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-o f-flight quadrupole mass spectrometry. At low fluences, plausibly cons idered as sub-threshold. we observe strong induction effect for the pa rent molecule and desorption of only one new species, namely, of HCl. Other products are most likely formed, but they start desorbing only a s parent peak induction becomes important. AT higher fluences, we obse rve three main additional species, namely (C6H5)(2), C6H4Cl2 and C6H5- C6H4Cl. Yet, there are specific differences between the various produc ts as far as their desorption efficiency and translational distributio ns are concerned. In particular, the phenyl photoproducts are describe d by the same velocity distribution as the parent molecule, whereas HC l is generally nearly energetically equilibrated with the parent molec ule. We suggest that these differences derive from the different effic iencies of entrainment of the species in the plume. (C) 1998 Elsevier Science B.V.