Jahn–Teller Effect in the Excited State: Anomalous Temperature Dependence of the Zero-Phonon Line

We are studying a zero-phonon line (ZPL) in the optical spectra of impurity centers in crystals, taking into account the strong Jahn–Teller effect in the excited state. We have found that, if this effect brings the system close to the dynamical instability, then at low temperatures the usual T7-dependence of the homogeneous width of the ZPL is replaced by the T3- dependence. One example of such a case is given by a center with a strong linear E–e Jahn–Teller effect. In this case the dynamical motion in the relaxed E-state is on the verge of the dynamical instability with respect to a shift along the bottom of the trough of the adiabatic potential. The first excited state of the nitrogen-containing centers in diamond crystals belongs to this type of states. Our experimental study of the emission of this center from E-state indeed has revealed the T3-type temperature broadening of the ZPL at low temperature.