Laser-Induced Electron Diffraction in Chiral Molecules

Strong laser pulses enable probing molecules with their own electrons. The oscillating electric field tears electrons off a molecule, accelerates them, and drives them back toward their parent ion within a few femtoseconds. The electrons are then diffracted by the molecular potential, encoding its structure and dynamics with angstrom and attosecond resolutions. Using elliptically polarized laser pulses, we show that laser-induced electron diffraction is sensitive to the chirality of the target. The field selectively ionizes molecules of a given orientation and drives the electrons along different sets of trajectories, leading them to recollide from different directions. Depending on the handedness of the molecule, the electrons are preferentially diffracted forward or backward along the light propagation axis. This asymmetry, reaching several percent, can be reversed for electrons recolliding from two ends of the molecule. The chiral sensitivity of laser-induced electron diffraction opens a new path to resolve ultrafast chiral dynamics

Spectroscopie d'émission par transformation de fourier des radicaux SiC et HCSi

Le but de cette thèse était d'étudier des radicaux siliciés d'intérêt pour la production de matériaux et couches minces ainsi que d'intérêt astrophysique, notament le radical SiC, en utilisant la technique de spectroscopie d'émission parTransformée de Fourier.Etant connue la difficulté d'obtenir un signal spectroscopique du radical SiC, il a fallu dans un premier temps trouver une source à la fois capable de le produire et appropriée à notre technique d'enregistrement. Les résultats présentés dans ce mémoire montrent qu'en utilisant une décharge électrique continue établie dans untube à décharge de type Schuller dans un mélange d'hélium et d'hexamethyl disilane, il a été possible de produire de manière efficace et reproductible le radical SiC, ainsi qu'un nouveau radical HCSi.En ce qui concerne le radical SiC, nous avons mis l'accent sur l'interprétation du spectre de la transition C3Π-X3Π. L'examen de certaines constantes moléculaires effectives de l'état C3Π indique un comportement anormal de celles-ci avec le nombre quantique de vibration. Pour l'expliquer, nous avons proposé deux hypothèses : une dépendance des constantes d'interaction de spin avec la distance internucléaire et lamanifestation de perturbations de type spin-orbite entre l'état C3Π et d'autres états électroniques proches en énergie.Outre les deux transitions électroniques du radical SiC, grâce à cette source d'émission, nous avons détecté pour la première fois en phase gazeuse le radical silicié HCSi. Les ensembles de constantes moléculaires déterminées pour les niveaux vibroniques observés sont en bon accord avec les calculs ab initio. L'analysespectroscopique et les valeurs anormales de certaines constantes moléculaires indiquent l'existence de perturbations rotationnelles dans le niveau vibrationnel (100) de l'état excité et respectivement, d'une perturbation vibrationnelle dans le niveau vibrationnel (100) de l'état électronique fondamental, due à l'interaction de Fermi avec le niveau (020) du même état

Using high harmonic radiation to reveal the ultrafast dynamics of radiosensitiser molecules

International audience5-Fluorouracil (5FU) is a radiosensitiser molecule routinely used in combined chemo-and radio-therapies to enhance and localize cancer treatments. We have employed ultra-short XUV pulses produced by high harmonic generation (HHG) as a pump pulse to study the dynamics underlying the photo-stability and the radiation damage of this molecule. This work shows that it is possible to resolve individual dynamics even when using unselected HH. By comparing the results with those obtained in the multiphoton absorption at 400 nm, we were able to identify the frequencies of the HH comb relevant to the recorded dynamics: HH5 and HH3. The latter excites a high-lying Rydberg state interacting with a valence state and its dynamics is revealed by a 30 fs decay signal in the parent ion transient. Our results suggest that the same photoprotection mechanisms as those conferring photostability to the neutral nucleobases and to the DNA appear to be activated: HH5 excites the molecule to a state around 10.5 eV that undergoes an ultrafast relaxation on a timescale of 30 fs due to nonadiabatic interactions. This is followed sequentially by a 2.3 ps internal conversion as revealed by the dynamics observed for another fragment ion. These dynamics are extracted from the fragment ion signals. Proton or hydrogen transfer processes are required for the formation of three fragments and we speculate that the time scale of one of the processes is revealed by a H+ transient signal

Synthesis of Crystalline Structures of CNx Thin Films Deposited on Sapphire, Quartz and Alumina Substrates

International audienceCarbon nitride films raise current interest for their potential applications as ultrahard materials. We report on the formation of crystalline CN structures in films deposited at 248 nm on different substrates (sapphire, quartz, alumina) using the laser-induced CVD method and a gas mixture containing ethylene/nitrous oxide and ammonia as carbon and nitrogen source, respectively. The structural and morphological properties of the films were characterized by different analytical techniques (SEM, TEM, TED and XPS)

Deposition of carbon nitride thin films by IR-laser-induced reactions in carbon-nitrogen gas-phase compounds

International audienceSince the theoretical studies of Liu and Cohen who predicted the existence of a superhard phase of carbon nitride, a great deal of effort was underdone in order to synthesize this hypothetical material with a nitrogen content as high as the 57% present in a (beta) -C3N4 structure. This study presents an attempt to produce CNx thin films using the laser-induced CVD technique. CW CO2 laser was used for irradiating various carbon-nitrogen containing mixtures such as C2H4/N2O/NH3. The CNx films were grown alternatively on bare alumina ((alpha) - Al2O3) substrates and on pre-deposited Ti films. A comparative analysis of nitrogen incorporation in the films obtained in different experimental conditions was performed by means of the X-ray photoelectron spectroscopy. The same method was used to identify the chemical states of the CN system

Laser pyrolysis of carbon-nitrogen gas-phase compounds: An attempted approach to carbon nitride formation

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High-harmonic transient grating spectroscopy of NO2 electronic relaxation.

We study theoretically and experimentally the electronic relaxation of NO(2) molecules excited by absorption of one ∼400 nm pump photon. Semiclassical simulations based on trajectory surface hopping calculations are performed. They predict fast oscillations of the electronic character around the intersection of the ground and first excited diabatic states. An experiment based on high-order harmonic transient grating spectroscopy reveals dynamics occurring on the same time scale. A systematic study of the detected transient is conducted to investigate the possible influence of the pump intensity, pump wavelength, and rotational temperature of the molecules. The quantitative agreement between measured and predicted dynamics shows that, in NO(2), high harmonic transient grating spectroscopy encodes vibrational dynamics underlying the electronic relaxation.MOLCOTUV and ITNICONIC contract