Ab initio simulation of molecular Auger spectra: Nuclear dynamics effects in the spectra of carbonyl sulfide

Experimental and theoretical investigation of the Auger electron spectra of isothiocyanic acid, HNCS

We investigate isothiocyanic acid, HNCS, by resonant and nonresonant Auger electron spectroscopy at the K-edge of carbon and nitrogen, and the L2,3-edge of sulfur, employing soft X-ray synchrotron radiation. The C1s and N1s ionization energies as well as the S2s and S2p ionization energies are determined and X-ray absorption spectra reveal the transitions from the core to the virtual orbitals. Final states for all normal Auger electron spectra and the resonant ones recorded at the carbon and nitrogen edge are assigned and rationalized with theoretical spectra obtained with a wave-function based protocol. The latter is based on ab initio configuration-interaction representations of the bound part of the electronic wave functions, the one-center approximation for Auger intensities, and a moment theory for band shapes. The computed spectra are in very good agreement with the experimental data and most of the relevant signals are assigned. The double ionization energy is derived from the S2p1/2 spectrum and in good agreement with a recently determined value. The Auger electron spectra are compared with those of the congener HNCO. A similar shape of the normal Auger electron spectra was found for the low binding energy final states, while intensities differed. Similarities are less pronounced in the resonant Auger electron spectra.

Three body dissociation of CS2(2+) subsequent to various S(2p) Auger transitions

Fragmentation kinematics of CS2 following various S(2p) Auger transitions is studied. Employing a combination of electron energy analysis and recoil ion momentum spectroscopy, changes in the dissociation channel yields, as well as the differences in the kinematical parameters for various bands of Auger hole states are presented. The fragmentation mechanism for dissociative channels leading to complete atomization of CS2(2+) molecular ion is studied in detail. We find that CS2(2+) does not retain linear geometry and is bent before undergoing concerted break-up. It is also observed that different geometric configurations of the CS2(2+) precursor result in different kinetic energy release values.

The OCS S L3MM auger spectrum and angular distributions studied by photoelectron-auger electron coincidence experiments

The selectivity of the photoelectron-Auger electron coincidence technique has been used to isolate the S L(3)MM contribution to the L(2,3)MM Auger spectrum in OCS. In this way, a direct comparison of the energies, widths, and relative intensities of the final OCS(2+) states with theoretical calculations has been achieved. Moreover, the angular distributions of some selected Auger electrons have been measured in coincidence with the photoelectron at two different photon energies. In contrast with the results of noncoincidence measurements, the coincidence angular distributions show a significant asymmetry and a dependence on the photon energy and OCS(2+) final state.

The Auger spectroscopy of pyrimidine and halogen-substituted pyrimidines

The C 1s and N 1s Auger spectra of pyrimidine, 2-chloropyrimidine, and 5-bromopyrimidine have been measured in an electron impact experiment at 1000 eV. In the case of the halogen-substituted pyrimidines, also the Cl 2p and Br 3d Auger spectra have been recorded. We have thoroughly analyzed and interpreted all the Auger spectra recorded here with the aid of accurate Green's function calculations with a large basis set. The spectra are extremely complex with thousands of states contributing and almost no single-state feature even near the double ionization threshold. Besides reproducing and explaining with great detail nearly all the main spectral features observed, the calculations have successfully unraveled the interplay among the different C 1s core hole chemical shifts in each molecule and how this affects some fingerprinting details in the composite C 1s Auger spectra.

Effects of nuclear dynamics in the low-kinetic-energy Auger spectra of CO and CO2

The CO and CO(2) carbon and oxygen Auger spectra have been measured by electron impact and compared with accurate theoretical calculations accounting for the effects of the dynamics of the nuclei on the energy and linewidth of the Auger bands. The calculations for CO were previously published [L. S. Cederbaum et al., J. Chem. Phys. 95, 6634 (1991)], while for CO(2) they are new and presented here for the first time. For both molecules, particular attention has been paid to the low-kinetic-energy region of the spectra, which corresponds to doubly charged ion states with the two holes mainly localized in the inner valence region. New bands have been observed. It is shown that a proper consideration of the vibrational broadening and shift of the bands due to the dynamics of the nuclei is needed to assign these features. For CO, very large energy shifts between corresponding features in the C 1s and O 1s spectra have been observed, confirming the theoretical predictions of 1991. The new computed spectra of CO(2) allow a very accurate analysis of the experiments over the whole energy range.

Complete valence double photoionization of SF6

Single photon double ionization of SF(6) has been investigated at the photon energies 38.71, 40.814, and 48.372 eV by using a recently developed time-of-flight photoelectron-photoelectron coincidence spectroscopy technique which gives complete two-dimensional e(-)-e(-) spectra. The first complete single photon double ionization electron spectrum of SF(6) up to a binding energy of approximately 48 eV is presented and accurately interpreted with the aid of Green's function ADC(2) calculations. Spectra which reflect either mainly direct or mainly indirect (via interatomic coulombic decay of F 2s holes) double ionization of SF(6) are extracted from the coincidence map and discussed. A previous, very low value for the onset of double ionization of SF(6) is found to energetically coincide with a peak structure related to secondary inelastic scattering events.