The partitioning of energy amongst vibration, rotation, and translation during the dissociation of p-difluorobenzene–Ar neutral and cation complexes

Abstract

The dissociation dynamics of p-difluorobenzene–Ar and p-difluorobenzene-Ar+ have been investigated from the 51¯ level in S1 and the 292¯ level in D0, respectively. The technique of velocity map imaging has been used to determine the translational energy release distributions. In the case of 51¯ p-difluorobenzene–Ar, dispersed fluorescence spectra provide the distribution of vibrational energy in the p-difluorobenzene fragment. A significant fraction of the p-difluorobenzene products are formed in the 00 level. From the translational energy release data the rotational energy distribution within 00 can be inferred. The results show that the average rotational energy is 380 cm−1, >5 times the average translational energy of 70 cm−1. This rotational excitation infers that dissociation occurs with the Ar atom significantly displaced from its equilibrium position above the center of the aromatic ring. From the average rotational energy it is determined that the Ar atom is, on average, displaced by 1.8–3.7 Å from the center of the aromatic ring at dissociation, i.e., the Ar atom is beyond the carbon atoms. In the case of dissociation from the 292¯ level of p-difluorobenzene-Ar+, the vibrational distribution within the p-difluorobenzene+ product is not known, however it can be inferred from previous studies of dissociation within S1. As for the 51¯ p-difluorobenzene–Ar case, the evidence suggests that dissociation leads to significant rotational excitation of p-difluorobenzene+. There are a limited number of destination vibrations within the p-difluorobenzene and p-difluorobenzene+ fragments for dissociation from 51¯ (S1) and 292¯ (D0), respectively. Hence there are only a few, widely separated, values for the combined translational and rotational energy available. Despite this, the translational energy release distributions in both cases are smooth and structureless. In the limit of no rotational excitation of the polyatomic fragment, the translational energy release distributions would show peaks only at energies corresponding to populated vibrational states of the product. The absence of such peaks indicates that rotational excitation of the product occurs for all vibrational states, reducing the average translational energy released and smearing the distribution.