Mechanisms of Non Resonance Vibronic Coupling in Polyatomic Molecules

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Abstract
The influence of non resonance vibronic coupling of two excited molecular states on the electronic absorption and emission spectrum is discussed in terms of low order perturbation theory. Transition probabilities are calculated taking into account a non totally symmetric coupling mode and a totally symmetric vibration with a displacement of equilibrium position in different electronic states. While band progressions in the emission and absorption spectra can be analyzed in terms of the Herzberg Teller (HT)‐theory, the ratio of HT‐intensities of orbitally “forbidden” transitions in absorption and emission has to be corrected by a factor due to the violation of the Born‐Oppenheimer (BO) approximation. The HT transition probabilities and the non‐BO correction factor are obtained in terms of spectroscopic properties. As a consequence it is possible to decide which vibronic coupling mechanism plays the dominant role in a given case. The calculated values are in good agreement with available experimental results.