SiF4 anomalous behaviour reassessed

The Si 1s−1, Si 2s−1, and Si 2p−1 photoelectron spectra of the SiX4 molecules with X = F, Cl, Br, CH3 were measured. From these spectra the Si 1s−1 and Si 2s−1 lifetime broadenings were determined, revealing a significantly larger value for the Si 2s−1 core hole of SiF4 than for the same core hole of the other molecules of the sequence. This finding is in line with the results of the Si 2p−1 core holes of a number of SiX4 molecules, with an exceptionally large broadening for SiF4. For the Si 2s−1 core hole of SiF4 the difference to the other SiX4 molecules can be explained in terms of Interatomic Coulomb Decay (ICD)-like processes. For the Si 2p−1 core hole of SiF4 the estimated values for the sum of the Intraatomic Auger Electron Decay (IAED) and ICD-like processes are too small to explain the observed linewidth. However, the results of the given discussion render for SiF4 significant contributions from Electron Transfer Mediated Decay (ETMD)-like processes at least plausible. On the grounds of our results, some more molecular systems in which similar processes can be observed are identified

Molecular movie of ultrafast coherent rotational dynamics of OCS

Recording molecular movies on ultrafast timescales has been a longstanding goal for unravelling detailed information about molecular dynamics. Here we present the direct experimental recording of very-high-resolution and -fidelity molecular movies over more than one-and-a-half periods of the laser-induced rotational dynamics of carbonylsulfide (OCS) molecules. Utilising the combination of single quantum-state selection and an optimised two-pulse sequence to create a tailored rotational wavepacket, an unprecedented degree of field-free alignment, 〈cos2θ2D〉 = 0.96 (〈cos2θ〉 = 0.94) is achieved, exceeding the theoretical limit for single-pulse alignment. The very rich experimentally observed quantum dynamics is fully recovered by the angular probability distribution obtained from solutions of the time-dependent Schrödinger equation with parameters refined against the experiment. The populations and phases of rotational states in the retrieved time-dependent three-dimensional wavepacket rationalises the observed very high degree of alignment

a tool to disentangle overlapping core-excited states

We have measured resonant-Auger decay following Cl 1s−1 excitations in HCl and CH3Cl molecules, and extracted the pseudo-cross sections of different Cl 2p−2 final states. These cross sections show clear evidence of shake processes as well as contributions of electronic state-lifetime interference (ELI). To describe the spectra we developed a fit approach that takes into account ELI contributions and ultrafast nuclear dynamics in dissociative core-excited states. Using this approach we utilized the ELI contributions to obtain the intensity ratios of the overlapping states Cl 1s−14pπ/1s−14pσ in HCl and Cl 1s−14pe/1s−14pa1 in CH3Cl. The experimental value for HCl is compared with theoretical results showing satisfactory agreement

Hard-X-Ray-Induced Multistep Ultrafast Dissociation

Creation of deep core holes with very short (τ≤1  fs) lifetimes triggers a chain of relaxation events leading to extensive nuclear dynamics on a few- femtosecond time scale. Here we demonstrate a general multistep ultrafast dissociation on an example of HCl following Cl 1s→σ∗ excitation. Intermediate states with one or multiple holes in the shallower core electron shells are generated in the course of the decay cascades. The repulsive character and large gradients of the potential energy surfaces of these intermediates enable ultrafast fragmentation after the absorption of a hard x-ray photon

Molecular movie of ultrafast coherent rotational dynamics of OCS

Recording molecular movies on ultrafast timescales has been a longstanding goal for unravelling detailed information about molecular dynamics. Here we present the direct experimental recording of very-high-resolution and -fidelity molecular movies over more than one-and-a-half periods of the laser-induced rotational dynamics of carbonylsulfide (OCS) molecules. Utilising the combination of single quantum-state selection and an optimised two-pulse sequence to create a tailored rotational wavepacket, an unprecedented degree of field-free alignment, 〈cos2θ2D〉 = 0.96 (〈cos2θ〉 = 0.94) is achieved, exceeding the theoretical limit for single-pulse alignment. The very rich experimentally observed quantum dynamics is fully recovered by the angular probability distribution obtained from solutions of the time-dependent Schrödinger equation with parameters refined against the experiment. The populations and phases of rotational states in the retrieved time-dependent three-dimensional wavepacket rationalises the observed very high degree of alignment

Atomic-resolution imaging of carbonyl sulfide by laser-induced electron diffraction

Measurements on the strong-field ionization of carbonyl sulfide molecules by short, intense, 2 µm wavelength laser pulses are presented from experiments where angle-resolved photoelectron distributions were recorded with a high-energy velocity map imaging spectrometer, designed to reach a maximum kinetic energy of 500 eV. The laser-field-free elastic-scattering cross section of carbonyl sulfide was extracted from the measurements and is found in good agreement with previous experiments, performed using conventional electron diffraction. By comparing our measurements to the results of calculations, based on the quantitative rescattering theory, the bond lengths and molecular geometry were extracted from the experimental differential cross sections to a precision better than ±5 pm and in agreement with the known values

Argon KLL Auger spectrum: Initial states, core-hole lifetimes, shake, and knock-down processes

State-of-the-art argon KLL Auger spectra measured using photon energies of hν=3216 and 3400 eV are presented along with an Ar [1s] photoelectron spectrum (square brackets indicate holes in the respective orbital). The two different photon energies used for measuring the Auger spectra allow distinguishing between the shake transitions during the Auger decay and the Auger transitions of the photoelectron satellites. A complete assignment of satellite transitions is provided, partially based on configuration-interaction calculations. In addition, Ar [1s3(s,p)]n′l′→[2p2(1D2)] transitions are observed, which can be explained by knock-down transitions leading to a direct exchange of angular momentum between the excited electron and the Auger electron. The lifetime broadenings of the Ar [2s] single-core-hole state and the [2s2] and [2s2p] double-core-hole states are also determined, confirming previously observed trends for double-core-hole states

Processus de corrélations électroniques dans la photoionisation d'atomes et de molécules en couche profonde

Synchrotron radiation in the tender x-ray energy range (2-13 keV) allows deep core-shell excitation/ionization of atoms and molecules. The electronic states populated have ultrashort lifetimes, in the order of one femtosecond. The atoms will then relax through emission of a photon or an electron. In this thesis, we used electron spectroscopy as a tool to study the different processes implied by the interaction between the matter and highly energetic radiation. In the first part, the lifetime of the excited electronic states is used as an intern clock allowing to measure nuclear dynamics in the sub-femtosecond timescale. The lifetime broadenings of the populated electronic states are large enough so that these states overlap, thus allowing their coherent excitation which may lead to interferences phenomena during the relaxation step. This is the subject of the second part of this work, in which we present a model that allows the extraction of these interference terms. In the last part, we show it is also possible to form multiply excited/ionized electronic states, and that our experimental setup allows to measure their lifetimes, and the disentanglement of the many contributions overlapping in the electron spectra. The Ariadne’s thread of this work is to try to apprehend the different electronic correlation processes following the excitation of the studied system by a highly energetic photon, such as how electrons share the incident excess energy or the angular momentum transferred by the incident photon.Le rayonnement synchrotron dans la gamme d'énergie des rayons X tendres (2-13 keV) permet l'excitation/ionisation d'atomes et de molécules en couche profonde. Les états ainsi peuplés ont des durées de vie ultra-courtes, de l'ordre de la femtoseconde. Les atomes vont alors se relaxer par émission d'un photon ou d'un électron. Dans cette thèse, nous avons utilisé la spectroscopie d'électrons afin d'étudier les différents processus induits par l'interaction entre la matière et un rayonnement très énergétique. Dans la première partie, la durée de vie des états électroniques excités est utilisée comme une horloge interne permettant la mesure du mouvement nucléaire à l'échelle de temps sub-femtoseconde. Les élargissements naturels dus à la durée de vie des états électroniques peuplés sont suffisamment grands pour que ces états se recouvrent, permettant l'excitation simultanée de plusieurs états intermédiaires pouvant causer des phénomènes d'interférences lors de l'étape de relaxation. C'est le sujet de la deuxième partie de cette thèse, où nous présentons un modèle permettant d'extraire ces termes d'interférences. Dans la dernière partie, nous montrons qu'il est également possible de peupler des états électroniques multiplement excités/ionisés, et que notre dispositif expérimental permet d'en extraire les durées de vie, ainsi que de résoudre toutes les contributions se recouvrant dans les spectres d'électrons. Le fil d'Ariane de ce travail est de tenter d'appréhender les différents processus de corrélations électroniques suite à l'excitation du système étudié via un photon de haute énergie, comme le partage d'excès d'énergie ou de moment angulaire entre plusieurs électrons

Electronic correlation processes in deep core-shell photoionization of atoms and molecules

Le rayonnement synchrotron dans la gamme d'énergie des rayons X tendres (2-13 keV) permet l'excitation/ionisation d'atomes et de molécules en couche profonde. Les états ainsi peuplés ont des durées de vie ultra-courtes, de l'ordre de la femtoseconde. Les atomes vont alors se relaxer par émission d'un photon ou d'un électron. Dans cette thèse, nous avons utilisé la spectroscopie d'électrons afin d'étudier les différents processus induits par l'interaction entre la matière et un rayonnement très énergétique. Dans la première partie, la durée de vie des états électroniques excités est utilisée comme une horloge interne permettant la mesure du mouvement nucléaire à l'échelle de temps sub-femtoseconde. Les élargissements naturels dus à la durée de vie des états électroniques peuplés sont suffisamment grands pour que ces états se recouvrent, permettant l'excitation simultanée de plusieurs états intermédiaires pouvant causer des phénomènes d'interférences lors de l'étape de relaxation. C'est le sujet de la deuxième partie de cette thèse, où nous présentons un modèle permettant d'extraire ces termes d'interférences. Dans la dernière partie, nous montrons qu'il est également possible de peupler des états électroniques multiplement excités/ionisés, et que notre dispositif expérimental permet d'en extraire les durées de vie, ainsi que de résoudre toutes les contributions se recouvrant dans les spectres d'électrons. Le fil d'Ariane de ce travail est de tenter d'appréhender les différents processus de corrélations électroniques suite à l'excitation du système étudié via un photon de haute énergie, comme le partage d'excès d'énergie ou de moment angulaire entre plusieurs électrons.Synchrotron radiation in the tender x-ray energy range (2-13 keV) allows deep core-shell excitation/ionization of atoms and molecules. The electronic states populated have ultrashort lifetimes, in the order of one femtosecond. The atoms will then relax through emission of a photon or an electron. In this thesis, we used electron spectroscopy as a tool to study the different processes implied by the interaction between the matter and highly energetic radiation. In the first part, the lifetime of the excited electronic states is used as an intern clock allowing to measure nuclear dynamics in the sub-femtosecond timescale. The lifetime broadenings of the populated electronic states are large enough so that these states overlap, thus allowing their coherent excitation which may lead to interferences phenomena during the relaxation step. This is the subject of the second part of this work, in which we present a model that allows the extraction of these interference terms. In the last part, we show it is also possible to form multiply excited/ionized electronic states, and that our experimental setup allows to measure their lifetimes, and the disentanglement of the many contributions overlapping in the electron spectra. The Ariadne’s thread of this work is to try to apprehend the different electronic correlation processes following the excitation of the studied system by a highly energetic photon, such as how electrons share the incident excess energy or the angular momentum transferred by the incident photon

Selenium in Proteins: Conformational Changes Induced by Se Substitution on Methionine, as Studied in Isolated Model Peptides by Optical Spectroscopy and Quantum Chemistry

International audienceThe side-chain of methionine residues is long enough to establish NH⋯S H-bonds with neighboring carbonyl groups of the backbone, giving rise to so-called intra-residue 6δ and inter-residue 7δ H-bonds. The aim of the present article is to document how the substitution of sulfur with a selenium atom affects the H-bonding of the Met system. This was investigated both experimentally and theoretically by conformation-resolved optical spectroscopy, following an isolated molecule approach. The present work emphasizes the similarities of the Met and Sem residues in terms of conformational structures, energetics, NH⋯Se/S H-bond strength and NH stretch spectral shifts, but also reveals subtle behavior differences between them. It provides evidence for the sensitivity of the H-bonding network with the folding type of the Sem/Met side-chains, where a simple flip of the terminal part of the side-chain can induce an extra 50 cm−1 spectral shift of the NH stretch engaged in a 7δ NH⋯S/Se bond