Photophysics of indole upon x-ray absorption

A photofragmentation study of gas-phase indole (C$_8$H$_7$N) upon single-photon ionization at a photon energy of 420 eV is presented. Indole was primarily inner-shell ionized at its nitrogen and carbon $1s$ orbitals. Electrons and ions were measured in coincidence by means of velocity map imaging. The angular relationship between ionic fragments is discussed along with the possibility to use the angle-resolved coincidence detection to perform experiments on molecules that are strongly oriented in their recoil-frame. The coincident measurement of electrons and ions revealed fragmentation-pathway-dependent electron spectra, linking the structural fragmentation dynamics to different electronic excitations. Evidence for photoelectron-impact self-ionization was observed.Comment: 11 pages, 6 figure

Auger electron angular distributions following excitation or ionization of the I 3d level in methyl iodide

Auger electron spectra following excitation or ionization of the I 3d level in CH3I have been recorded with horizontally or vertically plane polarized synchrotron radiation. These spectra have enabled the Auger electron angular distributions, as characterized by the β parameter, to be determined. The I 3d photoionization partial cross section of CH3I has been calculated with the continuum multiple scattering approach, and the results show that in the photon energy range over which Auger spectra were measured, the I 3d cross section exhibits an atomic-like behavior and is dominated by transitions into the εf continuum channel. In this limit, the theoretical value of the alignment parameter (A20) characterizing the core ionized state in an atom becomes constant, independent of photon energy. This theoretical value has been used to obtain the Auger electron intrinsic anisotropy parameters (α2) from the β parameters extracted from our normal (non-resonant) molecular Auger spectra. The resulting anisotropy parameters for the M45N45N45 transitions in CH3I have been compared to those calculated for the corresponding transitions in xenon, and the experimental and theoretical results are in good agreement. Anisotropy parameters have also been measured for the M45N1N45, M45N23N45, and M45N45O23 transitions. For the M45N1N45 and M45N23N45 Auger decays in CH3I, the experimentally derived angular distributions do not exhibit the strong dependence on the final ionic state that is predicted for these transitions in xenon. Resonantly excited Auger spectra have been recorded at 620.4 and 632.0 eV, coinciding with the I 3d5/2 → σ* and 3d3/2 → σ* transitions, respectively. The resulting Auger electron angular distributions for the M4N45N45 and M5N45N45 decays were found to exhibit a higher anisotropy than those for the normal process. This is due to the larger photo-induced alignment in the neutral core excited state. For a particular Auger transition, the Auger electron kinetic energy measured in the resonantly excited spectrum is higher than that in the normal spectrum. This shift, due to the screening provided by the electron excited into the σ* orbital, has been rationalized by calculating orbital ionization energies of I 3d excited and I 3d ionized states in CH3I

XUV double-pulses with femtosecond to 650 ps separation from a multilayer-mirror-based split-and-delay unit at FLASH

Extreme ultraviolet (XUV) and X-ray free-electron lasers enable new scientific opportunities. Their ultra-intense coherent femtosecond pulses give unprecedented access to the structure of undepositable nanoscale objects and to transient states of highly excited matter. In order to probe the ultrafast complex light-induced dynamics on the relevant time scales, the multi-purpose end-station CAMP at the free-electron laser FLASH has been complemented by the novel multilayer-mirror-based split-and-delay unit DESC (DElay Stage for CAMP) for time-resolved experiments. XUV double-pulses with delays adjustable from zero femtoseconds up to 650 picoseconds are generated by reflecting under near-normal incidence, exceeding the time range accessible with existing XUV split-and-delay units. Procedures to establish temporal and spatial overlap of the two pulses in CAMP are presented, with emphasis on the optimization of the spatial overlap at long time-delays via time-dependent features, for example in ion spectra of atomic clusters

Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser

In pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C6H3F2I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. We discuss in detail the necessary data analysis steps and describe the origin of the time-dependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment

Alignment, orientation, and Coulomb explosion of difluoroiodobenzene studied with the pixel imaging mass spectrometry (PImMS) camera

Citation: Amini, K., Boll, R., Lauer, A., Burt, M., Lee, J. W. L., Christensen, L., . . . Rolles, D. (2017). Alignment, orientation, and Coulomb explosion of difluoroiodobenzene studied with the pixel imaging mass spectrometry (PImMS) camera. Journal of Chemical Physics, 147(1). doi:10.1063/1.4982220Laser-induced adiabatic alignment and mixed-field orientation of 2,6-difluoroiodobenzene (C6H3F2I) molecules are probed by Coulomb explosion imaging following either near-infrared strong-field ionization or extreme-ultraviolet multi-photon inner-shell ionization using free-electron laser pulses. The resulting photoelectrons and fragment ions are captured by a double-sided velocity map imaging spectrometer and projected onto two position-sensitive detectors. The ion side of the spectrometer is equipped with a pixel imaging mass spectrometry camera, a time-stamping pixelated detector that can record the hit positions and arrival times of up to four ions per pixel per acquisition cycle. Thus, the time-of-flight trace and ion momentum distributions for all fragments can be recorded simultaneously. We show that we can obtain a high degree of one-and three-dimensional alignment and mixed-field orientation and compare the Coulomb explosion process induced at both wavelengths. © 2017 Author(s)

CAMP@FLASH: an end-station for imaging, electron- and ion-spectroscopy, and pump–probe experiments at the FLASH free-electron laser

The non-monochromatic beamline BL1 at the FLASH free-electron laser facility at DESY was upgraded with new transport and focusing optics, and a new permanent end-station, CAMP, was installed. This multi-purpose instrument is optimized for electron- and ion-spectroscopy, imaging and pump–probe experiments at free-electron lasers. It can be equipped with various electron- and ion-spectrometers, along with large-area single-photon-counting pnCCD X-ray detectors, thus enabling a wide range of experiments from atomic, molecular, and cluster physics to material and energy science, chemistry and biology. Here, an overview of the layout, the beam transport and focusing capabilities, and the experimental possibilities of this new end-station are presented, as well as results from its commissioning

Dynamique des processus de photoionisation d atomes et molécules excités en couches profondes

Dans cette thèse de doctorat, nous nous intéressons à la dynamique des processus induits par l'excitation résonante ou l'ionisation directe en couche interne à travers une étude expérimentale de systèmes modèles, l'atome d argon et la molécule de sulfure de carbonyle OCS. Afin de réaliser des mesures sur des systèmes isolés, nous avons développé une nouvelle expérience, CELIMENE, dans laquelle des espèces en phase gazeuse très diluée sont ionisées par interaction avec un rayonnement synchrotron. Lors de l'ionisation de l'atome Ar en couche 1s, nous avons mesuré le recul de l'ion Arn+ causé par l'émission d'électrons Auger ce qui a permis de déterminer le processus de relaxation majoritaire pour chaque état de charge et de déduire le taux d'émission radiative lors d'une seule mesure. Nous avons mesuré l'effet post collisionnel entre l'ion Arq+ (q=1-5) et le photoélectron en coïncidence, ce qui nous a permis de déduire la durée de vie des niveaux intermédiaires. Lors de l'ionisation de OCS en couche S 1s, la mesure de recul du centre de masse est utilisée afin de déterminer la voie dominante dans le processus de relaxation ce qui nous a permis d estimer la durée de vie des états intermédiaires dans un système moléculaire. Une comparaison entre les mesures expérimentales des MFPADs et des calculs théoriques a été faite pour la première fois dans le domaine des X-tendres, pour la photoionisation en couche 1s de l'atome de soufre. Finalement la mesure des moments appliquée au système OCS combinée à une simulation de l'explosion coulombienne de la molécule a permis d'étudier finement la cinématique de O++C++S+.In this PhD thesis, we study dynamical processes induced by resonant excitation or direct ionization in deep K shell through experimental measurement realized on model systems, the argon atom and the carbonyl sulfide molecule, OCS. We developed a new experimental setup, CELIMENE dedicated to coincidence measurement of charged fragments created after interaction between free atoms/molecules and synchrotron radiation. On Argon ionized at the K shell, the ion recoil of Ar n+ due to Auger electrons emission is measured for each ionic state and is used to disentangle relaxation pathways and the radiative decay rate in one measurement. This measurement performed in coincidence with the photoelectron is then used to estimate the lifetime of the intermediate state. For the OCS molecule ionized at the S1s shell, we used molecular mass center recoil to determine the main relaxation pathways for each fragmentation channel and measurement of the PCI effect on the photoelectron allowed a determination of the lifetime of the intermediate states in a molecular system. The comparison of the experimental and theoretical MFPADs has been done for the first time in the tender x-ray region for the OCS molecule ionized above S1s threshold. Finally, measurements of the momentum of the O+ C+ S+ fragments combined with a Coulomb explosion simulation allowed us to carefully study the dynamics the molecular three body fragmentation.PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

Femtosecond photoelectron diffraction: a new approach to image molecular structure during photochemical reactions

Continuing technical advances in the creation of (sub-) femtosecond VUV and X-ray pulses with Free-Electron Lasers and laser-based high-harmonic-generation sources have created new opportunities for studying ultrafast dynamics during chemical reactions. Here, we present an approach to image the geometric structure of gas-phase molecules with fewfemtosecond temporal and sub-Ångström spatial resolution using femtosecond photoelectron diffraction. This technique allows imaging the molecules “from within” by analyzing the diffraction of inner-shell photoelectrons that are created by femtosecond VUV and X-ray pulses. Using pump-probe schemes, ultrafast structural changes during photochemical reactions can thus be directly visualized with a temporal resolution that is only limited by the pulse durations of the pump and the probe pulse and the synchronization of the two light pulses. Here, we illustrate the principle of photoelectron diffraction using a simple, geometric scattering model and present results from photoelectron diffraction experiments on laser-aligned molecules using X-ray pulses from a Free-Electron Laser. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only