Effect of very high pressure on the optical absorption spectra in CsI

The effect of very high pressure on the fundamental optical absorption edges in CsI crystal was investigated between u.v. and near i.r. region by a diamond anvil type high pressure cell. With increasing pressure the fundamental absorption edge energy in CsI shifts from u.v. to i.r. region. At the pressure of 45 GPa the CsI becomes completely opaque. When the indirect band gap is assumed for CsI at high pressures, the insulator to metal transition in CsI is expected to occur at about 70 GPa from the pressure versus absorption edge curves. The availability of CsI as one of the pressure standard up to 50 GPa region is also discussed.