13 research outputs found
High-order harmonic transient grating spectroscopy of SF6 molecular vibrations
special issue : Ultrafast electron and molecular dynamicsInternational audienceStrong field transient grating spectroscopy has shown to be a very versatile tool in time-resolved molecular spectroscopy. Here we use this technique to investigate the high-order harmonic generation from SF6 molecules vibrationally excited by impulsive stimulated Raman scattering. Transient grating spectroscopy enables us to reveal clear modulations of the harmonic emission. This heterodyne detection shows that the harmonic emission generated between 14 to 26 eV is mainly sensitive to two among the three active Raman modes in SF6, i.e. the strongest and fully symmetric nu 1-A1g mode (774 cm-1, 43 fs) and the slowest mode nu5-T2g (524 cm-1, 63 fs). A time-frequency analysis of the harmonic emission reveals additional dynamics: the strength and central frequency of the nu 1 mode oscillate with a frequency of 52 cm-1 (640 fs). This could be a signature of the vibration of dimers in the generating medium. Harmonic 11 shows a remarkable behavior, oscillating in opposite phase, both on the fast (774 cm-1) and slow (52 cm-1) timescales, which indicates a strong modulation of the recombination matrix element as a function of the nuclear geometry. These results demonstrate that the high sensitivity of high-order harmonic generation to molecularvibrations, associated to the high sensitivity of transient grating spectroscopy, make their combination a unique tool to probe vibrational dynamics
Spectroscopie femtoseconde reésolue en temps dans les systÚmes polyatomiques étudieés par l'imagerie de vecteur vitesse et de génération d'harmoniques d'ordre élevé
Revealing the underlying ultrafast dynamics in molecular reaction spectroscopy demands state-of-the-art imaging techniques to follow a molecular process step by step. Femtosecond time-resolved velocity-map imaging is used to study the photodissociation dynamics of chlorine azide (ClN3). Here especially the co-fragments chlorine and N3 are studied on the femtosecond timescale in two excitation energy regions around 4.67 eV and 6.12 eV, leading to the formation of a linear N3 fragment and a cyclic N3 fragment, respectively. This work is the first femtosecond spectroscopy study revealing the formation of cyclic N3. Tetrathiafulvalene (TTF, C6H4S4) electronic relaxation is studied, while scanning the electronic excitation around 4 eV, by time resolved mass and photoelectron spectroscopy. As only few is known about the ion continuum about TTF the imaging photoelectron photoion coincidence (iPEPICO) technique is used in order to disentangle the complex ionic dissociation. The second part of the thesis is based on the generation and application of XUV light pulses by high-order harmonic generation with an intense femtosecond laser pulse in a molecular target. Two types of phase sensitive attosecond spectroscopy experiments were conducted to study the vibrational dynamics of SF6: one using strong field transient grating spectroscopy, where high-order harmonic generation takes place in a grating of excitation, and the second experiment using high-order harmonic interferometry using two intense XUV probe pulses. The temporal dependencies in phase and amplitude reveal the vibrational dynamics in SF6 and demonstrate that high-order harmonic generation is sensitive to the internal excitations. Last but not least, the use of high-order harmonics as a XUV photon source for the velocity-map imaging spectrometer is investigated. Using time-resolved photoelectron imaging, the relaxation dynamics initiated with 15.5 eV in argon and 9.3 eV in acetylene are revealed.Dans cette thÚse, la dynamique de photodissociation de l'azoture de chlore (ClN3) est étudiée dans le domaine temporel par imagerie de vecteur vitesse des photofragments, spécialement du chlore et de N3. Cette imagerie résolue à l'échelle femtoseconde permet d'extraire les temps de dissociation, l'établissement temporel de la balance d'énergie de la réaction ainsi que la conservation des moments. Cette étude a permis de différencier deux domaines d'énergie: l'un menant à la formation d'un fragment N3 linéaire (étude autour de 4.5 eV d'excitation électronique) et le plus intéressant aboutissant à la formation d'un fragment N3 cyclique (autour de 6 eV). Dans une seconde étude, la dynamique de relaxation électronique du tétrathiafulvalÚne (C6H4S4-TTF) est étudiée autour de 4 eV par spectroscopie de masse résolue en temps ainsi que par spectroscopie de photoélectron. Les seuils d'ionisation dissociative sont extraits d'une détection en coïncidence entre les photoélectrons de seuil et les fragments ionisés réalisée sur rayonnement synchrotron. Les deux derniÚres expériences sont basées sur la génération d'harmoniques d'ordre élevé dans l'XUV d'une impulsion femtoseconde à 800 nm ou à 400 nm. Dans la premiÚre expérience, les harmoniques sont couplées à un imageur de vecteur vitesse en tant que rayonnement secondaire VUV. Par imagerie de photoélectron résolue en temps, nous avons révélé ainsi les dynamiques de relaxation des états de Rydberg initiée par une impulsion femtoseconde XUV à 15.5 eV dans l'argon et à 9.3 eV dans l'acétylÚne. Dans la seconde expérience, couramment nommée spectroscopie attoseconde, les harmoniques constituent le signal pompe sonde. Deux types de spectroscopie attoseconde ont été réalisés pour étudier la dynamique vibrationnelle de SF6: une expérience en réseau transitoire créé par deux impulsions pompe Raman avec une impulsion sonde intense générant les harmoniques à partir du réseau d'excitation et une expérience d'interférence de deux rayonnement XUV en champ lointain créés par deux impulsions sonde intenses
Communication: Existence of the doubly excited state that mediates the photoionization of azulene
Femtosecond time-resolved Electronic Relaxation Dynamics in Tetrathiafulvalene
In the present paper the ultrafast electronic relaxation of tetrathiafulvalene (TTF) initiated around 4 eV is studied for the first time by femtosecond time-resolved velocity-map imaging. The goal is to enlighten the broad double structure observed in the absorption spectrum at such energy. By monitoring the transients of the parent cation and its fragments and by varying both the pump and probe wavelengths, two internal conversions are detected with typical time constants of 260 fs and of 600 fs as well as intramolecular vibrational relaxation. Photoelectron images strengthen the analysis of the relaxation process. In addition the formation of the dimer of TTF has been revealed
Comparing Femtosecond Multiphoton Dissociative Ionization of Tetrathiafulvene with Imaging Photoelectron Photoion Coincidence Spectroscopy
In this paper we describe femtosecond
photoionization and the imaging
photoelectron photoion coincidence spectroscopy of tetrathiafulvene,
TTF. Femtosecond photoionization of TTF results in the absorption
of up to twelve 808 nm photons leading to ion internal energies up
to 12.1 eV as deduced from the photoelectron spectrum. Within this
internal energy a variety of dissociation channels are accessible.
In order to disentangle the complex ionic dissociation, we utilized
the imaging photoelectron photoion coincidence (iPEPICO) technique.
Above the dissociation threshold, iPEPICO results show that the molecular
ion (<i>m</i>/<i>z</i> = 204) dissociates into
seven product ions, six of which compete in a 1.0 eV internal energy
window and are formed with the same appearance energy. Ab initio calculations
are reported on the possible fragment ion structures of five dissociation
channels as well as trajectories showing the loss of C<sub>2</sub>H<sub>2</sub> and C<sub>2</sub>H<sub>2</sub>S from high internal
energy TTF cations. A three-channel dissociation model is used to
fit the PEPICO data in which two dissociation channels are treated
as simple dissociations (one with a reverse barrier), while the rest
involve a shared barrier. The two lower energy dissociation channels, <i>m</i>/<i>z</i> = 146 and the channel leading to <i>m</i>/<i>z</i> = 178, 171, 159, 140, and 127, have <i>E</i><sub>0</sub> values of 2.77 ± 0.10 and 2.38 ±
0.10 eV, respectively, and are characterized by Î<i>S</i><sup>âĄ</sup><sub>600 K</sub> values of â9 ±
6 and 1 ± 6 J K<sup>â1</sup> mol<sup>â1</sup>,
respectively. Competing with them at higher internal energy is the
cleavage of the central bond to form the <i>m</i>/<i>z</i> = 102 fragment ion, with an <i>E</i><sub>0</sub> value of 3.65 ± 0.10 eV and Î<i>S</i><sup>âĄ</sup><sub>600 K</sub> = 83 ± 10 J K<sup>â1</sup> mol<sup>â1</sup>
Comparing Femtosecond Multiphoton Dissociative Ionization of Tetrathiafulvene with Imaging Photoelectron Photoion Coincidence Spectroscopy
In this paper we describe femtosecond
photoionization and the imaging
photoelectron photoion coincidence spectroscopy of tetrathiafulvene,
TTF. Femtosecond photoionization of TTF results in the absorption
of up to twelve 808 nm photons leading to ion internal energies up
to 12.1 eV as deduced from the photoelectron spectrum. Within this
internal energy a variety of dissociation channels are accessible.
In order to disentangle the complex ionic dissociation, we utilized
the imaging photoelectron photoion coincidence (iPEPICO) technique.
Above the dissociation threshold, iPEPICO results show that the molecular
ion (<i>m</i>/<i>z</i> = 204) dissociates into
seven product ions, six of which compete in a 1.0 eV internal energy
window and are formed with the same appearance energy. Ab initio calculations
are reported on the possible fragment ion structures of five dissociation
channels as well as trajectories showing the loss of C<sub>2</sub>H<sub>2</sub> and C<sub>2</sub>H<sub>2</sub>S from high internal
energy TTF cations. A three-channel dissociation model is used to
fit the PEPICO data in which two dissociation channels are treated
as simple dissociations (one with a reverse barrier), while the rest
involve a shared barrier. The two lower energy dissociation channels, <i>m</i>/<i>z</i> = 146 and the channel leading to <i>m</i>/<i>z</i> = 178, 171, 159, 140, and 127, have <i>E</i><sub>0</sub> values of 2.77 ± 0.10 and 2.38 ±
0.10 eV, respectively, and are characterized by Î<i>S</i><sup>âĄ</sup><sub>600 K</sub> values of â9 ±
6 and 1 ± 6 J K<sup>â1</sup> mol<sup>â1</sup>,
respectively. Competing with them at higher internal energy is the
cleavage of the central bond to form the <i>m</i>/<i>z</i> = 102 fragment ion, with an <i>E</i><sub>0</sub> value of 3.65 ± 0.10 eV and Î<i>S</i><sup>âĄ</sup><sub>600 K</sub> = 83 ± 10 J K<sup>â1</sup> mol<sup>â1</sup>