Reactivity characteristics in the UV ablation of molecular van der Waals solids

To address the factors that affect reactivity patterns in the UV ablation o f molecular solids, we have examined photoproduct formation in the 248 nm a blation of C6H5Cl films. The desorbates are probed as a function of the las er fluence via time-of-flight quadrupole mass spectrometry. Above the ablat ion threshold, we observe formation of four main species: HCl, (C6H5)(2), C 6H4Cl2 and C6H5-C6H4Cl, while CI and (C6H4Cl)(2) are detected, mainly at so mewhat higher fluences. All products can be accounted for by radical additi on/abstraction reactions of the C6H5 and Cl fragments that are produced by photolysis of the parent molecule. The reactions are fully compatible with the known gas-phase and solution chemistry of these fragments, indicating t hat no new reaction channels open up above the ablation threshold. However, the formation of the indicated products on the timescale of a single ablat ion event is inconsistent with the available rate cross-sections. The discr epancy suggests "hot" reactivity of the C6H5 and Cl photofragments. Plausib le mechanisms are discussed.