Photoproduct desorption dynamics in the UV ablation of model molecular solids. Ablation of chlorobenzene films at 248 nm

The 248-nm ablation of thick films of C6H5Cl is studied with emphasis on the desorption dynamics of the observed photoproducts. To this end, the desorbates are probed as function of the laser fluence via time-of-flight quadrupole mass spectrometry. At low fluences, plausibly considered as sub-threshold, we observe strong induction effect for the parent molecule and desorption of only one new species, namely, of HCl. Other products are most likely formed, but they start desorbing only as parent peak induction becomes important. At higher fluences, we observe three main additional species, namely C6H5.2, C6H4Cl2 and C6H5–C6H4Cl. Yet, there are specific differences between the various products as far as their desorption efficiency and translational distributions are concerned. In particular, the phenyl photoproducts are described by the same velocity distribution as the parent molecule, whereas HCl is generally nearly energetically equilibrated with the parent molecule. We suggest that these differences derive from the different efficiencies of entrainment of the species in the plume. q1998 Elsevier Science B.V.