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

Abstract

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 Cl 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.