Surface and Bulk σ*(C-D) States in NEXAFS Spectra of Condensed Formic Acid Layers

Abstract

C K-shell near edge X-ray absorption fine structure (NEXAFS) spectra of condensed layers of formic acid (DCOOD) have been recorded using the CKVV Auger electron yield (AEY) and the photon-stimulated ion desorption (PSID) yield of D + . It is found that the position of σ*(C-D) resonance in the AEY is much higher than that in the D + PSID yield which is similar to that in the AEY of formic acid monolayer. This indicates that PSID-NEXAFS is extremely surface sensitive while AEY-NEXAFS reflects a bulk state. The higher energy shift can be explained by the antibonding nature of σ*(C-D) excited state. The obtained results demonstrate that excitation of a strongly antibonding molecular orbital shifts to higher energy in the bulk compared with the surface by final-state interactions. These results have broad implications for the assignments of NEXAFS peaks in other systems.