A COMPARISON OF THE SURFACE ELECTRONIC-STRUCTURE OF TA(100) AND TAC(100) USING AUGER-PHOTOELECTRON COINCIDENCE SPECTROSCOPY

We present N7 VV (4f7/2 5d 5d) Auger spectra from TaC(111) taken in time coincidence with photoemitted bulk and surface-shifted 4f7/2 core-level electrons, and compare these data with self-convolutions of calculated valence band d densities of states (SCDOS) for the surface (first) and bulk (exponential sum of second, fourth, and sixth subsurface) atomic layers of a 13-layer slab. The Ta 4f levels of TaC(111) and Ta(100) both exhibit large surface core level shifts, but of opposite sign. Therefore, we compare the TaC(111) data and calculations with analogous N7 VV Auger coincidence spectra and calculated SCDOS that we have previously published for Ta(100). The bulk Ta(100) and surface TaC(111) spectra show favorable agreement with the calculations; the surface Ta(100) and bulk TaC(111) spectra, on the other hand, exhibit marked disagreement with the calculations. These discrepancies are consistent with and provide added support for the presence of our proposed 1 surface-to-bulk (bulk-to-surface) interatomic Auger decay mode which adds a bulk (surface) contribution to the surface (bulk) Auger line shape of Ta(100) (TaC(111)). These conclusions are supported by comparison of our data to incoherent sums of bulk and surface SCDOS.