Optical spectra of systems with nearly unstable excited states: modulated lambda-shaped spectra

Absorption spectra of faded small molecules in superfluid droplets of ⁴He display an oscillating structure of the zero-phonon lines. To understand the origin of these oscillations, we have examined the theory of optical spectra of impurity centres in condensed matter. The attention was paid to the case of the optical transition taking place in the vicinity of a nearly stable or an unstable state. In this case the ratio of the mean vibration frequencies in the final and initial states is small. 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 the shape of an asymmetrical lambda letter. It may also reveal Airy-type oscillations, characteristic of a nearly free motion in the final state. Such oscillations have been observed in the hot luminescence of self-trapped excitons in Xe crystals. In our opinion, of an analogous origin is the oscillating structure of the zero-phonon lines in superfluid droplets of ⁴He.