PACS: 63.20.Pw; 78.20.Ci The optical dephasing, caused by a quadratic interaction with acoustic phonons, is studied on the basis of the nonperturbative theory. The effect of a strong weakening of the elastic springs on the electronic transition is studied. It is found that, if the weakening brings the system close to the dynamical instability, then the usual / T 7 dependence of the homogeneous width of a zero-phonon line (ZPL) is replaced by the / T 3 dependence in a broad temperature interval. An equation is derived which describes the additional broadening of a ZPL caused by the dynamical instability of the final state. The model explains the temperature broadening and shift of the zero-phonon lines of some nitrogen-containing centers in diamond crystals.

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We are studying the optical spectra of impurity centers in the case when the optical transition takes place between nondegenerate and two-fold degenerate electronic E-states characterized by a strong vibronic coupling. In the latter state the adiabatic surface has a shape of a deformed Mexican hat with three flat minima on the trough of the “hat.” We have found that in this case the usual T7-dependence of the homogeneous width of the zero-phonon line at low temperatures is replaced by a dependence close to the T3-one. Besides, the usual T4-dependent redshift of the line is replaced by the T2-dependent shift. Depending on the flatness of the adiabatic surface, the latter shift may be to the red or to the blue. The theory is applied to the zero-phonon line 637 nm of the nitrogen-containing N-V center in a diamond. In this system the temperature dependence of the ZPL results from the common action of the Jahn–Teller effect and the pseudo-Jahn–Teller effect in the excited E-state: the pseudo-Jahn–Teller effect “helps” to get the system close to the dynamical instability due to the enlargement of the Jahn–Teller distortion.

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