Anomalous XANES Spectra of Octadecanethiol Adsorbed on Ag(111)

Abstract

We report on the sulfur K-edge X-ray absorption near-edge structure (XANES) spectra of self-assembled monolayers for a series of n-alkanethiols adsorbed on Ag(111). We investigate the effects of chain length and incidence angle on the XANES spectra of n-alkanethiols [CH3−(CH2)x-1−SH, x = 6, 12, 14, 16, 18, and 22; hereafter referred to as Cx]. Other than that of the C18 sample, the XANES spectra were similar to one another. The C18 spectrum displayed an above-threshold feature that was absent in all of the other spectra, and the shape of the principal subthreshold peak is different than that for all of the other compounds. Moreover, the intensities of these features exhibit an atypical angular dependence, as well. These data are consistent with recent EXAFS studies of thiol adsorption on silver, and suggest that the local environment of the sulfur adatom is different for the octadecanethiol adsorbate.