Vibronic transitions in the vicinity of the dynamical instability of the final state

Optical spectra of some centres in crystals display unusual shapes resembling an anisotropic lambda letter. We have found that this kind of spectra can be observed in case the final electronic state has soft vibrational dynamics. To describe these spectra we have proposed a method which allows one to consider an arbitrary quadratic vibronic coupling with phonons. The numerical calculations have been done in the case when the optical transition takes place in the vicinity of a nearly stable or an unstable state. We have found that in this case a strong enhancement of the low-energy phonon sideband appears. As a result, the spectrum (or its envelope) has indeed the shape of an asymmetrical lambda letter. It may also reveal the Airy-type oscillations characteristic of a nearly free motion in the final state.