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

To address the factors that affect reactivity patterns in the UV ablation of molecular solids, we have examined photoproduct formation in the 248 nm ablation of C6H5Cl films. The desorbates are probed as a function of the laser fluence via time-of-flight quadrupole mass spectrometry. Above the ablation threshold, we observe formation of four main species: HCl, (C6H5)(2), C6H4Cl2 and C6H5-C6H4Cl, while CI and (C6H4Cl)(2) are detected, mainly at somewhat higher fluences. All products can be accounted for by radical addition/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 that no new reaction channels open up above the ablation threshold. However, the formation of the indicated products on the timescale of a single ablation event is inconsistent with the available rate cross-sections. The discrepancy suggests "hot" reactivity of the C6H5 and Cl photofragments. Plausible mechanisms are discussed.