Electronic structure of ordered sulfur overlayers on Ni(001)

Angle-resolved photoelectron spectroscopy is used to measure the dispersion, linewidth, and photoionization cross section of the electronic valence levels of $p(2\mathrm{}\times 2)$ and $c(2\mathrm{}\times 2)$ S overlayers on Ni(001). Polarized light from a synchrotron is used to identify the symmetry of the individual S $3p$ states and to probe their intensity as a function of photon frequency. The data are compared to various theoretical calculations of the energy levels and of the differential cross section for a ${\mathrm{Ni}}_5$S cluster as well as for ordered S layers on a semi-infinite Ni(001) substrate. The $p$ states of the $c(2\mathrm{}\times 2)$ structure exhibit about 1.5 eV dispersion with ${\overrightarrow{\mathrm{k}}}_{\parallel }$ whose qualitative behavior is well reproduced by the theoretical results using the layer Korringa-Kohn-Rostoker scheme. The measured and calculated $p$ levels of the $c(2\mathrm{}\times 2)$ structure, on the other hand, show very little dispersion as a result of the relatively large S-S spacing. The observed linewidth of the S levels is considerably larger (1-2 eV) than the calculated single-particle broadening of the $3p$ levels due to hybridization with the Ni $\mathrm{sp}$ band (0-1 eV). Lifetime broadening associated with Auger decay appears to be the main origin of the measured linewidth. The photoionization cross section for normal emission from the $p(2\mathrm{}\times 2)$ structure exhibits a sharp resonance at about 18-eV photon energy which is not present in the case of the $c(2\mathrm{}\times 2)$ structure. Theoretical analysis of the cross section suggests that this resonance behavior is primarily due to the dominant S $p-\mathrm{to}-d$ transition coupled with strong multiple-scattering interferences of the outgoing wave.