The closing diamond anvil optical window in multimegabar research

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Abstract
The tetragonal distortion of a diamond anvil supporting a sample pressure of over 4 Mbars is such that the cubic crystal becomes elastically distorted to a tetragonal crystal with c/a ■0.69. These large distortions in the anvil greatly change its optical properties. The decrease of the band gap of diamond with pressure is described in terms of a dielectric model and in terms of experimental data to 4.05 Mbars. It is shown how this band gap decrease makes it impossible to excite ruby fluorescence using argon or He‐Cd lasers above about 250 GPa or so (depending on the wavelength). The radiation cannot get through the diamond anvil to the ruby. There is a very strong stress‐induced luminescence in Type Ia diamond in the red at pressures above 2 Mbars and in infrared above 2.5 Mbar. This latter fluorescence, if assumed to be due to ruby R1 fluorescence (no ruby is present) suggests that the pressure is 5.6 Mbars.