M. Kavčič

Resonant inelastic x-ray scattering at the limit of subfemtosecond natural lifetime

We present measurements of the resonant inelastic x-ray scattering (RIXS) spectra of the CH(3)I molecule in the hard-x-ray region near the iodine L(2) and L(3) absorption edges. We show that dispersive RIXS spectral features that were recognized as a fingerprint of dissociative molecular states can be interpreted in terms of ultrashort natural lifetime of ∼200 attoseconds in the case of the iodine L-shell core-hole. Our results demonstrate the capacity of the RIXS technique to reveal subtle dynamical effects in molecules with sensitivity to nuclear rearrangement on a subfemtosecond time scale.

Relative detection efficiency of back- and front-illuminated charge-coupled device cameras for x-rays between 1 keV and 18 keV

High-resolution x-ray measurements were performed with a von Hamos-type bent crystal spectrometer using for the detection of the diffracted photons either a back-illuminated charge-coupled device (CCD) camera or a front-illuminated one. For each CCD the main x-ray emission lines (e.g., Kalpha, Kbeta, Lalpha, and Lbeta) of a variety of elements were measured in order to probe the performances of the two detectors between 1 and 18 keV. From the observed x-ray lines the linearity of the energy response, the noise level, the energy resolution, and the quantum efficiency ratio of the two CCDs were determined.

Dissociation of chloromethanes upon resonant σ* excitation studied by x-ray scattering

The dissociation process following the Cl K-shell excitation to σ* resonances is studied by high resolution spectroscopy of resonant elastic and inelastic x-ray scattering on CH3Cl, CH2Cl2, CHCl3, and CCl4 molecules. Calculations employing the transition potential and Delta-Kohn-Sham DFT approach are in good agreement with the measured total fluorescence yield and show the presence of a second quasidegenerate group of states with σ* character above the lowest σ* unoccupied molecular orbital for molecules with more than one Cl atom. A bandwidth narrowing and a nonlinear dispersion behavior is extracted from the Kα spectral maps for both σ* resonances. The fitted data indicate that the widths of the Franck-Condon distributions for the first and second σ* resonances are comparable for all the molecules under study. In addition, an asymmetric broadening of the emission peaks is observed for resonant elastic x-ray scattering with zero detuning on both σ* resonances. This is attributed to the fast dissociation, transferring about 0.15 of the scattering probability into higher vibrational modes.

Dissociation dynamics of simple chlorine containing molecules upon resonant Cl K-σ* excitation

A theoretical analysis of dissociation dynamics of chlorine K-σ(*) core-excited molecules is performed. The potential energy surfaces of HCl, Cl2, CH3Cl, CH2Cl2, CHCl3, CCl4, CFCl3, CF2Cl2, and CF3Cl are calculated along the normal vibrational modes of the ground electronic state yielding the widths of the corresponding Franck-Condon distributions. An insight into the potential energy surface of 1st σ(*) resonances shows that the initial dissociation dynamics of chloro(fluoro)methanes mainly involves the distancing of the carbon and the core-excited chlorine atom and is practically independent of other atoms in the molecule, which is in agreement with the recent experimental findings. The carbon atom pulls out the remaining three atoms shortly after piercing the three-atom plane resulting in a high vibrationally excited state of the fragment if the reconnection time is smaller than the lifetime of the L shell.

A 2D position sensitive germanium detector for spectroscopy and polarimetry of high-energetic x-rays

We report on a first prototype 2D μ-strip germanium detector, developed at IKP-Julich, and its performance test at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Beside an accurate determination of the detector response function, the polarization sensitivity has been addressed in this study. For this purpose photon beams at energies of 60 keV and 210 keV have been used.

Structural and dynamical properties of chlorinated hydrocarbons studied with resonant inelastic x-ray scattering

We present a theoretical and experimental study of resonant inelastic x-ray scattering on a large group of chlorinated hydrocarbons: CH3Cl, CH2Cl2, CHCl3, CCl4, CH3CH2Cl, ClCH2CH2Cl, CH3CHCl2, CH3CCl3, C2H2Cl2-iso, C2H2Cl2-cis, C2H2Cl2-trans, and C6H5Cl. Differences in structural and dynamical properties of the molecules generated by diverse chemical environments are observed in the measured Cl(K(α)) spectral maps as well as in the Cl(K) total fluorescence yield spectra. The energy position, relative intensity, and the width of the Franck-Condon distribution of low-lying σ* and π* resonances are extracted by a fitting procedure taking into account the experimental broadening. The theoretical values obtained with the transition potential and Δ Kohn-Sham methods are in good agreement with the experimental parameters indicating subtle differences due to variations in the molecular structure.

Synchrotron-radiation-based determination of Xe L-subshell Coster-Kronig yields: A reexamination via high-resolution x-ray spectroscopy

The xenon L-subshell Coster-Kronig (CK) transition yields were revisited via high-resolution measurements of the Lalpha{sub 1,2} (L{sub 3}-M{sub 4,5}) and Lbeta{sub 1} (L{sub 2}-M{sub 4}) x-ray emission lines. The L x-ray spectra were measured employing a Johansson-type curved crystal spectrometer and energy-tunable synchrotron radiation. The CK yields were derived from the relative L x-ray intensity jumps at the L edges by fitting the fluorescence intensities as a function of the photon energy to the L-subshell photoionization cross sections. The latter were obtained from the measured L-edge photoabsorption spectrum. Values of 0.118+-0.029, 0.383+-0.037, and 0.096+-0.016 were found for the f{sub 23}, f{sub 13}, and f{sub 12} CK yields, respectively. Thanks to high resolution, the L{sub 1} fluorescence yield of 0.059+-0.002 was also determined from intensity ratios of the well-resolved Lbeta{sub 4} (L{sub 1}-M{sub 2}) and Lbeta{sub 1} (L{sub 2}-M{sub 4}) lines.

Single-photon double K-shell ionization of low-Z atoms

The photon energy dependence of the double K-shell ionization of light atoms is reported. Experimental double-to-single photoionization cross section ratios for Mg, Al, Si and Ca were obtained from measurements of high-resolution x-ray emission spectra. The double photoionization (DPI) cross-sections for K-shell hollow atom production are compared to convergent close-coupling calculations (CCC) for neutral atoms and He-like ions. The relative importance of the initial-state and final-state electron-electron interactions to the K-shell DPI in many-electron atoms and two-electron ions is addressed. Physical mechanisms and scaling laws of the K-shell double photoionization are examined. A semiempirical universal scaling of the DPI cross sections with the effective nuclear charge for neutral atoms 2≤Z≤47 is established.

High energy resolution measurements of the radiative decay of double K-shell vacancies in 20 ≤ Z ≤ 29 elements bombarded by fast C and Ne ions

We report on high energy resolution measurements of the Kα hypersatellite x-ray spectra of Ca, V, Fe and Cu induced by impact with 144 MeV C and 180 MeV Ne ions.

Electron dynamics in the core-excited CS2 molecule revealed through resonant inelastic x-ray scattering spectroscopy

We present an experimental and theoretical study of resonant inelastic x-ray scattering (RIXS) in the CS2 molecule near the S 1s edge. We show that localization of the S 1s core-hole occurs in CS2 during the RIXS process due to the orientational dephasing of interference between the waves scattering on the two sulfur atoms. Strong evolution of the RIXS profile with the excitation energy far below the first absorption resonance reflects the onset of electron dynamics triggered by a coherent excitation of multiple electronic states.