Optical absorption spectrum of trigonal selenium under high pressure

Optical absorption spectra of trigonal selenium have been measured at pressure up to 5 GPa using a diamond anvil cell with two kinds of pressure-transmitting media, that is, a mixture of methanol, ethanol and water, and a potassium bromide, to confirm no effects of them. With increasing pressure, the absorption edge shifts to a lower energy and the pressure coefficients of the absorption edge at atmospheric pressure obtained by using both pressure-transmitting media agree with each other. At high pressure, as more clearly observed in the spectra obtained by using a potassium bromide, the edge shifts to a lower energy become moderate near 3 GPa, where an abrupt decrease in intramolecular covalent bond length was observed. This result must be related to the transition of the electronic band structure in trigonal selenium at approximately 3 GPa.