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

The dissociation dynamics of p-difluorobenzene-Ar and p-difluorobenzene-Ar+ have been investigated from the (5(1)) over bar level in S-1 and the (29(2)) under bar level in D-0, respectively. The technique of velocity map imaging has been used to determine the translational energy release distributions. In the case of (5(1)) over bar 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 0(0) level. From the translational energy release data the rotational energy distribution within 0(0) 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 Angstrom 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 (29(2)) under bar 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 S-1. As for the (5(1)) over bar 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 (5(1)) over bar (S-1) and (29(2)) under bar (D-0), 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. (C) 2003 American Institute of Physics.