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.

Computer Simulations of Laser Ablation of Molecular Substrates

“…A similar observation of a lower threshold fluence and higher ablation rates with photochemistry has been observed in experiments. 109,162 As with the results presented in Figure 6, the yield vs fluence data in Figure 12 has been fit to the Arrhenius dependence, eq 1, in the desorption regime and to the critical energy density model, eq 2, in the ablation regime. The values of the activation energy in the desorption model, E s , are 0.17 and 0.45 eV for calculations with and without photochemistry, respectively.…”

“…The reaction patterns in our model are based on photochemistry of chlorobenzene. The selection of chlorobenzene as the basic system for modeling photochemical events is based on the well-known photochemistry of the compound and extensive experimental studies [108][109][110][111][112][113] that make a detailed interpretation/verification of the simulation results possible. Photofragmentation of chlorobenzene occurs via scission of the C-Cl bond to yield C 6 H 5 and Cl radicals, which in solution and static gas cell experiments react with each other and with the parent molecule to form a number of different products.…”

“…Since the transition from desorption to ablation is not reflected in the number of ejected monomers, it can easily remain unnoticed in a post-ionization experiment. An interpretation of the physical mechanisms of material ejection should not be based solely on the mass spectrometry data, 15,21 but should be complemented by measurements of other characteristics, such as shapes of the acoustic waves propagating from the absorption region [31][32][33][34][35][36][37][38][39]115 or cluster detection in fast imaging 109,116,162,163 and trapping plate 28 experiments.…”

“…Photofragmentation of chlorobenzene occurs via scission of the C-Cl bond to yield C 6 H 5 and Cl radicals, which in solution and static gas cell experiments react with each other and with the parent molecule to form a number of different products. [108][109][110]114 To represent the photochemical processes in chlorobenzene, we chose reactions that are thermodynamically favorable and are observed in gas-phase or solution chemistry of chlorobenzene. In total, there are 12 reactions considered, a sample of which are delineated below.…”

The role of the photochemical fragmentation in laser ablation: a molecular dynamics study

“…Photofragmentation of chlorobenzene occurs exclusively by scission of the C-Cl bond to yield C 6 H 5 and Cl radicals, which in solution and static gas cell experiments react with the precursor molecules to form a number of different products [10]. The photochemistry of C 6 H 5 Cl has been investigated by numerous experimental techniques [1,2,9,[11][12][13][14]25]. Using a C 6 H 5 Cl molecular beam [13], it was found that excitation of chlorobenzene by 248 nm laser light can lead to either photochemical dissociation or vibrational relaxation.…”