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. © 2003 Elsevier B.V. All rights reserved.