Analysis of resonancelike peaks in angle-resolved ultraviolet photoemission spectroscopy from adsorbed atoms

The physical origin of resonancelike peaks in the emission spectra of angle-resolved ultraviolet photoemission is analyzed. Emission from the $3d$ core level of Ni(001)-$c(2\mathrm{}\times 2)$ Se is studied. Evidence is found that the emission peaks exhibit strong dependence to the adsorbate-substrate geometry. Contrary to the gas-phase explanation in terms of the formation of a temporary "excited atom," we suggest that emission peaks from adsorbed species are due to constructive interference of the final-state electron wave between overlayer and substrate layers. Because of this strong dependence to the adsorbate-substrate spacing, it is recognized that the resonancelike peak can be used to determine surface bonding geometry.