On the vibronic level structure in the NO3 radical Part III. Observation of intensity borrowing via ground state mixing

Abstract

The òE″← ²A′₂ absorption spectrum exhibits vibronically allowed transitions from the ground state of NO₃ to upper state levels having a″₁ and e′ vibronic symmetries. This paper explores the coupling mechanisms that lend intensities to these features. While transitions to e′ vibronic levels borrow intensity from the very strong ²E′← ²A′₂ electronic transition, those to a″₁ levels involve only negligible upper-state borrowing effects. Rather, it is the vibronic mixing of the ground vibronic level of NO₃ with vibrational levels in the ²E′ electronic state that permit the a″₁ levels to be seen in the spectrum. These ideas are supported by vibronic coupling calculations. The fact that the intensities of features corresponding to the two different vibronic symmetries are comparable is thus accidental.