Analysis of the vibronic spectrum of chromium doped strontium titanate

The fluorescence spectrum of chromium-doped strontium titanate was measured as a function of temperature. The low-energy vibronic sideband of the $R$ lines at low temperatures was analyzed to determine the one-phonon and multiphonon contributions to the observed spectral profile. The one-phonon spectrum appears to be predominantly forced electric dipole in nature. Vibronic selection rules appropriate for such transitions were determined, and by comparison with neutron scattering and infrared absorption data more than 30 peaks in the vibronic spectrum can be tentatively identified with transitions involving specific phonon modes. Numerous low-frequency peaks are observed in the high-energy vibronic sideband, many of which cannot be associated with known vibrational modes. Using a simple long-wavelength phonon approximation, a phonon density of states is obtained and found to compare quite well to that determined from analyzing neutron scattering data. The temperature dependences of the widths of the zero-phonon lines and local mode were investigated using several phonon distributions including the effective density of phonon states obtained from the vibronic sideband. Low-frequency modes appear to make the dominant contribution to the broadening of the zero-phonon lines, whereas both low- and high-frequency phonons are active in broadening the local mode.