Optical properties of methane to 288 GPa at 300 K

Optical properties of solid methane (CH4) were studied at high pressure and room temperature using a diamond anvil cell. Reflectivity and transmission measurements were used to measure the refractive index to 288 GPa. Fabry–Perot interferometery was used to measure the sample thickness to 172 GPa. This data was fitted to the derived expression of thickness vs. pressure that was then used to calculate the thickness to 288 GPa. This in turn was combined with optical absorption experiments to obtain the absorption coefficient and hence the extinction coefficient k*k*. From combined reflection and absorption experiments the refractive index n=ns+ik*n=ns+ik* was obtained. The index of refraction and the ratio of molar refraction to molar volume showed a large increase between 208 and 288 GPa. This behavior indicated that a phase transformation of insulator–semiconductor might have occurred in solid CH4 by 288 GPa.