ADSORPTION OF OXYGEN AND HYDROGEN ON PT(111) STUDIED WITH 2ND-HARMONIC GENERATION

The adsorption of oxygen and hydrogen on a Pt(111) surface has been studied with phase-sensitive Second-Harmonic Generation (SHG) signal detection. The SHG-signal change measured with pin, p-out polarization during the adsorption of oxygen and hydrogen was found to be different in amplitude and phase for the two adsorbates. At the wavelength used (1064 nm), only a localized interaction between adsorbate and substrate is seen, leading to a linear dependence of the susceptibility on the coverage. Sticking coefficients, s(theta), and their coverage dependence were determined. For hydrogen, a linear decrease in s(theta) with coverage was found; the initial sticking coefficient being s(0) = 0.06 at a temperature of T = 130 K. For oxygen, s(theta) shows a quadratic decrease with coverage, strongly dependent on temperature, with s(0) = 0.05 at T = 350 K. A method based on these results is proposed, which would allow the determination of adsorbate coverages of coadsorption systems with SHG using phase-sensitive signal detection.