Vibronic intensities in the absorption spectra of Yb3+

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The oscillator strengths and relative vibronic intensity distribution of the ${(}^{2}$${\mathit{F}}_{5/2}$)${\mathrm{\ensuremath{\Gamma}}}_{8}$,${\mathrm{\ensuremath{\Gamma}}}_{7}$\ensuremath{\leftarrow}${\mathrm{\ensuremath{\Gamma}}}_{6}$${(}^{2}$${\mathit{F}}_{7/2}$) transitions of the ${\mathrm{YbCl}}_{6}^{3\mathrm{\ensuremath{-}}}$ complex ion at an octahedral symmetry site in the ${\mathrm{Cs}}_{2}$${\mathrm{NaYbCl}}_{6}$ lattice have been measured experimentally and calculated using a combined vibronic crystal-field--ligand polarization approach. The vibronic crystal-field contribution to the total transition dipole moment of the various excitations was worked out both with and without invoking closure over the central metal ion intermediate electronic states and the intensity was assumed to be derived from both a parity and a spin-allowed d\ensuremath{\leftarrow}f transition with the cooperation of the odd-parity vibrational modes of the complex ion. Quadrupole and hexadecapole terms have been included in the ligand polarization contribution. Attention has been given to the correct choice of phases for both the electronic and the vibrational wave functions in order to ensure the right sign for the cross term which couples together the crystal field and the ligand polarization transition dipole vectors. The ab initio formalism employed avoids the use of any adjustable parameters in calculating the vibronic intensities. The calculated oscillator strengths of vibronic transitions are within order of magnitude agreement with experimental values. The sensitivity of the calculated values to the use of different force fields has been investigated. The experimentally measured total oscillator strengths for the ${(}^{2}$${\mathit{F}}_{5/2}$)${\mathrm{\ensuremath{\Gamma}}}_{8}$,${\mathrm{\ensuremath{\Gamma}}}_{7}$\ensuremath{\leftarrow}${\mathrm{\ensuremath{\Gamma}}}_{6}$${(}^{2}$${\mathit{F}}_{7/2}$) transitions of ${\mathrm{YbCl}}_{6}^{3\mathrm{\ensuremath{-}}}$ diluted into the transparent ${\mathrm{Cs}}_{2}$${\mathrm{NaGdCl}}_{6}$ host remain constant with change in ${\mathrm{Yb}}^{3+}$ concentration although deviation of the chromophore from octahedral symmetry is evident at intermediate concentrations. A comparison with the vibronic sidebands of ${\mathrm{Cs}}_{2}$${\mathrm{LiYbCl}}_{6}$ and ${\mathrm{Cs}}_{2}$${\mathrm{KYbF}}_{6}$ is made. \textcopyright{} 1996 The American Physical Society.