Ground-state photochemistry of tetramethyldioxetane. 1. Energy distribution among molecules excited by infrared multiple photon absorption

Tetramethyldioxetane (TMD) was dissociated under nearly collision-free conditions following infrared multiple-photon excitation (IRMPE) or overtone excitation (OTE) of the C-H vibrational stretching mode. The kinetics of the ensuing chemiluminescence were observed in real time. A quantitative comparison of the rise and decay curves allows the determination of the approximate energy distribution among molecules excited by IRMPE to beyond the dissociation limit. In addition, the actual yield of dissociating molecules is determined as a function of fluence. These data are compared with models based on the widely used rate equations approach. In these models, energy-dependent absorption cross sections σ(E) are used, which are allowed to vary smoothly with energy. It is found that the yield can be fitted by a number of different functional forms of σ(E). In contrast, none could be used to reproduce the energy distributions. This result indicates that yield data alone are insufficient to derive population distributions generated by IRMPE. More sensitive experimental parameters need be determined, which in turn will lead to a better description of the molecular parameters contributing to IRMPE.