OPTICAL 2ND-HARMONIC ELECTROREFLECTANCE FROM AG(111)

We have used an electrochemical sample chamber to modulate the surface charge of Ag(111) in spectral measurements (over a range of the incident photon energy from 1.4 to 2.23 eV) of the isotropic contribution to the second-harmonic reflectance. When the charge modulation was positive, and the harmonic energy was below the onset for interband transitions, the results compared favorably with a previously published prediction of the same phenomenon which was based upon time-dependent local-density-functional theory for jellium having the same bulk electron density as Ag. The correlation between this theory and our experiment was poor for negative charge modulation. However, this was not unexpected, since the jellium model makes no allowance for the influence of the d bands. We have tentatively assigned a localized feature in the spectrum for negative charge modulation, which appears at a harmonic energy of 3.4 eV, as a two-photon resonance involving crystal-potential and image-potential surface states.