Vibration-mediated uv photodesorption: Ammonia on GaAs

A surprisingly large isotope effect is observed in nonthermal uv photodesorption of ammonia from GaAs(100). The cross section for ${\mathrm{NH}}_3$ photodesorption is 4 times larger than for ${\mathrm{ND}}_3$, a fact which cannot be explained using existing photodesorption models, e.g., the conventional Menzel-Gomer-Redhead model. We take this as the first evidence for uv-driven photodesorption from electronically quenched, vibrationally excited ground-state adsorbates.