Zero-phonon line: Effect of quadratic electron–phonon coupling

The recently proposed theory of vibronic transitions in impurity centres of solids [10] is applied to zero-phonon lines in optical spectra. New equations for the relative intensity, position and width of the line are derived, which take into account both the linear and the quadratic electron–phonon couplings. The joint effect of these couplings on the line intensity is found to be different for absorption and luminescence.