PhD subjects

Many complex molecular systems absorbing light in the near UV spectral range, including those of paramount biological importance, like DNA bases or proteins, are endowed with mechanisms of excited-state deactivation following UV absorption. These mechanisms are of major importance for the photochemical stability of these species since they provide them a rapid and efficient way to dissipate the electronic energy in excess into vibrations, thus avoiding photochemical processes to take place and then structural damages which affect the biological function of the system. In this context, the study of gas phase bio-relevant systems such peptides as proteins building blocks should lead to a better understanding of the photophysical phenomena involved in the relaxation mechanisms of life components. This Ph. D project aims at both investigating the electronic dynamics of bio-relevant model systems, i.e. building block of life components, and documenting the basic phenomena controlling the lifetime of the excited states, through a dual approach using most recent methodological tools, consisting of:



i) An experimental characterization of i) the lifetimes, in nano-, pico- and femtosecond pump-probe experiments, and ii) the nature of the electronic states formed. Sophisticated diagnostic techniques, such as a photo-electron velocity map imaging diagnosis, will be used. These experiments will allow us to identify the relaxation pathways followed by the system, and in particular to assess the role of the several excited states together with the effect of its environment.



ii) A theoretical modeling of the processes involved, in particular to assess the role of specific regions of the potential energy surface (PES), namely the conical intersections, and to determine the motions that trigger deactivation. The systems’ size, their flexibility, the non-covalent interactions, which govern the structures, and the nature of the excited states require the implementation of a computational strategy using sophisticated quantum chemical methods dedicated to excited states (non-adiabatic dynamic, coupled cluster method and multireference configuration interaction method) in order to characterize the first excited states, simulate their PES and eventually determine the relaxation pathways.



Moreover, this work will take place in the following of the ANR project, ESBODYR, for "Excited States of BiO-relevant systems: towards ultrafast conformational Dynamics with Resolution" (Coord V. Brenner, 2014-2018). Finally, the theoretical part will benefit from an access to the national High Performance Computing resources (GENCI/TGCC and DRF/CCRT) as well as from access to both the femtosecond ATTOLab server (Orme des Merisiers) and the Laser Center of the University Paris-Sud (CLUPS).

Many complex molecular systems absorbing light in the near UV spectral range, including those of paramount biological importance, like DNA bases or proteins, are endowed with mechanisms of excited-state deactivation following UV absorption. These mechanisms are of major importance for the photochemical stability of these species since they provide them a rapid and efficient way to dissipate the electronic energy in excess into vibration, thus avoiding photochemical processes to take place and then structural damages which affect the biological function of the system. In this context, the study of gas phase bio-relevant systems such peptides as proteins building blocks should lead to better understanding the photophysical phenomena involved in the relaxation mechanisms of life components. The size of the systems, their flexibility, the existence of non-covalent interactions which governs structures and the nature of the excited states require the use of sophisticated theoretical models in order to characterize the preferentially formed conformations in gas phase as well as to investigate the electronic deactivation mechanisms of the first excited states. The focus of the PhD project concerns the implementation of a computational strategy to both characterize the first excited states and simulate their potential energy surfaces in order to determine the relaxation pathways. This theoretical research project contains then the development, evaluation and validation of modern quantum chemical methods dedicated to excited states. It will be backed up by key gas phase experiments performed in our group on bio-relevant systems using recent spectroscopic techniques which provide precise data on their spectroscopic properties and their electronic dynamics of relaxation. Moreover, it will take place in the context of the following of the ANR project, ESBODYR or "Excited States of BiO-relevant systems: towards ultrafast conformational Dynamics with Resolution" (Coord V. Brenner, 2014-2018) and will benefit from an access to the national High Performance Computing resources (GENCI/TGCC and DRF/CCRT).

Laser technology now makes it possible to generate coherent light pulses with durations down to a few tens of femtosecondes only, with an energy per pulse of up to several Joules. These laser beams are likely to exhibit spatio-temporal coupling, i.e. a spatial dependence of their temporal properties across the beam, which can considerably degrade their performances. Our team has developed over the last few years different techniques to measure the full spatio-temporal structure of such lasers. These advanced measurement techniques have been demonstrated on different lasers, including some for the most powerful systems in operation to date. The objectives of this PhD work will be twofold: 1- to exploit these new advanced techniques to characterize different laser sources, of increasing complexity; 2- to improve these measurement techniques, in particular by reducing the number of required laser shots. This second point will be achieved by both using the modern techniques of compressed sensing, and designing new schemes to encode the relevant information in the measured data. The ultimate goal is to obtain all information on the spatio-temporal structure of the beam in a single laser shot, in contrast to the hundreds of shots required with the present techniques.

Ion pairs are ubiquitous in Nature, from sea water and aerosols, to living organisms. The scientific program of this thesis aims at investigating, at the microscopic scale, neutral ion pairs, isolated and microsolvated in the gas phase, by using an approach combining conformer-selective IR and UV spectroscopy, molecular dynamics simulations and quantum chemistry calculations. Three directions will be explored:

- Spectroscopic characterization of each type of pairs, and application to the dissociative role of the solvent.

- Description of the first steps of crystallisation of ionic compounds.

- Analysis of the influence of counter-ions on the structure of charged biomolecules.



A CV, a motivation letter and the contacts of the Master interships supervisors must be sent before April 22nd, 2019. The selected candidate will access the MESRI 2019 Competition of the 2MIB doctoral School of Paris-Saclay University.

Flexible molecules are ubiquitous in Nature (proteins, sugars, ...) and are at the origin of many applications (drugs, molecular machines, ...). By definition, these molecules exist in several conformations that each have physical, chemical or biological properties which can vary greatly from one conformation to another. Among these properties, photoexcitation and relaxation of the electronic states are particularly sensitive to conformation: for example, the lifetime of the first excited electronic state can vary with the conformation by several orders of magnitude. However, the experimental characterisation of such conformational effects on the excited states remains rare due to the difficulty to specifically study one conformation present in a conformational mixture. This thematic is still poorly documented despite i) a need for experimental results to help the development of theoretical models, and ii) a lack of knowledge in a field where fundamental (conical intersections, ultrafast phenomena) and application (photostability, energy transfer) issues are important.



In this context, the LIDYL laboratory brings together several experimental apparatus allowing an original multi-scale (ns-fs) and conformation-resolved study of the dynamics of the electronic relaxation in flexible molecular systems. The research program will focus on biologically-relevant systems and molecular complexes, and will target:



- the observation of conformer-dependent dynamic processes and their rationalisation.

- the characterization of species previously inaccessible to conventional detection techniques.

A vast range of topics in physics such as the star internal structure, the X-ray emission of accretion disks, the dynamics of inertial confinement fusion, or new radiation sources requires an accurate knowledge of the radiative properties of hot plasmas. Such plasmas exhibit spectra consisting of a large number of lines often merging in unresolved arrays. Statistical methods are used for the interpretation of such spectra. Using second quantization and tensor algebra techniques, one may obtain quantities such as the average and variance of transition energies inside these unresolved arrays. Though a wide literature exists on this subject, certain types of transitions, e.g., magnetic dipole transitions inside a given configuration or processes involving several electron jumps, have not been considered up to now. In addition to this analytical study, a numerical work involving the Flexible Atomic Code will be proposed for this thesis. This study will concern plasmas either at thermodynamic equilibrium, or out of equilibrium, where level population is obtained by solving a system of kinetic equations. Several applications may be foreseen: interpretation of recent opacity measurements performed on LULI2000 laser at Ecole Polytechnique, determination of radiative power losses in a tungsten plasma to characterize tokamak operation, or the open subject of the characterization of photoionized silicon plasma analyzed in Sandia Z-pinch in connection with astrophysical observations.

Light in the extreme ultraviolet (XUV) is a universal probe of mater, may it be in diluted or condensed phase: photons associated with this spectral range carry energy of 10 to 100 eV, sufficient to directly ionize atoms, molecules or solids. Large scale instruments such as synchrotrons or the lately developed free electron lasers (FEL) work in this spectral range and are used to both study fundamental light matter interaction and develop diagnosis tools. However these instruments do not offer the temporal resolution require to study light matter interactions at their ultimate timescales, which is in the attosecond range (1as = 10-18s). An alternative is offered by the recent development of XUV sources based on high order harmonic generation (HHG). They are based on the extremely nonlinear interaction of a femtosecond intense laser beam with a gas target. Our laboratory has pioneered the development, control and design of these sources providing XUV attosecond pulses.



During this PhD project, we will develop specific setups putting at play two beams forming an active grating to generate attosecond pulses with controlled angular momenta, may it be spin or orbital angular momenta. This will open new applications roads through the observations of currently ignored spectroscopic signatures. On the one hand, the fundamental aspects of the coupling of spin and orbital angular momentum of light in the highly nonlinear regime will be investigated, and on the other hand, we will tack attosecond novel spectroscopies, may it be in diluted or condensed phase. In particular, we will chase helical dichroism, which manifest as different absorptions of beams carrying opposite orbital angular moments. These effects are largely ignored to date.

The student will acquire practical knowledge about lasers, in particular femtosecond lasers, and hands on spectrometric techniques of charged particles. He/she will also study strong field physical processes which form the basis for high harmonic generation. He/she will become an expert in attosecond physics. The acquisition of analysis skills, computer controlled experiments skills will be encouraged although not required.



Detailed presentation of the subject : http://iramis.cea.fr/LIDYL/Pisp/thierry.ruchon/