REFLECTANCE EFFECTS CAUSED BY REFRACTIVE-INDEX GRADIENTS OF H2 SAMPLES LOADED WITH 0.5-MU-M AL2O3 POWDER IN DIAMOND-ANVIL CELLS

This paper shows that there is doubtful basis for the claim of metalli zation of hydrogen as a result of optical studies in the diamond-anvil cell to date. The presence of fine Al2O3 particles in the hydrogen pr ecludes a simple hydrogen-diamond interface; the graded index must be included and is shown to account for the observed behavior in H. K. Ma o et al. [Phys. Rev. Lett. 65, 484 (1990)] and M. Hanfland et al. [Phy s. Rev. B 43, 8767 (1991)]. The small reflectivity found on the publis hed experiment (having only a tiny fraction of the diamond covered wit h Al2O3) at 161 GPa and a mentioned result at 160 GPa (with no Al2O3), both at room temperature, is very likely due to an absorption peak of hydrogen in diamond at 0.38 eV broadened by the stresses in the diamo nd anvil. Therefore, it must be shown that refractive-index gradients and/or hydrogen in diamond are not responsible for the observed reflec tance before other claims are made.