Ruby at high pressure. I. Optical line shifts to 156 GPa

We have measured the pressure shifts of the ruby R lines and several surrounding lines to 156 GPa in a quasihydrostatic xenon pressure medium. We fit these lines to a cubic crystal-field scaling theory, including trigonal field distortions and spin-orbit coupling as perturbations. By scaling with volume, rather than pressure, we obtained good agreement with the observed curvature in the line shifts with pressure. Measurements of the R₃' line to 140 GPa in a nonhydrostatic environment yielded R₃' line shifts similar to those observed in the quasihydrostatic measurements. We also measured the U absorption band shift to 106 GPa in a nonhydrostatic environment and found good agreement with our scaling theory. We infer a U-band--B-line anticrossing near 70 GPa and predict an R₃'-R line crossing near 200 GPa. The implications of these results to pressure measurements using the ruby pressure scales are discussed.