Development of ultrahigh pressure apparatus based on high pressure produced by constraining thermal expansion

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An ultrahigh pressure apparatus based on high pressure produced by constraining thermal expansion is developed. In the apparatus, a sample compressed between two diamond anvils is irradiated with a high-power YAG laser pulse through diamond window. Ultrahigh pressure can be produced by constraining thermal expansion due to ultrahigh temperature generated in the central region of the sample. However, the existence of ultrahigh temperature region in the sample gives rise to difficulties in the pressure measurements. In the apparatus, solidified gas having very small thermal conductivity is used as the sample in order to produce very large gradients in temperature across the sample. Then it is demonstrated by calculations using the finite difference method that the lower temperature region can appear in the corner of the sample chamber. The high pressure can be measured by observing laser-induced fluorescence of a ruby crystal placed in the lower temperature region. The apparatus is equipped with a temperature-and-pressure measuring system that is designed to satisfy the following requirements : (1) radiation spectra from three arbitrary positions in the sample can be simultaneously observed ; (2) excitation laser beam can be focused at point of preselected ruby crystal and the laser-induced fluorescence spectrum of the ruby crystal can be observed ; (3) simultaneous time-resolved measurements for these spectra can be achieved by using cooled CCD in kinetics mode. A cryogenic technique for loading gases in the diamond anvil cell is also devised, which is applied to the H_2 loading. Furthermore, it is confirmed experimentally that the optical system and the diamond anvil cell are not damaged by the high-power laser pulses. Thus it is demonstrated that the ultrahigh-pressure experiments using solidified H_2 as the sample can be executed by the present apparatus.