On the anisotropy and CO coverage dependence of SHG from Pd(1 1 1)

Second harmonic non-linear susceptibilities of Pd(1 1 1) are derived at 800 nm fundamental wavelength from measurements of second harmonic generation (SHG) as a function of azimuth and input and output polarizations. By contrast to other metal surfaces and to other wavelengths, Pd(1 1 1) does not exhibit any anisotropy related to azimuth at 800 nm. SHG is also measured during CO adsorption at 1064 and 800 nm. Opposite trends are observed at the two wavelengths. The results are discussed in the light of previously reported experiments on Pd(1 1 1) at other wavelengths and on other metals. At 800 nm, free electrons and/or the Shockley surface state probably dominate the SHG response of Pd(1 1 1). CO chemisorption probably reduces the density of the d-band states probed at 1064 nm, and on the contrary enhances the density of the states probed at 800 nm.