X-ray absorption in hydrides of the 3p and 4p elements

Photoabsorption spectra of gaseous hydrides of 3p elements (PH$_3$, H$_2$S, HCl) are measured in the energy region of photoexcitations pertaining to K edge. The analysis is extended to hydrides of 4p series (GeH$_4$, AsH$_3$, H$_2$Se, HBr) from an earlier experiment, and to noble gases Ar and Kr as well as the published spectrum of SiH$_4$, inaccessible to our measurement. Edge profiles and sharp features immediately above the edges reflect single- and multi-electron transitions into molecular and modified atomic orbitals. In the pre-edge analysis, the published data on the homologous 2p series (CH$_4$, NH$_3$, H$_2$O and HF) with Ne in the XUV region are included in the comparison. The edge profiles are modelled with a linear combination of lorentzian components, describing excitations to individual bound states and to continuum. Transition energies and probabilities are also calculated in the non-relativistic molecular model of the ORCA code, in good agreement with the experiment. Edge profiles in the heavier homologues are closely similar, the symmetry of the molecule governs the transitions to the lowest unoccupied orbitals. In 2p series the effect of the strong nuclear potential prevails. Transitions to higher, atomic-like levels remain very much the same as in free atoms. For analysis of valence coexcitations in the post-edge region the deconvolution procedure is used to compensate the increased natural widths of spectral structures in the 4p group. The similarity of the shape of the sharp structures in analog pairs of spectra proves that the coupling mechanism of the outer electronic configurations is mostly the same. We have shown that high-energy part of coexcitation spectra in hydrides can be modelled satisfactorly with a linear combination of multiplied Ar or Kr coexcitation spectra. The total cross-section for a single-electron ionization and transition of the second electron to the bound state shows a slightly decrease with an increasing charge of the central atom in both series, the transition of valence electron to the quasi-atomic orbitals dominate. The latter is in contrast with the results of multi-electron excitations higher up above the edge with prevailing transitions to molecular orbitals. Our findings are confirmed by some recent results in emission spectroscopy. The analysis and good agreement of parameters obtained in a two-steps calculation with measured values indicate that a valence co-excitation in gaseus hydrides of 3p and 4p elements can be described in the sudden approximation: the excitation of core electron is followed by the molecule relaxation with shake-up and shake-off transition of the outer electron