Vibronic intensity borrowing and the isotope dependence of vibronic structure

In recent years it has become clear that the conventional Herzberg-Teller theory is not capable of explaining the breakdown in intensity mirror symmetry between the "forbidden" vibronic structure in absorption and fluorescence spectra of many polyatomic molecules. Furthermore, in a number of systems the isotope dependence of vibronic intensitles predicted by the Herzberg-Teller theory is not obeyed. A theory of vibronic structure is presented which includes the so-called Born-Oppenheimer vibronic coupling and the Duschinsky and isotope dynamic mixing adiabatic corrections to the conventional Herzberg-Teller theory. The results of illustration calculations for a three-mode system axe presented and discussed.