Since 2004, SLIC is a partner of the FP6 LASERLAB-EUROPE I3 (Integrated Initiative of Infrastructures) consortium in which it had a significant implication in the Access activity. In particular, SLIC had 2 participants in the LASERLAB Access Board (including the Chairman position for Didier Normand) whose role was to monitor the whole LASERLAB access activity (3975 days of access from 2004 to 2007).
SLIC had also an individual commitment to provide at least 75 days of Transnational access to European scientists per year. This objective has been more than fulfilled: SLIC has provided 310 days of access, which is 10 days more than the contractual commitment. Twenty scientific projects were carried out involving 51 different users from 19 laboratories. Nearly one third of our visitors were students; this illustrates the educational value of the transnational access activity. To date, 11 papers have been published following European experiments.
The LASERLAB-EUROPE access activity has been continued in 2008, within a specific one year FP7 contract (LASERLAB-Continuation). It should then continue from 2009 to 2012 within an enlarged LASERLAB-EUROPE2 I3 consortium.

European Research Groups are encouraged to apply for access to SLIC Laboratory within its IHP-contract.
 
Contract period : January , 2004 - December , 2007.

The application form can be found at the LASERLAB-EUROPE web page
Click on "Transnational Access" and then "Submit your Research Proposal"

Do not forget to complete on line the "Users Questionnaire" within two weeks of the end of the period of access, and a "Project Summary Report" and return it to us as soon as possible

SLIC Laboratory location

SLIC is located in France near Paris. The mailing address is :
Service des Photons, Atomes et Molécules
Bâtiment 522
CEA/Saclay
91191 Gif-Sur-Yvette Cedex, FRANCE
Phone : (33) 1.69.08.74.09
Fax : (33) 1.69.08.87.07

Le SLIC (Saclay Laser-Matter Interaction Center, IRAMIS-CEA), le LOA (Laboratoire d'Optique Appliquée, ENSTA Paris Tech) et le CELIA (CEntre Lasers Intenses et Applications, Université de Bordeaux 1) offrent conjointement aux laboratoires et équipes de recherche français la possibilité d'effectuer des expériences sur leurs installations laser. Les appels d'offres sont annuels et ont lieu dans la période mi-octobre à début novembre. La procédure à suivre et les formulaires sont disponibles ci-dessous.

Appel d'offre 2011(doc,pdf)

contact:
Pascal d'Oliveira
IRAMIS/SPAM/SLIC
CEA/Saclay 91191 Gif Sur Yvette
Tel: 33 (0)169088260
pascal.doliveira@cea.fr

Undoubtedly, the acceleration of charged particles has been one of the most active research fields in the physics of laser-matter interaction all along the last ten years. In itself, laser driven ion acceleration was already a well known phenomenon although essentially circumscribed to the thermal expansion of the coronal plasma typical of nano and sub-nanosecond low intensities laser pulses interaction regimes.

Thanks to the CPA technique, introduced in the ending 80' it has been possible to dispose of short, more and more intense laser beams. From that time on, intensities ≥ 10¹⁸W/cm² have been available allowing to explore laser-matter interaction in the so-called "relativistic domain" as the motion of electrons in such an electromagnetic field is indeed relativistic. The huge electric fields produced in plasmas due to the laser-induced charge separation open new fields of research in the domain of charged particle acceleration. The admitted scenario for proton acceleration, called Target Normal Sheath Acceleration (TNSA), involves three consecutive steps. First, the laser pre-pulse creates a thin plasma layer at the surface of the foil.

Then, the intense part of the pulse interacting with this thin layer accelerates electrons toward the foil. Finally, the electron beam reaches the rear surface and creates a strong electrostatic field which first ionizes and then accelerates protons and ions to high energies. The excellent emission properties these ion beams show make them particularly suitable for a wide number of applications including high resolution probing of electric fields in plasmas, fast ignition applications, induction of nuclear phenomena, isotope production for medical applications, and proton therapy.

Next steps in research on laser driven ion acceleration in Saclay

Relying on a rich set of human resources, technical expertise and equipment, we are going to implement an ambitious program of research.

a) Energy scaling laws

Most of the existing models wich are supposed to give scaling laws about  proton beams main features (maximum energy, number of accelerated particles) seem better suited to high-energy and "long" duration (~ ps) laser pulses. This is explained by the fact that these models mainly go by data acquired in experiments carried out on such facilities. However, we can understand the interest, mainly economic, for future laser facilities to achieve very high levels of intensity but using a very short pulse duration and relatively little energy. As a consequence, it becomes important to check and modify existing laws scale to better fit them to this kind of interaction regime. This is what we propose to do by studying the influence of pulse duration and the intensity of illumination on the maximum energy and on the spectral distribution of protons.

 b)  Nano-structured targets

Recent experiments in Saclay showed the coupling efficiency increase potential of micro and nano-structured targets. Boosted by our results, we will extend the exploration of this research field using targets with characteristics better suited to our interaction parameters. Moreover, we will also study the emission of electron bunches we observed using  targets showing a regular periodic grating-like pattern on the laser exposed surface.

c) The ALPS project

As a logical continuation of the ANR project “GOSPEL”, ended in 2012, ALPS project is in the framework of the SAPHIR project (see below), to which our team takes part. This strategic industrial innovation project, a collaboration between industrial and academic partners, aims to define the necessary scientific and technological building blocks for the realization of a protontherapy device using a laser driven proton source. Three main research objectives are pursued:
  - Development of scaling laws for proton energies of different laser intensities up to the ultra-relativistic regime, I> 10²¹ W cm^-2, in order to build a laser ion source of medical interest;
  - Getting a deepen insight into the knowledge of the fundamental aspects of laser-generated plasmas, to identify and control the more suitable acceleration paths for practical use of the produced ion beams;
- Quantifying the effects that a dose deposited by laser-accelerated particles have on biological samples, aiming to confirm or deny their usefulness for proton therapy.
The CEA / DAM-IDF and the LOA (project leader) are the other partners of the project.

d)  The PROPAGATE project

The PROPAGATE project aims to perform a systematic experimental study of the basic laser-absorption mechanism on atomic clusters as a function of the cluster average size, the electron density and the laser parameters (energy, pulse length, contrast ratio…) in order  to measure the absolute rates of particle production. PROPAGATE has built a strong interdisciplinary collaboration between chemists with an expertise in nano-object production processes, who will provide also the characterisation of these nano-objects, atomic physicists, who will build the spray source necessary for the introduction of the nano – objects into the laser reaction chamber, nuclear physicists for the neutron and high-energy particle detection, laser plasma physicists and astrophysicists. The experimental activity will be held on the ECLIPSE laser at CELIA and on the UHI100 TW laser system in Saclay.

e) Ions energy transfer to matter

Laser accelerated ion bunches emittance and duration are orders of magnitude smaller than bunches one can get using a conventional accelerator. We have shown that the shot to shot reproducibility can rise up to 5% RMS using UHC laser pulses. All these features make laser accelerated ions the perfect device to study the energy deposition in matter over extremely short time scales, in particular through pump-probe experiments. Such information is of primary interest for the development of numerical models of induced fluorescence in matter by ionizing particles. This research program is carried out  in collaboration with the CEA / IRAMIS / Nanosciences et Innovation pour les Matériaux la Biomédecine et l'Energie (NIMBE).

f) Laser accelerated protons for protontherapy: the SAPHIR consortium

Among all the possible applications of these proton bunches, the most interesting from a social point of view is, by far, the possibility of using them for cancer treatment. In this context, the aim of SAPPHIRE consortium is to study the feasibility of a laser-generated source expected to be more compact, flexible and chip than existing facilities based on standard compact proton accelerators. This project, which recently received the enthusiastic support of OSEO, gathers partners of recognized competence in the world of research and business: the LOA, CEA-DAM, Amplitude Technologies, Institute Gustave Roussy, the Curie Institute, the Orsay Proton Therapy Center, Propulse sas, Imagine Optic and Dosisoft.

Bertrand Carré

Head of Attophysic group

CEA- Saclay
DSM/IRAMIS
Service of Photons, Atoms and Molécules
Bât. 522 p. 113
91191 Gif sur Yvette Cedex
France

Tél : +33 1 69 08 58 40
Fax : +33 1 69 08 12 13

e-mail : bertrand.carre@cea.fr

Whereas tunnel single ionization is an attosecond process taking place within a fraction of an optical cycle, multiple ionization occurs within at least one half-cycle in the case of non sequential multiple ionization or within several cycles. In molecules, multiple ionization leads to the fragmentation into multicharged atomic fragments and is usually labelled Coulomb explosion. In principle Coulomb explosion allows one to image the positions of the atoms within the molecule provided that multiple ionization times remain small in comparison with the timescales of nuclear motions. In these experiments, the time resolution is a major issue in order to compete with accelerator-based techniques such as foil-induced and ion-induced multiple ionizations which occur during a few tens of attoseconds. The advantage of the laser excitation lies in the compactness of the experimental set-ups and more importantly in the possibility to perform pump-probe excitation schemes for imaging excited molecules. Using few-cycle laser pulses to multiply ionize molecules, we study the simultaneous electronic and nuclear relaxations as a powerful probe of the ultrafast ionization sequence at the femtosecond time scale. The molecular systems range from the hydrogen molecule to simple polyatomic molecules. The molecular response is analysed using the multicharged atomic fragments time-of-flight detection, ions correlations, energy analysis, and fluorescence detection.

The project proposes to develop a new gas-phase electron diffraction technique using state-of-the-art femtosecond laser technology and the related attosecond physics. In principle, the method allows to analyze structures of transient molecular species with a resolution approaching a few femtoseconds. Nowadays the spectacular progress in laser physics has opened the way to a full control of optical tunnel ionization of atoms and molecules in the attosecond time domain using carrier-envelope-phase-locked few-cycle pulses. Since its proposal in the 1990s by Kuchiev, Schafer and Kulander, and Corkum, the so-called rescattering model has been successful in explaining many new highly non-linear effects such as non sequential double ionization and XUV attosecond pulse generation in terms of quasi-classical orbits of the ionized electron which is driven back onto the ion core by the laser electric field. Following these advances, we propose to control the attosecond dynamics of the first ionization step and the electronic rescattering paths in aligned molecules using carrier-envelope-phase-locked few-cycle laser pulses. The main objective will be to process time-resolved electronic diffraction patterns of molecules using well-established unimolecular experimental diagnostics such as ion and electron spectroscopy. In this scheme, the linearly-polarized laser field drives the corresponding electron flux back onto the ion core. The associated De Broglie wavelength is of the order of internuclear distances thus opening the way to interference effects and structural analysis. Rescattering following tunnel ionization may be elastic or inelastic depending on the energy exchange with the remaining ion core. Our main interest will be focused on elastic rescattering which is the operating physical effect for photoelectron diffraction.

In the 1990s, Coulomb explosion of small molecules has been extensively studied using pulses durations of a few tens of femtoseconds. The stretching of the internuclear separation R was observed during multiple ionization and lead to the important concept of Charge Resonance Enhanced Ionization discovered by A. Bandrauk et al. We have shown that the excitation of N₂ or CO₂ with 10-fs pulses inhibits any significant stretching. In addition, the fragmentation yields are lower in few-cycle pulses because of the 1/R scaling of the molecular multi-ionization thresholds. These results open the way to femtosecond time-resolved Coulomb explosion imaging of molecules using few-cycle laser pulses.

Thomas-Fermi theory of multiple ionization
Main investigator: M. Brewczyk (University of Bialystok, Poland)
Molecular multiple ionization is studied using a hydrodynamic model which allows one to deal with many-electron systems in intense laser fields. The predicted kinetic energy releases of the fragmentation channels are in good agreement with the measured energies in CO₂ and N₂O.

Ph. Hering, Thèse de doctorat, Paris (1999)
Ph. Hering, M. Brewczyk, and C. Cornaggia, Phys. Rev. Lett. 85, 2288 (2000)

At intensities below 10¹⁴ Wcm^-2, double ionization of H₂ is dominated by recollision excitation of H₂⁺ followed by field ionization of transient excited states. A full quantum non-Born-Oppenheimer model shows that rescattering produces a coherent superposition of excited states which present a pronounced transient H⁺H^- character. This excitation is followed by either field-induced double ionization or the formation of short-lived attosecond autoionizing states. In the first case, electrons are ejected in the same direction. In the second case, electrons are ejected in opposite directions. See the butterfly pattern in the following figure.

We have implemented pulse compression on the Sofockle laser (800 nm, 600 µJ, 40 fs) after testing different schemes to produce a spatially-homogeneous spectral broadening through self-phase modulation in argon. We obtained pulse durations between 8 and 10 fs, energies up to 200 µJ with intensities up to 10¹⁶ Wcm^-2.

The room-temperature fluorescence properties of DNA nucleoside and nucleotide aqueous solutions are studied by steady-state and time-resolved spectroscopy. The steady-state fluorescence spectra, although peaking in the near-UV region, are very broad, extending over the whole visible domain. Quantum yields are found to be mostly higher and the fluorescence decays faster than those reported in the literature. The fluorescence spectra of the 2‘-deoxynucleosides are identical to those of the 2‘-deoxynucleotides, with the exception of 2‘-deoxyadenosine, for which a difference in the spectral width is observed. The steady-state absorption and fluorescence spectra do not show any concentration dependence in the range 5 × 10-6 to 2 × 10-3 M. All fluorescence decays are complex and cannot be described by monoexponential functions. From the zero-time fluorescence anisotropies recorded at 330 nm, it is deduced that after excitation at 267 nm the largest modification in the electronic structure is exhibited by 2‘-deoxyguanosine. In the case of purines, the fluorescence decays and quantum yields are the same for 2‘-deoxynucleosides and 2‘-deoxynucleotides. In contrast, for pyrimidines, the fluorescence quantum yields of nucleotides are higher and the fluorescence decays slower as compared to those of the corresponding nucleosides showing that the phosphate moiety affects the excited-state relaxation.

DNA monomeric units in solution

In parallel with our studies of model helices we have measured, by femtosecond UV fluorescence spectroscopy , the fluorescence lifetimes and anisotropy decays of all the bases, the nucleosides and the nucleotides in aqueous solution, with the exception of guanine which is not sufficiently soluble. In general, the fluorescence decays of the monomeric DNA components are very fast (< 1 ps ). The observed deactivation rates do not depend on the wavelength and decrease slightly when going from the base to nucleotide. Due to the time-resolution of our setup, we have found that, contrary to what was reported in the literature, the fluorescence decays cannot be described correctly by mono-exponential functions. This shows that the relaxation processes occurring in the excited state(s) are complex. A significant part of total fluorescence (up to 75 % for dA and dAMP) decays at times faster than our temporal resolution (< 100 fs).

Publications and abstracts

We report a comparison of the steady-state absorption and fluorescence spectra of three representative uracil derivatives (uracil, thymine and 5-fluorouracil) in alcoholic solutions. The present results are compared with those from our previous experimental and computational studies of the same compounds in water and acetonitrile. The effects of solvent polarity and hydrogen bonding on the spectra are discussed in the light of theoretical predictions. This comparative analysis provides a more complete picture of the solvent effects on the absorption and fluorescence properties of pyrimidine nucleobases, with special emphasis on the mechanism of the excited state deactivation.

The first comprehensive quantum mechanical study of solvent effects on the behavior of the two lowest energy excited states of uracil derivatives is presented. The absorption and emission spectra of uracil and 5-fluorouracil in acetonitrile and aqueous solution have been computed at the time-dependent density-functional theory level, using the polarizable continuum model (PCM) to take into account bulk solvent effects. The computed spectra and the solvent shifts provided by our method are close to their experimental counterpart. The S0/S1 conical intersection, located in the presence of hydrogen-bonded solvent molecules by CASSCF (8/8) calculations, indicates that the mechanism of ground-state recovery, involving out-of-plane motion of the 5 substituent, does not depend on the nature of the solvent. Extensive explorations of the excited-state surfaces in the Franck−Condon (FC) region show that solvent can modulate the accessibility of an additional decay channel, involving a dark n/π* excited state. This finding provides the first unifying explanation for the experimental trend of 5-fluorouracil excited-state lifetime in different solvents. The microscopic mechanisms underlying solvent effects on the excited-state behavior of nucleobases are discussed.

The excited-state dynamics of 5-fluorouracil in acetonitrile has been investigated by femtosecond fluorescence upconversion spectroscopy in combination with quantum chemistry TD-DFT calculations ((PCM/TD-PBE0). Experimentally, it was found that when going from water to acetonitrile solution the fluorescence decay of 5FU becomes much faster. The calculations show that this is related to the opening of an additional decay channel in acetonitrile solution since the dark n/π* excited state becomes near degenerate with the bright π/π* state, forming a conical intersection close to the Franck−Condon region. In both solvents, a S1−S0 conical intersection, governed by the out-of-plane motion of the fluorine atom, is active, allowing an ultrafast internal conversion to the ground state.

Ultrafast Excited-State Deactivation of 8-Hydroxy-2′-Deoxyguanosine Studied by Femtosecond Fluorescence Spectroscopy and Quantum-Chemical Calculations
P. Changenet-Barret, T. Gustavsson, R. Improta, D. Markovitsi
J. Phys. Chem. A 2015, 119, 6131–6139.

A joint experimental/theoretical study of the ultrafast excited state deactivation of deoxyadenosine and 9-methyladenine in water and acetonitrile
T. Gustavsson, N. Sarkar, I. Vayá, M.C. Jiménez, D. Markovitsi, R. Improta
Photochem. Photobiol. Sci. 2013, 12, 1375 - 1386.

The effect of methylation on the excited state dynamics of aminouracils  
T. Gustavsson, R. Improta, Á. Bányász, I. Vaya, D. Markovitsi
J. Photochem. Photobiol. A 2012, 234, 37-43.

Femtosecond fluorescence studies of DNA/RNA constituents
T. Gustavsson, A. Banyasz, R. Improta, D. Markovitsi
J. Phys. : Conference Series 2011, 261, 012009.

Excited-State Dynamics of dGMP Measured by Steady-State and Femtosecond Fluorescence Spectroscopy
F. A. Miannay, T. Gustavsson, A. Banyasz, D. Markovitsi
J. Phys. Chem. A 2010, 114,  3256-3263.

DNA/RNA Building Blocks of Life Under UV Irradiation
T. Gustavsson, R. Improta, D. Markovitsi
J. Phys. Chem. Letts. 2010, 1, 2025-2030.

The Peculiar Spectral Properties of Amino-Substituted Uracils: A Combined Theoretical and Experimental Study
A. Banyasz, S. Karpati, Y. Mercier, M. Reguero, T. Gustavsson, D. Markovitsi, R. Improta
J. Phys. Chem. B 2010, 114, 12708–12719

Assessing solvent effects on the singlet excited state dynamics of uracil derivatives:
A femtosecond fluorescence upconversion study in alcohols and D2O
T. Gustavsson, A. Banyasz, N. Sarkar, D. Markovitsi, R. Improta
Chem. Phys. 2008, 350, 186-192.

Effect of amino substitution on the excited state dynamics of uracil
Á. Bányász, T. Gustavsson, E. Keszei, R. Improta, D. Markovitsi
Photochem. Photobiol. Sci. 2008, 7, 765-768.

Solvent effects on the steady-state absorption and emission spectra of the three pyrimidine bases uracil, thymine and 5-fluorouracil
T. Gustavsson, N. Sarkar, Á. Bányász, D. Markovitsi, R. Improta
Photochem. Photobiol. 2007, 83, 595-599.

Solvent effect on the singlet excited state lifetimes of nucleic acid bases: a computational study of 5-fluorouracil and uracil in acetonitrile and water
F. Santoro, V. Barone, T. Gustavsson, R. Improta
J. Am. Chem. Soc. 2006, 128, 16312-16322.

Singlet excited state dynamics of uracil and thymine derivatives. A femtosecond fluorescence upconversion study in acetonitrile
T. Gustavsson, N. Sarkar, E. Lazzarotto, D. Markovitsi, R. Improta
Chem. Phys. Lett. 2006, 429, 551-557.

Solvent effect on the singlet excited state dynamics of 5-fluorouracil in acetonitrile as compared to water
T. Gustavsson, N. Sarkar, E. Lazzarotto, D. Markovitsi, V. Barone, R. Improta
J. Phys. Chem. B 2006, 110, 12843 - 12847.

Singlet excited state behavior of uracil and thymine in aqueous solution: a combined experimental and computational study of 11 uracil derivatives
T. Gustavsson, A. Banyasz, E. Lazzarotto, D. Markovitsi, G. Scalmani, M. J. Frisch, V. Barone, R. Improta
J. Am. Chem. Soc. 2006, 128, 607-619.

Cytosine excited state dynamics studied by femtosecond absorption and fluorescence spectroscopy 
A. Sharonov, T. Gustavsson, V. Carré, E. Renault, D. Markovitsi
Chem. Phys. Lett., 2003, 380, 173.

Photophysical properties of 5-methylcytosine 
A. Sharonov, T. Gustavsson, S. Marguet , D. Markovitsi
 Photochem. Photobiol. Sci. 2003, 2, 1.

Fluorescence properties of DNA nucleosides and nucleotides: a refined steady-state and femtosecond investigation
D. Onidas, D. Markovitsi, S. Marguet, A. Sharonov, T . Gustavsson
J. Phys. Chem. B, 2002, 106, 11367.

Thymine, thymidine and thymidine 5'-monophosphate studied by femtosecond fluorescence upconversion spectroscopy
T . Gustavsson, A. Sharonov, D. Markovitsi
Chem. Phys. Lett. 2002, 351, 195.

Adenine, deoxyadenosine and deoxyadenosine 5'-monophosphate studied by femtosecond fluorescence upconversion spectroscopy 
T . Gustavsson, A. Sharonov, D. Onidas, D. Markovitsi
Chem. Phys. Lett. 2002, 356, 49.

We report fluorescence measurements of DNA components performed on the femtosecond time-scale using the fluorescence upconversion technique. Aqueous solutions of thymine (T), thymidine (dT) and thymidine 5′-monophosphate (TMP) were studied in room temperature by excitation at 267 nm and detection at wavelengths between 310 and 380 nm. The fluorescence decays are complex and cannot be described by single exponentials. About 25% of the fluorescence disappears within 150 fs and the remaining part decays more slowly when going from the base through the nucleoside to the nucleotide. The initial fluorescence anisotropy was found to be 0.35±0.03 and did not show any drastic change on the examined time interval.

Aqueous solutions of adenine (A), deoxyadenosine (dA) and deoxyadenosine 5′-monophosphate (dAMP) were studied in room temperature by femtosecond fluorescence upconversion. The fluorescence decays cannot be described by single exponentials. They consist of an ultrafast component (230 fs for A, <100 fs for dA and dAMP) and a slower one (8 ps for A, 0.5 ps for dA and dAMP). The slow component constitutes 95% of the total fluorescence (time-integrated) for the base while only 24% for the nucleoside or the nucleotide. The initial fluorescence anisotropy is 0.30±0.03 for A, 0.25±0.05 for dA and dAMP. The anisotropy of the A fluorescence partially decays during its lifetime due to rotational diffusion.

The excited state lifetimes of uracil, thymine and 5-fluorouracil have been measured using femtosecond UV fluorescence upconversion in various protic and aprotic polar solvents. The fastest decays are observed in acetonitrile and the slowest in aqueous solution while those observed in alcohols are intermediate. No direct correlation with macroscopic solvent parameters such as polarity or viscosity is found, but hydrogen bonding is one key factor affecting the fluorescence decay. It is proposed that the solvent modulates the relative energy of two close-lying electronically excited states, the bright ππ* and the dark nπ* states. This relative energy gap controls the non-radiative relaxation of the ππ* state through a conical intersection close to the Franck–Condon region competing with the ultrafast internal conversion to the ground state. In addition, an inverse isotope effect is observed in D2O where the decays are faster than in H2O.

We report a femtosecond spectroscopic study of the DNA base cytosine in aqueous solution at room temperature. Two different experimental techniques were used, fluorescence upconversion and transient absorption, providing complementary information on the excited state relaxation. While the fluorescence decay is clearly bi-exponential, with an ultrafast (0.2 ps) and a slower (1.3 ps) component, the decay of the transient absorption signal is mono-exponential with a 1.1 ps characteristic time. In addition, the fluorescence anisotropy is also found to decay in a bi-exponential manner. The results are discussed in terms of possible non-radiative relaxation processes that may intervene in the deactivation of the excited state.

The excited state deactivation of two amino-substituted uracils, 5-aminouracil (5AU) and 6-aminouracil (6AU) in aqueous solution was studied by femtosecond fluorescence upconversion. The fluorescence of 6AU decays as fast as that of uracil with a unique time constant of about 100 femtoseconds. The fluorescence of 5AU exhibits a more complex behavior, fundamentally different from what we found in any other uracils: the decays are globally slower (up to several picoseconds) and depend strongly on the wavelength. This difference is attributed to the particular character of the amino group, affecting the out-of-plane motion of the 5-substituent which has been shown to be crucial for the ultrafast internal conversion occurring in uracils. Our observations indicate instead the formation of a transient fluorescent state which in turn is deactivated by a different relaxation process specific to the amino group

Cytosine methylation, which determines the hot spots for DNA photo-damage, is shown to induce a red-shift of the nucleoside absorption spectrum, making the chromophore more vulnerable to solar radiation, and a tenfold increase of the fluorescence lifetime, making excited state reactions more probable. A femtosecond investigation of the excited state deactivation reveals a quite complex mechanism.

The excited-state properties of uracil, thymine, and nine other derivatives of uracil have been studied by steady-state and time-resolved spectroscopy. The excited-state lifetimes were measured using femtosecond fluorescence upconversion in the UV. The absorption and emission spectra of five representative compounds have been computed at the TD−DFT level, using the PBE0 exchange-correlation functional for ground- and excited-state geometry optimization and the Polarizable Continuum Model (PCM) to simulate the aqueous solution. The calculated spectra are in good agreement with the experimental ones. Experiments show that the excited-state lifetimes of all the compounds examined are dominated by an ultrafast (<100 fs) component. Only 5-substituted compounds show more complex behavior than uracil, exhibiting longer excited-state lifetimes and biexponential fluorescence decays. The S0/S1 conical intersection, located at CASSCF (8/8) level, is indeed characterized by pyramidalization and out of plane motion of the substituents on the C5 atom. A thorough analysis of the excited-state Potential Energy Surfaces, performed at the PCM/TD−DFT(PBE0) level in aqueous solution, shows that the energy barrier separating the local S1 minimum from the conical intersection increases going from uracil through thymine to 5-fluorouracil, in agreement with the ordering of the experimental excited-state lifetime.

The excited state properties of uracil, thymine and four analogous uracil compounds have been studied in acetonitrile by steady-state and time-resolved spectroscopy. The excited state lifetimes were measured using femtosecond UV fluorescence upconversion. The excited state lifetimes of uracil and its 1- and 3-methyl substituted derivatives are well described by one ultrafast (100 fs) component. Five substituted compounds show a more complex behavior, exhibiting longer excited state lifetimes and bi-exponential fluorescence decays. These longer decays are substantially faster in acetonitrile than in aqueous solution showing that the excited state deactivation mechanism is in part governed by the solvent.

Photoinduced biological processes are complex and can often be reduced to a series of sequential events after the absorption of the photon. Each of these steps, if separable, can be compared with the evolution of a much simpler system, mimicking its essential characteristics, a biomimetic system. This stems from the very local properties of the initial event where a small reaction centre has been excited, spatially limited within the biomolecule. Gas phase conditions provide the unique way to study these model systems.

Time-resolved photoion and photoelectron velocity mapped images from NO₂ excited close to its first dissociation limit [to NO(X²P) + O(³P₂)] have been recorded in a two colour pump–probe experiment, using the frequency-doubled and frequency-tripled output of a regeneratively amplified titanium–sapphire laser. At least three processes are responsible for the observed transient signals; a negative pump–probe signal (corresponding to a 266 nm pump), a very shortlived transient close to the cross-correlation of the pump and probe pulses but on the 400 nm pump side, and a longer-lived positive pump–probe signal that exhibits a signature of wavepacket motion (oscillations). These transients have two main origins; multiphoton excitation of the Rydberg states of NO₂ by both 266 and 400 nm light, and electronic relaxation in the ¹²B₂ state of NO₂, which leads to a quasi-dissociated NO2 high in the ¹²A₁ electronic ground state and just below the dissociation threshold.

The detection and analysis of toxic gases are of great importance in many areas, including the monitoring of air quality, the safety of workers in industrial plants, the control of industrial processes and the protection of the population against terrorists’ chemical weapons. Due to the large number and variety of toxic pollutants and the lack of a universal detection system, numerous methods and techniques have been proposed. However, most of the available techniques have several drawbacks in terms of the required high sensitivity and/or selectivity, and/or fast sampling time, especially when it is needed to detect low level of gas in the part per billion (ppb) range. Moreover, when these requirements are met, the resulting price is often prohibitive. Therefore, the development of versatile sensors which can be easily adapted to the detection of a wide range of pollutants is an important and significant challenge.
To this end, the “Chemical Sensors” team had proposed new ideas based on chemical sensors and had developed new strategies aimed at improving the trapping and detection of the pollutants.

Contact: thu-hoa.tran-thi@cea.fr

Introduction

This research deals with the elaboration and study of solid state nanocomposite structures built using pre-synthesized objects handled in liquids (bottom-up approach). This approach affords a way to study the impact of nanoscale modifications on the macroscopic properties of nanostructures which can be therefore optimized. It is developed on model systems based on platinum nanoparticles chemically functionalized with 4-mercaptoaniline (mother particles) from which chemical modification performed on this organic crown affords new nano-objets with controlled features and composition. Thanks to the organic crown grafted on the platinum core (2nm diameter), the different nanoparticles can be handled in solution like molecules.
 

Collaborations : A. Huetz et al., Laboratoire d’Interaction des X Avec la Matière, CNRS Université Paris-Sud (Orsay, France)

Two-color XUV/IR ionization of rare gases has been investigated in so-called “complete experiments”, where all the momenta of electrons and ions from the same ionization event are simultaneously measured (vectorial correlations), using a momentum imaging spectrometer in the coincidence mode (developed by the LIXAM). In single photoionization (PI) of He, angular distribution evidences interferences between open channels [O. Guyétand et al., J. Phys. B 41, 051002 (2008)]. Interestingly, the interference depends on the attosecond delay between harmonics and laser pulses, as illustrated in the simulations in the figure, in good agreement with the delay-averaged experimental points. Conversely, angular distributions in PI can be used to characterize the XUV spectral phase, besides other methods such as RABBITT (see High Harmonic Generation and Attosecond physics).
 

The excited state dynamics of the tert-butyl radical, t-C₄H₉, was investigated by femtosecond time-resolved photoionization and photoelectron spectroscopy. The experiments were supported by ab initio calculations. Tert-butyl radicals, generated by flash pyrolysis of azotert-butane, were excited into the A ²A₁ (3s) state between 347 and 307 nm and the 3p band at 274 and 268 nm and ionized by 810 nm radiation, in a [1+2’] or [1+3’] process. Electronic structure calculations confirm that the two states are of s- and p-Rydberg character, respectively. The carbon framework becomes planar and thus ion-like in both states. 

Understanding the primary photophysical processes in molecules is essential for interpreting their photochemistry, because molecules rarely react from the initially excited electronic state. In this study the ultrafast excited-state dynamics of chlorophenylcarbene (CPC) and trifluoromethylphenylcarbene (TFPC), two species that are considered as models for carbene dynamics, were investigated by femtosecond time-resolved pump probe spectroscopy in the gas phase. Their dynamics was followed in real time by time-resolved photoionization and photoelectron imaging. CPC was excited at 265 nm into the 3 1A′ state, corresponding to excitation from a π-orbital of the aromatic ring into the LUMO. The LUMO contains a contribution of the p-orbital at the carbene center. Three time constants are apparent in the photoelectron images: A fast decay process with τ1 ≈ 40 fs, a second time constant of τ2 ≈ 350 fs, and an additional time constant of τ3 ≈ 1 ps. The third time constant is only visible in the time-dependence of low kinetic energy electrons. Due to the dense manifold of excited states between 3.9 and 5 eV, known from ab initio calculations, the recorded time-resolved electron images show broad and unstructured bands. A clear population transfer between the states thus can not directly be observed. The fast deactivation process is linked to either a population transfer between the strongly coupled excited states between 3.9 and 5.0 eV or the movement of the produced wave packet out of the Franck−Condon region. Since the third long time constant is only visible for photoelectrons at low kinetic energy, evidence is given that this time constant corresponds to the lifetime of the lowest excited A 1A′ state. The remaining time constant reflects a deactivation of the manifold of states in the range 3.9−5.0 eV down to the A 1A′ state.

J. Am. Chem. Soc., 2008, 130 (45), 14908-14909

The time evolution of electronically excited vitamin B₁₂ (cyanocobalamin) has been observed for the first time in the gas phase. It reveals an ultrafast decay to a state corresponding to metal excitation. This decay is interpreted as resulting from a ring to metal electron transfer. This opens the observation of the excited state of other complex biomimetic systems in the gas phase, the key to the characterisation of their complex evolution through excited electronic states.

Chem. Phys. (2007)

Collaborations : S. Guizard et al., CEA-IRAMIS-Laboratoire des Solides Irradiés (Palaiseau, France)
A. Belski, Laboratoire de Physico-Chimie des Matériaux Luminescents, Université Lyon 1, (Villeurbanne, France)
P. Martin et al., Centre d’Etudes des Lasers Intenses et Applications (Bordeaux, France)
M. Kirm et al., Institute of Physics, University of Tartu (Tartu, Estonia)
A. Vasil’ev, Department of Optics and Spectroscopy, Moscow Lomonosov University (Moscow, Russia)
L. Juha et al., Institute of Physics, Academy of Sciences of the Czek Republic (Prague, Czek Republic)

As compared with IR-visible laser excitation, intense XUV pulses induce specific excitation in solids. The main feature is that single-photon excitation is localized at the surface (absorption length ~ nm), where high density of electronic excitation can be obtained (~1020 /cm3). No further multi-photon heating of the electrons takes place so that the energy is deposited in a controlled manner, in space, time and amount. This type of excitation is particularly relevant for studying relaxation dynamics at femtosecond scale.
As an example, we have investigated the electronic relaxation after XUV excitation in dielectric tungstate crystals, e.g., CdWO4 and CaWO4, which are used as fast scintillators. Long term relaxation – at ns to µs scale – is monitored through the time-resolved luminescence of self-trapped excitons (STE). We evidence that non radiative decay channels can efficiently quench the excitons at short time (< ns) and high excitation density; they involve STE-STE (dipole-dipole) interaction [M. Kirm et al. Physica Status Solidi (c) 4, 870 (2007) ; M. De Grazia et al., Proc. SPIE Vol. 6586, 65860I (2007)].
 

Atoms in a strong laser field: an electron wave packet is launched and driven by the field over one optical cycle. The EWP can return to its parent ion and be scattered as an outgoing electron wave or an attosecond burst of XUV light. The EWP recollision has therefore a double interest: it can be exploited either as a probe of the system with an extreme resolution, or as an ultra-short source of XUV light.

The dynamics of atomic and molecular electrons in a strong laser field is particularly rich and has been a central research topic at LIDYL for more than thirty years. This program is experimentally and theoretically continued in the Attophysics group.

Basically, the ultra-fast electron dynamics in a strong laser field can be described from both quantum and semi-classical concepts, as for instance electron wave packets and electron trajectories, respectively. The semi-classical picture has popularized the elementary dynamical process under the so-called “three-step” model [P. Corkum, Phys. Rev. Lett. 71, 1994 (1993)].

Its profound physical content is illustrated in the figure. When the atom (molecule) is submitted to a laser field – in the intensity range 10¹³-10¹⁶ W/cm², that is strong enough to distort significantly the core potential -, an electron initially in a valence orbital can escape the core (step 1). This electron subsequently "rides" the laser field and may return back to its parent core within one optical cycle (of duration 2.7 fs = 2.7 10^-15 s with the infra-red lasers we use), after it has gained kinetic energy in the field up to a few tens or even hundreds of electronvolts (step 2). In the recollision with the core, the electron can be quasi-elastically scattered (electron diffraction) or inelastically but coherently scattered (step 3). In the latter inelastic recollision, the electron can either further ionize the core or recombine radiatively with it, releasing its energy as an attosecond burst of extreme–UV light. The above three steps including the attosecond emission constitute the elementary sequence in High Harmonic Generation or HHG, first observed in 1987 simultaneously in Chicago and Saclay [A. Mc Pherson et al., J. Opt. Soc. Am. B 4, 595 (1987), M. Ferray et al., J. Phys. B 21, L31 (1988)]. Each optical cycle drives two recollisions so that a train of attosecond pulses in produced in HHG; their temporal characterisation was first achieved in Saclay in 2001 [P.-M. Paul et al., Science 292, 1689 (2001)].

The atomic/molecular electron dynamics in the strong field encompass basic processes, such as ionization and EWP scattering in the different channels, which are studied for themselves along by two research lines. They are detailed in the Multiple ionization & Molecular Imaging and High Harmonic Generation and Attosecond physics pages.

Now, speaking quantum mechanics, the electron is better described as an electron wave packet (EWP) that dynamically splits into two parts, respectively bound and quasi-free, in the laser field, where the quasi-free component undergoes the recollision and scattering onto the core. The free EWP has a de Broglie wavelength in the Angstrom range, which makes it a very appropriate local probe of the system which extends over a comparable scale. Since the EWP probe has attosecond temporal resolution, it can in principle image ultra-fast motion of electrons and nuclei in molecules. Two research lines, described in the Ultra-fast imaging of molecules from electron diffraction and High Harmonic Generation and Attosecond physics pages, build on this "self-probing" paradigm.

Recolliding EWP in a strong laser field. The two coherent scattering channels, EWP diffraction and EWP radiative recombination keep an imprint of the nuclear structure and the electronic orbital in the molecule.

Besides the fundamental studies of the electron dynamics in strong field, and its use as a probe of transient systems, HHG provides with a source of ultra-short coherent pulses in the XUV (from 100 nm down to a few nm). The source's brightness, which reflects the high instantaneous flux and coherence in both the "narrowband" femtosecond and "broadband" attosecond ranges and its natural synchronization with a driving laser, make it very attractive for a number of applications. Among the Examples of applications we have performed multi-color Photoionization in the gas phase, and studies of XUV/solid interaction in the solid state. The coherence properties and partial tunability of the HHG source make it attractive for Seeding a Free Electron Laser, which constitutes another research line. A promising new application concerns the Coherent diffraction imaging of nanometric objects. Most of the applications are developed in collaboration with expert groups, either in France or in Europe, USA, Canada, Japan,…

Eventually, we pursue a theoretical activity to support the several experimental programs. It focuses on microscopic aspects of the gas phase-strong field interaction, i.e., the electron dynamics in atoms and molecules, including Strong Field Approximation (SFA) models in HHG. It also deals with the macroscopic aspects of the interaction, with the development of 3D propagation codes for the laser and XUV fields.

PERPETUALLY UNDER CONSTRUCTION

Interacting magnetic (single-domain) nanoparticles

Single domained ferro- or ferri-magnetic nanoparticles with unique anistropy axis (easy-magnetization axis) are superparamagnetic (SPM) in the absence of inter-particle interactions. That is, the magnetic moment of a particle can fluctuate randomly by thermal fluctuations at high enough temperatures, just as an atomic spin in a paramagnetic material. At low temperatures, the thermal energy becomes smaller than the anisotropy barrier energy inducing particles' magnetic moments to be blocked in the direction of the easy-magnetization axis. This blocking of magnetic moments occur at the temperature T_B determined by the particle's size and its composition.

When nanoparticles are sufficiently close to one another, the random, long-range dipole-dipole interactions create a collective phase at low temperatures. Such concentrated nanoparticle assemblies can be made into regular crystal lattice or in a completely random configuration when dispersed in fluid media such as water, oil, or glycerol. In these systems, the dipole-dipole interaction energy added to the individual partciles' anisotropy energy pushes the 'blocking' temperature higher. In some cases of concentrated, monodisperse nanoparticles in frozen media (called ferrofluids) a magnetic state of Superspin Glass has been witnessed. This state is analogous to atomic spin glass states in which the randomness of spin interactions create frustration among them such that a true ground state can never be reached. The name 'superspin' has its origin in the individual nanoparticle's large magnetic moment (e.g. 10⁴ μ_B per particle of γ-Fe2O3 with 8.6nm diameter). Emblematic sign of spin-glass behavior such as the critical slow down near transition temperature as well as the aging and memory effects (although small) have been observed in frozen ferrofluids.

In our group, we have focused our research effort on the Out-of-Equilibrium dynamics in the superspin glass state of concentrated maghemite ferrofluids. More specifically, we have examined the aging behavior through the thermoremanant magnetization, the AC susceptibility relaxation and zero-field cooled magnetization measurements through which the growing number of correlated superspins has been extracted. Recently, we have investigated the effect of textruization (the anisotropy axis alignment) on the aging dynamics of ferrofluid superspin glass. These experimental studies were conducted using a bulk SQUID magnetometer (CRYOGENIC^(TM) S600).