REFLECTANCE EFFECTS CAUSED BY REFRACTIVE-INDEX GRADIENTS IN DIAMOND CELL SAMPLES OF H-2 AND AL2O3

At high pressures, optical spectroscopic techniques are the principal means for probing dense solid hydrogen, including both its vibrational and electronic excitations. Ruoff et al. [Phys. Rev. B47, 11039 (1993)] proposed that refractive index gradients of Hz samples loaded with 5% Al2O3 powder in diamond anvil cells could cause a 4% rise in infrared reflectivity which interfered with the measurements on hydrogen reported by Mao et al. [Phys. Rev. Lett. 65, 484 (1990)]. We have analyzed this alternative proposal, including computer simulations of the appropriate sample configuration. Our calculations show that effects of a graded index on the rise of reflectivity from 1.0 to 0.5 eV are less than 0.1% and are therefore insignificant.