Soft dynamics of the excited state: Lambda-shaped optical spectra

Anomalous optical spectra of centres with strongly reduced local elastic constants in the excited electronic state are studied. In this case, transitions take place to the vicinity of a flat minimum of the potential energy in the configurational coordinate space. The existence of a flat minimum results in the appearance of the resonant (pseudo-local) mode of a low frequency. This leads to a strong enhancement of the low-frequency part of the phonon wing, causing a formation of the possibly modulated lambda-shaped optical spectra; the modulation comes from the Airy oscillations of the quantum accelerated motion.