Comment on ‘‘Optical absorption measurements of hydrogen at megabar pressures’’

Using the results of Fabry-Pérot measurements on hydrogen in a diamond-anvil cell, we show that the assumption made in Fig. 3 of Hanfland, Hemley, and Mao [Phys. Rev. B 43, 8767 (1991)] that ${\mathit{n}}_{\mathrm{H}}^{\mathrm{\infty }}$<${\mathit{n}}_{\mathit{D}}$ for P>130 GPa and ${\mathit{n}}_{\mathit{D}}$=2.42 at 161 GPa (as at atmospheric pressure) violates the combined first and second laws of thermodynamics in that it leads to an increase in the sample thickness with increasing pressure. If, instead of using the assumption that ${\mathit{n}}_{\mathit{D}}$=2.42 at 161 GPa, the experimental variation of ${\mathit{n}}_{\mathit{D}}$ is extrapolated from 40 GPa over the entire range an even larger violation is found.