Isotope effects in UV-photostimulated desorption of ammonia from Cu (1 1 1)

Abstract

For UV photoinduced desorption of ammonia and deuterated ammonia from Cu (1 1 1) at hv = 6.4 eV, strong isotope effects are found, which increase from σ NH 3 σ ND 3 = 1.9 ± 0.5 to 4.1 ± 1.2 when the coverage is decreased from 0.14 to 0.02 monolayer (ML), respectively. The coverage dependence of the isotope effect is attributed to a variation of the desorption probability with adsorbate binding energy. The magnitude of the isotope effect suggests that nuclear motion along an intramolecular coordinate during a short-lived electronic excitation facilitates desorption. We propose that after deexcitation to the electronic ground state potential energy surface, efficient coupling of the highly vibrational excited inversion mode with the molecule-surface coordinate leads to desorptio. Trajectory calculations performed on a two-dimensional model potential energy surface predict that desorption occurs rapidly within a few vibrational periods of the umbrella mode ( T vib ∼ 35 fs) — with comparable energy release into the translational and vibrational degrees of freedom.