THE CLOSING DIAMOND ANVIL OPTICAL WINDOW IN MULTIMEGABAR RESEARCH

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 equal-sign-with-dot-above-and-below 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.