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.

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).