Ionization current detection of soft X-ray photoabsorption: Sulfur and chlorine k-shell spectra of SO2F2, SO2FCl and SO2Cl2

An apparatus for recording inner-shell excitation spectra by detecting currents produced by the X-ray ionization of gases is described. The equivalence of ion current and true photoabsorption (log(I0/I)) detection for the case of S 1s spectroscopy of molecules is demonstrated. The ion current detection has been used to record the S 1s and Cl 1s spectra of SO2FCl and SO2Cl2 and the S 1s spectrum of SO2F2. The spectral features are analysed in terms of excitations to σ* orbitals characteristic of the local bonding to the K-shell excited atom. The correlation between SX bond lengths and the positions of S 1s→σ*(SX) excitations relative to the S 1s ionization threshold is discussed and related to corresponding correlations for OX bonds.