LASER-INDUCED INTERACTION OF AMMONIA WITH GAAS(100) .2. DESORPTION DYNAMICS

UV laser irradiation of ammonia adsorbed on GaAs(100) leads to molecular desorption, with a mean translational temperature of [E(trans)/2k] = 300 K, independent of photon energy and isotope substitution. However, the photodesorption cross section depends strongly on isotope substitution: sigma(NH3)/sigma(ND3) = 4.1 at hv = 6.4 eV. This isotope effect is too large to be accounted for by the mass difference in the leaving particles (NH3 Vs ND3), but can be successfully explained in terms of an isotope effect in the internal N-H(D) coordinates. We take this as evidence for uv-driven photodesorption from electronically quenched, but vibrationally hot ground state ammonia.