The interpretation of the near infrared spectrum of ytterbium in the gallium and aluminum garnets is greatly simplified by the observation that a majority of the lines can be identified as vibronic by virtue of their symmetric pairing as summation and difference sidebands about the transition connecting the lowest ${}_{}{}^2{}_{}{}^{}F_{\frac{7}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}F_{\frac{5}{2}}^{}{}_{}{}^{}$ ytterbium levels. The few lines which cannot be so identified are readily assigned as electronic transitions connecting the remaining ${}_{}{}^2{}_{}{}^{}F_{\frac{7}{2}}^{}{}_{}{}^{}$ and ${}_{}{}^2{}_{}{}^{}F_{\frac{5}{2}}^{}{}_{}{}^{}$ levels. This procedure provides a complete identification of all the $4f$-electron energy levels of trivalent ytterbium based on experimental evidence alone. It is found that the first excited ${}_{}{}^2{}_{}{}^{}F_{\frac{7}{2}}^{}{}_{}{}^{}$ level occurs above 540 ${\mathrm{cm}}^{-1\mathrm{}}$ in the gallium garnets and above 600 ${\mathrm{cm}}^{-1\mathrm{}}$ in the aluminum garnets. New assignments are made for the ${}_{}{}^2{}_{}{}^{}F_{\frac{5}{2}}^{}{}_{}{}^{}$ levels in the aluminum garnets, these assignments being more similar to those already made for the gallium garnets. No spectral evidence is found in the gallium garnets for ytterbium ions occupying other than normal sites. However, there is some indication in the aluminum garnets that a small percentage of the ytterbium ions occupy abnormal sites.

• Received 20 June 1966

DOI:https://doi.org/10.1103/PhysRev.159.245