Formation and desorption dynamics of photoproducts in the ablation of van der Waals films of chlorobenzene at 248 nm

Abstract

The ablation of thick C6H5Cl films at 248 nm is studied with emphasis on the formation efficiency and desorption dynamics of the observed photoproducts. To this end, the desorbates are probed as a function of the laser fluence via time-of-flight quadrupole mass spectrometry. At low fluences, indicated to represent the subablation regime, we observe a strong induction effect for the parent molecule and desorption of only one new species, namely of HCl. Phenyl products are indicated to be formed, but they start desorbing only as parent peak induction becomes significant. In contrast, at higher fluences, desorption of the parent molecule from freshly deposited films is intense from the very first laser pulse, and a number of additional photoproducts are observed. Three main species, namely (C6H5)2, C6H4Cl2, and C12H9Cl, are observed, while Cl and C12H8Cl2 are detected mainly at higher fluences. All products are compatible with the known gas phase and solution chemistry of C6H5Cl, indicating that no new reaction channels open up above the ablation threshold. Over the full fluence range, the phenyl photoproducts differ significantly from HCl in their desorption observables. Specifically, the phenyl photoproducts are described by nearly the same velocity distribution as the parent molecule and their desorption efficiency closely correlates to that of C6H5Cl. In contrast, HCl exhibits a much more complicated translational behavior, but generally it is nearly equilibrated with the parent molecule. We argue that these differences derive mainly from the much higher volatility of HCl as compared with that of the phenyl photoproducts. Thus, for the phenyl derivatives, the “driving force” for their ejection is evidently their entrainment in the C6H5Cl jet, whereas for HCl, being quite volatile, its desorption appears to be largely independent of that of the parent molecule.