The mechanism of photo-induced desorption of molecules weakly adsorbed on a metal surface

Using mass spectrometry flight-time measurements, photodesorption of HFCO, H2CO, CH2CO and CH3Cl from Ag(lll) under pulsed nanosecond laser irradiation has been investigated in the experimental photon energy range of 1.17 eV less than or equal to hv less than or equal to 4.67 eV. All these molecules are physisorbed on Ag(lll). No threshold behaviour has been established within this energy range. The translational energy distribution of the desorbing molecules is characterized by a Maxwell-Boltzmann temperature in the range 110-150 K. The low translational temperatures and low photon energy thresholds, though in striking contrast to the high average translational temperatures and threshold behaviour reported for photodesorption of chemisorbed molecules from metal surfaces, can be understood in terms of the prevailing electron attachment model. However, it requires that the substrate electrons attach to the molecules with positive electron affinities. In an alternative model, electron scattering excites the high frequency, v=1 molecular vibrational levels, possibly through dipole interactions. In this case, desorption results from vibrational predissociation in the adsorbate-surface bond.