Absorption Spectra of Cr^(3+) in Al_2O_3 Under Shock Compression

Abstract

Unpolarized absorption spectra of single crystals of Cr^(3+) doped Al_2O_3 (synthetic ruby) have measured using a new, time-resolving, dispersive, streak photographic system over the range ∼350 to ∼700 nm during a series of shock loading experiments. The crystal field absorptions assigned to the transition ^4A_(2g) → ^4T_(2g) were observed to shift in a series of experiments from 555±1 nm at atmospheric pressure to 503±5 nm at 46 GPa. In a single experiment at 32 GPa the ^4A_(2g) → 4 T_(1g) transition was observed to shift from 405±1 to 386±5 nm. The present data extrapolate downwards in compression toward the 10 GPa data of Stephens and Drickamer (1961) although both crystal field absorption energies increase considerably less with compression than predicted by the simple ionic point charge model. The single datum observed for the Racah parameter B, 588±38 cm^(−1) at 32 GPa, is consistant with previous results to 10 GPa and the trend of decreasing B, with compression expected from the divergence of the data from the point charge model due to increasing covalancy.