High-resolution Core-level Spectroscopies Of Molecular Silicon Compounds Using Monochromatized Synchrotron Radiation

High-resolution core-level photoabsorption, photoelectron and photoionization mass spectra of numerous substituted silane molecules have been measured in the gas phase around the Si 2p ionization edges using monochromatized synchrotron radiation. Photoabsorption spectra were also measured around the Si 2s ionization edges and the Cl 2p ionization edges of the chlorine-containing molecules. Multiple-scattering X{dollar}\alpha{dollar} calculations were performed to aid in the assignment of the peaks observed in the photoabsorption spectra. The chemical series approach, where the effects of small systematic chemical changes in the composition of the molecule under study on the observed spectra provide useful aids for assignments, has been used throughout this work.;Photoabsorption spectra of the core electronic levels are reported for three series of silicon molecules, the fluoromethylsilane compounds: Si(CH{dollar}\sb3{dollar}){dollar}\sb{lcub}\rm x{rcub}{dollar}F{dollar}\sb{lcub}\rm 4-x{rcub}{dollar}, the chloromethylsilane compounds: Si(CH{dollar}\sb3{dollar}){dollar}\sb{lcub}\rm x{rcub}{dollar}Cl{dollar}\sb{lcub}\rm 4-x{rcub}{dollar}, and the chlorosilane compounds: SiH{dollar}\sb{lcub}\rm x{rcub}{dollar}Cl{dollar}\sb{lcub}\rm 4-x{rcub}{dollar}, x = 0-4. Theoretical results from MS-X{dollar}\alpha{dollar} calculations along with trends observed in the experimental spectra facilitate the identification and assignment of numerous features in the spectra. Below the ionization edges, the observed peaks are assigned to result from electronic transitions of the core electrons into virtual molecular orbitals and Rydberg orbitals. Above the ionization edges, peaks in the photoabsorption cross sections are assigned to transitions into a variety of quasibound states in the continuum: virtual molecular orbitals and Rydberg orbitals. Above the ionization edges, peaks in the photoabsorption cross sections are assigned to transitions into a variety of quasibound states in the continuum: virtual molecular orbitals supported by the shape of the molecular potential, shape resonances, and delayed-onsets resulting from centrifugal barriers to the exiting electron.;Using a newly constructed photoelectron spectrometer, Si 2p photoelectron spectra of eleven substituted silane molecules were measured with unprecedented experimental resolution. The spectra exhibit vibrational broadening, and for silane, tetrafluorosilane and ethylsilane, individual vibrational bands are resolved on the Si 2p photoelectron lines. Very high experimental resolution is required to resolve individual vibrational bands. Extensive use is made of the equivalent-cores approximation to aid in the interpretation of the observed vibrational structure. Chemical effects on the lifetimes of the Si 2p core holes are also investigated. Total ion yield spectra, total electron yield spectra, photoionized mass spectra and mass-resolved photoion yield spectra of the fluoromethylsilane compounds, Si(CH{dollar}\sb3{dollar}){dollar}\sb{lcub}\rm x{rcub}{dollar}F{dollar}\sb{lcub}\rm 4-x{rcub}{dollar}; x = 0-4, measured using photon energies around the Si 2p core ionization edges are also reported. In the fragmentation patterns observed following irradiation of the sample with monochromatized synchrotron radiation, the methyl groups are found to be more labile than the fluorine atoms at photon energies below and above the Si 2p ionization edges. Partial ion yields of the SiF{dollar}\sp-{dollar} and SiMe{dollar}\sp+{dollar} fragment ions are seen to exhibit specific enhancement at the Si 2{dollar}p\to\sigma{dollar}* resonances below the ionization edge

Reconstruction of the time-dependent electronic wave packet arising from molecular autoionization

Autoionizing resonances are paradigmatic examples of two-path wave interferences between direct photoionization, which takes a few attoseconds, and ionization via quasi-bound states, which takes much longer. Time-resolving the evolution of these interferences has been a long-standing goal, achieved recently in the helium atom owing to progress in attosecond technologies. However, already for the hydrogen molecule, similar time imaging has remained beyond reach due to the complex interplay between fast nuclear and electronic motions. We show how vibrationally resolved photoelectron spectra of H2 allow one to reconstruct the associated subfemtosecond autoionization dynamics by using the ultrafast nuclear dynamics as an internal clock, thus forgoing ultrashort pulses. Our procedure should be general for autoionization dynamics in molecules containing light nuclei, which are ubiquitous in chemistry and biologyThis work was supported by European Research Council advanced grant 290853-XCHEM within the seventh framework program of the European Union. We also acknowledge the financial support from MINECO projects FIS2013-42002-R and FIS2016-77889-R, and the European COST (Cooperation in Science and Technology) Action XLIC CM1204, and the computer time from the Centro de Computación Científica de la Universidad Autónoma de Madrid and Marenostrum Supercomputer Center. A.P. acknowledges a Ramón y Cajal contract from the Ministerio de Economía y Competitividad (Spain). F.M. acknowledges support from the “Severo Ochoa” Programme for Centres of Excellence in R&D (MINECO, grant SEV-2016-0686) and the “María de Maeztu” Programme for Units of Excellence in R&D (MDM-2014-0377). S.E.C. acknowledges funding from the Helmoltz Recognition Award. The Extreme Light Infrastructure Attosecond Light Pulse Source project (GINOP-2.3.6-15-2015-00001) was financed by the European Union and cofinanced by the European Regional Development Fun

Dynamical effects in the vibrationally resolved C 2s-1 photoionization cross section ratios of Methane

The vibrationally resolved C 2s photoionization cross-section of methane was investigated both theoretically and experimentally. When compared to that of C 1s photoionization, a rather different pattern has been observed, suggesting a strong interplay between the electron diffraction and interference effects

Reconstruction of the time-dependent electronic wave packet arising from molecular autoionization

The time evolution of an electronic molecular wave packet is determined using nuclear motion as an internal clock of the system

Photofragmentation of \u3ci\u3ecloso\u3c/i\u3e-Carboranes Part 1: Energetics of Decomposition

The ionic fragmentation following B 1s and C 1s excitation of three isomeric carborane cage compounds [closo-dicarbadodecaboranes: orthocarborane (1,2-C2B10H12), metacarborane (1,7-C2B10H12), and paracarborane (1,12-C2B10H12)] is compared with the energetics of decomposition. The fragmentation yields for all three molecules are quite similar. Thermodynamic cycles are constructed for neutral and ionic species in an attempt to systemically characterize single-ion closo-carborane creation and fragmentation processes. Lower energy decomposition processes are favored. Among the ionic species, the photon-induced decomposition is dominated by BH+ and BH2+ fragment loss. Changes in ion yield associated with core to bound excitations are observed

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

Vibronic interaction in trans-dichloroethene studied by vibration- and angle-resolved photoelectron spectroscopy using 19–90 eV photon energy

Valence photoelectron spectra and photoelectron angular distributions of trans-dichloroethene have been measured with vibrational resolution at photon energies between 19 eV and 90 eV. Calculations of photoelectron anisotropy parameters, β, and harmonic vibrational modes help provide initial insight into the molecular structure. The photon energy range encompasses the expected position of the atomic Cl 3p Cooper minimum. A corresponding dip observed here in the anisotropy of certain photoelectron bands permits the identification and characterization of those molecular orbitals that retain a localized atomic Cl character. The adiabatic approximation holds for the X 2Au state photoelectron band, but vibronic coupling was inferred within the A–B–C and the D–E states by noting various failures of the Franck–Condon model, including vibrationally dependent β-parameters. This is further explored using the linear vibronic coupling model with interaction parameters obtained from ab initio calculations. The A/B photoelectron band is appreciably affected by vibronic coupling, owing to the low-lying conical intersection of the A 2Ag and B 2Bu states. The C 2Bg band is also affected, but to a lesser extent. The adiabatic minima of the D 2Au and E 2Ag states are almost degenerate, and the vibronic interaction between these states is considerable. The potential energy surface of the D 2Au state is predicted to have a double-minimum shape with respect to the au deformations of the molecular structure. The irregular vibrational structure of the resulting single photoelectron band reflects the non-adiabatic nuclear dynamics occurring on the two coupled potential energy surfaces above the energy of their conical intersection

Photoelectron spectroscopy and dissociative photoionization of fulminic acid, HCNO

We report a joint experimental and computational study of the photoelectron spectroscopy and the dissociative photoionization of fulminic acid, HCNO. The molecule is of interest to astrochemistry and astrobiology as a potential precursor of prebiotic molecules. Synchrotron radiation was used as the photon source. Dispersive photoelectron spectra were recorded from 10~eV to 22~eV, covering four band systems in the HCNO cation and an ionization energy of 10.83~eV was determined. Transitions into the Renner-Teller distorted $X^+{}^2\Pi$ state of the cation were simulated using wavepacket dynamics based on a vibronic coupling Hamiltonian. Very good agreement between experiment and theory is obtained. While the first excited state of the cation shows only a broad and unstructured spectrum, the next two higher states exhibit a well-resolved vibrational progression. Transitions into the excited electronic states of \cation{HCNO}{+} were not simulated, due to the large number of electronic states that contribute to these transitions. Nevertheless, a qualitative assignment is given, based on the character of the orbitals involved in the transitions. The dissociative photoionization was investigated by photoelectron-photoion coincidence spectroscopy. The breakdown diagram shows evidence for isomerization from \cation{HCNO}{+} to \cation{HNCO}{+} on the cationic potential energy surface. Zero Kelvin appearance energies for the daughter ions \cation{HCO}{+} and \cation{NCO}{+} have been derived