K-edge absorption spectra of isoelectronic gaseous hydrides: a combination ofatomic and molecular channels

The fine detail in the x-ray absorption spectra in the energy region of absorption edgesprovides the insight into the mechanism of inner-shell photoexcitation: in particular in spectra offree atoms or simple molecules, the simplest being gaseous hydrides [1-2].Measured K edge absorption spectra of hydrides of 3p (PH3, H2S in HCl) and 4p (GeH4,AsH3, H2Se, HBr) elements, and published data of 2p hydrides (CH4, NH3, H2O, HF) as well asSiH4 [3-6] and the noble gases at the end of the isoelectronic series (Ne, Ar, Kr) are compared tothe respective calculated spectra, obtained by atomic HF86, GRASP codes [7] and molecularDFT (Density functional theory) ORCA code [8]. For a clearer view of intraatomic processes,the weak and simple structural (XAFS) signal of the molecule is removed from the spectra.Among the spectral features below the continuum limit, those with the lowest energybelong to the transition of the core electron to the lowermost free orbitals with the molecularcharacter. They are, as a rule, wider than the transitions to the higher orbitals with prevailingatomic character. The theoretical description with DFT code without specific adaptations issufficient for a qualitative picture of the pre-edge structure. The fine structure immediately abovethe K edge stems from the coexcitation of valence electrons. We have proved that thecoexcitations can be explained as a two-step process: the inner-shell photoeffect followed by theshake-up of a valence electron predominantly to a free atomic orbital. This process is markedlydifferent from coexcitations of more tightly bound electrons [9].In the collection of consecutive and homologous data, analyzed by a common procedure,the reaction channels can be identified with better precision and reliability than in analysis ofindividual spectra. Our analysis showed that the energies and probabilities of single-electrontransitions into the molecular orbitals are strongly affected by the symmetry of the molecule,essentially in the same way in 3p and 4p homologues, but not in 2p homologues with a strongerinfluence of the core charge. In transitions to atomic orbitals the influence of the molecular fieldis negligible