Effects of static non-hydrostatic stress on the R lines of ruby single crystals

The R-line fluorescence spectroscopy of ruby (Cr:Al2O3) single crystals has been investigated under controlled deviatoric stresses at high confining pressures in a diamond anvil cell. Full stress tensors of the ruby samples were determined based on free-slip boundary condition and cylindrical symmetry. The shift of the R(2) line is insensitive to deviatoric stress and represents local mean stress (pressure). When ruby is loaded along the c-axis, the R(1)-R(2) splitting changes linearly with deviatoric stress at a rate of -0.241 +/- 0.012 Angstrom/GPa and does not depend on the confining pressure. The splitting increases non-linearly for ruby loaded along the a-axis. The dependencies of the splittings on deviatoric stress are in excellent agreement with theoretical calculations constrained by shock-wave data. The widths of the fluorescence lines increase with growing inhomogeneity in pressure, but are not affected by the deviatoric stresses.