Depth-dependent C K-NEXAFS spectra for self-assembled monolayers of 4-methylbenzenethiol and 4-ethylbenzenethiol on Au(1Â 1Â 1)

Depth analysis using X-ray absorption fine-structure spectroscopy at the near carbon K-edge (C K-NEXAFS) was performed for self-assembled monolayers of 4-methylbenzenethiol (4-MBT) and 4-ethylbenzenethiol (4-EBT) on Au(1 1 1). On the basis of the polarization dependence of π* resonance intensity, the benzene rings of 4-MBT and 4-EBT were found to be tilted by 30 ± 10° and 60 ± 5° with respect to the Au(1 1 1) surface, respectively. The spectra were separated into two components according to their depth by fitting the intensity decay curves of the electrons emitted by X-ray absorption. The spectra for lower segments exhibited bands at almost identical energies with different intensities depending on the polarization between 4-MBT and 4-EBT. In contrast, the spectra for upper segments including the alkyl groups showed a marked difference in the location of the σCC* resonance. The σCC* resonance of the ethyl group predominantly observed in the 15° incidence spectrum of 4-EBT indicated that the ethyl group was directed perpendicularly.