LONG RADIATIVE LIFETIMES OF SMALL MOLECULES

In this paper, we apply a theory of electronic relaxation in polyatomic molecules for the study of the anomalously long radiative lifetimes of NO 2, CS2, and SO2. We have classified medium-size molecules which exhibit intramolecular vibronic coupling into two intermediate cases, which we call the sparse intermediate case and the dense intermediate case, characterized by low and high spacing of the vibronic levels relative to the radiative width. The radiative decay in the sparse intermediate case was considered in detail, taking advantage of the coarse level spacing and the extremely short intramolecular recurrence time encountered in this case. From our model calculations, we conclude that: (a) in the sparse case, the radiative decay rate is characterized by a superposition of slowly varying exponentials; (b) the mean radiative lifetime is expressed as a radiative lifetime calculated from the integrated oscillator strength and diluted" by the number of states within the half-width of the manifold of coupled levels; (c) no intramolecular electronic relaxation processes are encountered in this case; (d) a qualitative interpretation of the complex molecular spectra is provided."