X-ray and low energy electron induced damage in alkanethiolate monolayers on Au-substrates

In a previous communication we presented X-ray photoelectron (XP) spectra of alkanethiolate monolayers deposited from solution onto Au-substrates. In the S2p XP-spectra two different sulfur species were detected, and interpreted as evidence for disulfide formation, in agreement with an earlier report using grazing incidence X-ray diffraction (GIXRD) [P. Fenter, A. Eberhardt and P. Eisenberger, Science 266 (1994) 1216]. Here we present new experiments revealing that alkanethiolate monolayers adsorbed on Au exhibit an unexpectedly high cross section for X-ray induced damage. This observation makes a reinterpretation of the previous XPS data necessary, which - together with new data reveals that the second S-species observed in the previous experiments (which was interpreted to indicate disulfide formation) is due to beam damage. The relevance of this X-ray induced beam-damage for the interpretation of the GIXRD-data will be discussed. Data obtained with soft X-ray absorption spectroscopy (NEXAFS) for alkanethiolate monolayers exposed to low energy electrons in the typical energy range of secondary electrons (E = 0 - 50 eV) reveal the formation of C = C double bonds and provides evidence that the X-ray induced damage is caused by the secondary electrons resulting from the photoemission process.