The vibrational force constant and transition stress for diatomic molecules

Abstract

The approximate orbital additivity of vibrational force constants for diatomic molecules is demonstrated. For a given molecule or molecular ion, vibrational force constants of excited electronic states associated with a particular configuration are approximately equal, provided configuration interaction can be neglected. The transition stress associated with an electronic transition strongly affects the vibrational motion. The vibrational force constant is shown to depend on the transition stress. The general relation between the force constant and the bond length for excited states is discussed. Molecular constants of excited states of diatomic molecules and molecular ions can be calculated from the transition stress. These calculated values are in good agreement with experimental results or with the theoretical values obtained by ab initio calculation.