On the equivalence of EELS and IRAS: the case of O-Ag(110)

The coverage dependence of electron-energy loss intensity (EELS) and of the frequency of the O-Ag(110) stretching vibration are re-examined and compared with other EELS and infrared reflection absorption spectroscopy (IRAS) data. The analysis of the EELS intensity in our experiment yields a charge transfer q=0.51+/-0.03 e(-), which compares well with independent EELS estimates but is at variance with the much smaller IRAS value reported by Peng and Reutt-Robey [Surf. Sci. 336 (1995) L755]. Our value can account for most of the observed frequency shift with coverage, indicating that no chemical shift occurs; in contrast with the recent theory of Schimizu and Tsukada [Surf. Sci. 295 (1993) L1017], the opposite conclusion would be supported by IRAS. The (surprising) inconsistency of EELS in dipole scattering and IRAS could be indicative that the physical basis of the two spectroscopies is not unique, as assumed so far.