Optical absorption spectra of ruby and periclase at high shock pressures

Abstract

A spectrographic system is described that is capable of measuring optical absorption spectra in solids to shock pressures of several hundred kilobars. The system utilized light from a ‘point’ source at about 60,000°K. Spectra have a resolution of about 40 A and cover the visible range. With a streak camera, time resolution of about 75 nsec can be obtained. The spectrum of MgO is observed to remain featureless at 450 kb and upon unloading from this pressure. The color centers observed in shock-recovered material must result from either higher pressures or other processes. The optical absorption spectra of ruby under shock compressions of almost 15% have been measured in the range 375–600 nm. Below the elastic limit the large anisotropic strains are evident from the splitting of the ^4A_2 → ^4T_2 (F) absorption band by 3730 cm^(−1). Above the elastic limit this splitting is not resolved (but must be less than 800 cm^(−1)), indicating considerable loss of shear strength in such cases. Above the Hugoniot elastic limit up to pressures of 530 kb (15% volume compression) the measured value of the crystal field parameter agrees, within experimental error, with the value calculated from a point charge model (Dq α r^(−5)) if the local compressibility is equal to the bulk compressibility. This result agrees with Stephens and Prickamer's absorption data up to 150 kb and suggests that the point charge model is useful in predicting crystal field effects in mantle minerals, especially those having similar oxygen anion packings such as corundum.