Study of Nonadiabatic Transitions in Triatomic Molecules

Abstract

Nonadiabatic coupling between electronic states in triatomic molecules has been studied with special consideration given to the possibility of internal conversion. Approximate calculations of electronic and vibrational coupling elements and density of states, which determine the transition probability, have been performed for H2O and some other molecules. The results suggest that the statistical model mechanism for large molecules should not be applied to triatomics. Under favorable conditions, e.g., sufficiently different shape or position of the two potential curves involved, strong coupling might occur which is large enough to explain anomalously long fluorescence lifetimes in small molecules. However, observations of lifetime lengthening of the first excited states of NO2 and SO2 cannot easily be explained because internal conversion through vibrational modes is symmetry forbidden. In this case, the role and relative importance of intersystem crossing and Renner—Teller coupling need further investigation.