Branching ratio deviations from statistical behavior in core photoionization

Abstract

Accurate calculations of carbon 1s photoionization cross sections have been performed at the density functional level with the B-spline linear combination of atomic orbitals approach. The molecules considered are FC[triple bond]CH, FC[triple bond]CCH3, FC[triple bond]CCN, F2C=CH2, CF3COOCH2CH3, and C3H5O. The variation of the branching ratios relative to inequivalent C 1s ionizations have been evaluated from threshold to about 100 eV photoelectron kinetic energy. Large deviations from the statistical ratios are observed at low energies, which remain often significant several tens of eV above threshold. The importance of taking into account core branching ratios for peak deconvolution and quantitative analysis, as well as an additional tool for structural information, is pointed out. Strong shape resonant effects are found to largely cancel in branching ratios. Their nature and variation along the series is analyzed in the framework of excitations into sigma* valence orbitals.

Intensity oscillations in the carbon 1s ionization cross sections of 2-butyne

“…Such effects were predicted theoretically for carbon 1s ionization in a number of molecules by Di Tommaso and Decleva 9 and observed experimentally by Söderström et al in chlorine-substituted ethanes. 6 These results showed that there is an oscillatory behavior of the cross sections as the photon energy increases and that these oscillations can be seen several hundred eV above the ionization threshold.…”

“…is predicted to be a common feature for substituted ethyne molecules such as FCCH, FCCCH 3 , and FCCCN. 9 The origin of this feature is the presence of a continuum σ * u resonance, associated with the ionization of the σ C2C3 antibonding orbital, similar to that seen in the core ionization of N 2 . Such a feature also appears in the 1a 1 ionization, but with much reduced intensity, because of the smaller dipole transition moment from the adjacent methyl C1s orbital.…”

“…10,11 While it may be reasonable to expect pronounced scattering effects in the presence of heavy atoms such as chlorine, it might not be anticipated that such effects would also play a significant role in molecules containing only first-row atoms. Guided by some preliminary observations of nonstoichiometric intensity ratios in 2-butyne (CH 3 C≡CCH 3 ) and, also, by the predictions of Di Tommaso and Decleva, 9 we have investigated the carbon 1s photoelectron spectra of 2-butyne over a range of photon energies from threshold to 150 eV above threshold. Over this energy range we see oscillations in the intensity ratio (C2,3/C1,2) that range from 0.8 to 1.15, indicating that nonstoichiometric cross section ratios will be found even when there is no high-Z scattering center.…”

“…These are the scattering model used by Rehr et al 10,11 and the density-functional, B-spline model used by Di Tommaso and Decleva. 9 The model used by Di Tommaso and Decleva is essentially the density-functional equivalent of the "static-exchange" 12,13 approach to continuum orbitals combined with an atom-centered B-spline basis. Rehr's model uses a Green's function formalism with the scattering contribution to the propagator obtained in an expansion up to a given order of the scattering process, with the atomic 0021-9606/2013/138(23)/234310/5/$30.00 © 2013 AIP Publishing LLC 138, 234310-1 scatterers described in terms of muffin-tin potentials.…”

Energy-Dependent Relative Cross Sections in Carbon 1s Photoionization: Separation of Direct Shake and Inelastic Scattering Effects in Single Molecules

“…The theoretical β parameters for chloroethanes and the ESCA molecule are available in the literature only for relatively low kinetic energies up to 300 and 100 eV, respectively, where asymptotic limits are not reached yet. 45,49 To estimate the correction factor at high energies, we have calculated β parameters up to electron kinetic energies 570 and 530 eV for chloroethanes and the ESCA molecule, respectively. The details of the calculations are to be discussed below.…”