We report a quantum wavepacket study on the characteristic bimodal translational energy distribution of photostimulated desorbed Xe from an oxidized silicon (001) surface observed by Watanabe and Matsumoto, Faraday Discuss. 117 (2000) 203. We have simulated the theoretical translational energy distributions based on wavepacket calculations with a sudden transition and averaging model to reproduce the experiment. We discuss the desorption mechanism and suggest a very strong position dependence of the deexcitation processes for Xe/oxidized Si(001).