Zero-point vibration of hydrogen adsorbed on Si and Pt surfaces

Hydrogen atoms adsorbed on Si(111) and Pt(111) were investigated by nuclear reaction analysis ( NRA) using H-1(N-15, alphagamma)C-12. From measurements of the NRA spectrum at normal and tilted ion incidences the zero-point vibrational energies of H on Si(111) in the perpendicular and parallel directions were found to be 123.4 +/- 4.6 and 44.6 +/- 6.2 meV, respectively, which are consistent with harmonic potentials. The zero-point energies obtained for Pt(111)-H were 80.8 +/- 3.9 and 62.1 +/- 6.0 meV for perpendicular and parallel directions, respectively. These results indicate that the stretching mode is harmonic, while the bending mode is strongly anharmonic.