High-resolution measurements of near-edge resonances in the core-level photoionization spectra of SF6.

Abstract

The x-ray-absorption near-edge structure (XANES) of sulfur hexafluoride, ${\mathrm{SF}}_{6}$, in the gas phase has been measured with high energy resolution at the sulfur ${\mathit{L}}_{2,3}$ and fluorine K ionization thresholds using synchrotron radiation from the SX700/II monochromator at BESSY. Besides dominant transitions to core-excited inner-well states, several series of Rydberg states with vibrational fine structure were resolved below the sulfur ${\mathit{L}}_{2,3}$ thresholds. Using the Rydberg formula, quantum defects of ${\mathrm{\ensuremath{\delta}}}_{\mathit{s}}$=1.80 and ${\mathrm{\ensuremath{\delta}}}_{\mathit{d}}$=-0.03 were obtained for the s and d Rydberg orbitals of the central sulfur atom. A Franck-Condon analysis was used to determine the vibrational spacing and the S-F bond length of the (S 2${\mathit{p}}_{3/2}$${)}^{\mathrm{\ensuremath{-}}1}$4${\mathit{s}}^{1}$ core-excited state. The presence of vibronically coupled transitions below the sulfur ${\mathit{L}}_{2,3}$ edges was confirmed. The derived natural linewidths of the (S 2${\mathit{p}}_{1/2,3/2}$${)}^{\mathrm{\ensuremath{-}}1}$ Rydberg states were found to be strikingly narrower than those of the (S 2${\mathit{p}}_{1/2,3/2}$${)}^{\mathrm{\ensuremath{-}}1}$ inner-well resonances. Line-shape analysis also revealed significant inhomogeneous broadening of the (S 2${\mathit{p}}_{1/2,3/2}$${)}^{\mathrm{\ensuremath{-}}1}$${\mathit{a}}_{1\mathit{g}}^{1}$ states, which is attributed to unresolved vibrational structure. Large Lorentzian contributions to the line shapes of the (S 2${\mathit{p}}_{1/2,3/2}$${)}^{\mathrm{\ensuremath{-}}1}$${\mathit{t}}_{2\mathit{g}}^{1}$ resonances suggest that vibrational effects are relatively small for those states.