Near sulfurL-edge X-ray absorption spectra of methanethiol in isolation and adsorbed on a Au(111) surface: a theoretical study using the four-component static exchange approximation

Abstract

The relativistic four-component static exchange approach for calculation of near-edge X-ray absorption spectra has been reviewed. Application of the method is made to the Au(111) interface and the adsorption of methanethiol by a study of the near sulfur L-edge spectrum. The binding energies of the sulfur 2p_1/2 and 2p_3/2 sublevels in methanethiol are determined to be split by 1.2 eV due to spin–orbit coupling, and the binding energy of the 2p_3/2 shell is lowered from 169.2 eV for the isolated system to 167.4 and 166.7–166.8 eV for methanethiol in mono- and di-coordinated adsorption sites, respectively (with reference to vacuum). In the near L-edge X-ray absorption fine structure spectrum only the σ*(S–C) peak at 166 eV remains intact by surface adsorption, whereas transitions of predominantly Rydberg character are largely quenched in the surface spectra. The σ*(S–H) peak of methanethiol is replaced by low-lying, isolated, σ*(S–Au) peak(s), where the number of peaks in the latter category and their splittings are characteristic of the local bonding situation of the sulfur.