High-resolution photoabsorption near the sulfur L2,3 thresholds: H2S and D2S.

Abstract

The x-ray-absorption near-edge structure of gas-phase ${\mathrm{H}}_{2}$S, hydrogen sulfide, and ${\mathrm{D}}_{2}$S, deuterium sulfide, at the sulfur ${\mathit{L}}_{2,3}$ ionization thresholds has been measured using synchrotron radiation with high-energy resolution from the SX700/II soft-x-ray monochromator at the Berliner Elektronenspeicherring-Gessellschaft f\ur Synchrotronstrahlung m.b.H. Previously unobserved fine structure is resolved. The spectra of both molecules are characterized by multielectron excitations 8--14 eV above the ${\mathit{L}}_{2}$ threshold energy, broad valence-shell absorption features 5--8 eV below the ${\mathit{L}}_{2}$ edge, and many narrower Rydberg excitations 0--5 eV below the ${\mathit{L}}_{2}$ edge. Comparison of the ${\mathrm{H}}_{2}$S and ${\mathrm{D}}_{2}$S spectra allows the identification of transitions which include vibrational excitation, due to the isotopic dependence of vibrational energies. For the (2p${)}^{\mathrm{\ensuremath{-}}1}$ core-excited Rydberg states, a least-squares analysis was employed, which deconvoluted the core-level and excited-orbital splittings. Results show that the twofold degeneracy of the sulfur 2${\mathit{p}}_{3/2}$ core level is removed by the molecular field, with a resulting splitting of 115 meV for the higher-energy core-excited Rydberg states. The energies of the higher Rydberg states were well described by the Rydberg formula with the quantum defects ${\mathrm{\ensuremath{\delta}}}_{\mathit{p}}$=1.63 and ${\mathrm{\ensuremath{\delta}}}_{\mathit{d}}$=0.32. Fine structure was resolved in the transitions to the dissociative (2p${)}^{\mathrm{\ensuremath{-}}1}$ core-excited valence-shell states. The regular spacing and isotopic dependence of this structure clearly identifies it as a vibrational progression.