Vibrational excitations in chloromethyl radical formed by the photodissociation of chlorobromomethane

Abstract

Using velocity map ion imaging, the photodissociation of chlorobromomethane (CH2BrCl) at 233-234 nm has been studied. The total translational energy distributions and the anisotropy parameters have been determined from the ion images of the photofragments Br ((2)P1/2) (denoted as Br(*)) and Br ((2)P3/2) (denoted as Br) for the dominant CH2BrCl + hv → CH2Cl + Br(*) and CH2BrCl + hv → CH2Cl + Br channels. Using an impulsive model invoking angular momentum conservation, the vibrational energy distributions of the chloromethyl radicals have been derived from the total translational energy distributions for the two channels. The study suggests that there are a number of vibrational modes of the chloromethyl radical to be excited in both of the two photodissociation channels. In the Br* channel, the CH2 s-stretch mode v1 has the most probability of excitation. While in the Br channel, the CH2 scissors mode ν2 is attributed to the highest peak of the vibrational energy curve of the chloromethyl radical. The results further imply that, following absorption of one UV photon of 234 nm, other vibrational modes besides v5 (C-Br stretch mode) are also excited in the parent molecule.

Photodissociation dynamics of ICH2Cl → CH2Cl + I*/I: photofragment translational spectroscopy at 304 and 277 nm

“…From the above equation of the simple impulsive model, it gives E R /E T E 1. 19. This shows that the rotational excitation of the CH 2 Cl fragment is very important following the photodissociation process and the rotational energy E R has to be seriously considered in the partitioning of E avl .…”

“…But the photodissociation dynamics of di-halogenated methanes (CH 2 XY) has been studied much less, because of their complicated excited states and complex photodissociation processes. In the photodissociation of CH 2 ClBr 18,19 and CH 2 I 2 , [20][21][22][23] several photodissociation pathways and much higher E int /E avl have been reported. Due to the asymmetrical structure of CH 2 XY, the rotational energy E R of the photofragment CH 2 X (with a heavy halogen atom) would be highly excited following the photodissociation process.…”

Dynamics and yields for CHBrCl2photodissociation from 215–265 nm

“…This behavior may arise via contributions from both mechanisms suggested above, but we do not observe a clear bimodal distribution of recoil energies. Some experiments on similar alkyl halides show structure in the E int distribution that permits assignments of product vibrational excitations, 18,36,58 but we resolve no such structure. The width of the E int distribution as a fraction of E avl decreases monotonically, albeit modestly, with increasing photolysis energy, and we cautiously regard this trend as evidence that curve crossing is playing a larger role at lower excitation energies.…”

“…Nonetheless, there are some notable similarities. Other studies 25,36,58 invoke two distinct mechanisms to account for the greater spread of internal energy observed in the Br channel: (1) direct dissociation from excited states following the diabatic path to ground-state products and (2) indirect dissociation via curve crossing from the 3 Q 0 + state. In accord with the results of other halomethane studies, the latter case should promote vibrational excitation more effectively because of the sudden change in the reaction coordinate following curve crossing.…”

“…The vertical scale in Fig. 7 shows measured translational energy partitioning following photodissociation of CH 2 I 2 , 28 CHBrCl 2 (this work), ICH 2 Cl, 31,58 CH 2 ClBr, 25,26,29,33,36 and CH 3 CH 2 I. 18,48,[64][65][66] For each species, we plot an average over all photolysis energies of the published experimental data.…”