Optical second-harmonic generation from surfaces as a monitor for adsorbate induced work function changes

A quantitative correlation is found between the measured work function change and the coherent laser second-harmonic (SH) response of Re(0001) at 80 K upon gas adsorption. It is shown that radiation at 1.06 microm generates a SH signal which decreases when the surface is covered by electron withdrawing adsorbates that increase the work function of the metal, e.g. O₂ and CO. An increase of the SH response occurs when electron donating molecules adsorb on the surface, which also decrease the work function, e.g. H₂ and NH₃. At saturation coverage, oxygen increases the work function by 0.45 eV and the SH signal decreases by a factor of 5 with respect to the bare metal signal. Ammonia decreases the work function by 2.1 eV and increases the SH response by a factor of 55. An empirical expression based on the Drude model for the frequency response of the polarizability tensor, accounts quantitatively for the magnitude and direction of the SH response as a function of the changing work function of the metal upon gas adsorption. This expression accounts also for the adsorption kinetics and for the saturation fractional coverage of the gas molecule.