Absorption spectroscopy in solids under shock compression

Abstract

Experimental techniques that allow measurement of the optical absorption spectra of solids during the short time interval that they can be compressed dynamically offer the opportunity to supplement the more familiar equation of state data, which are usually reported in the form of a Hugoniot curve [1], with knowledge of the transition element (cation) coordination environment, charge transfer energy spectrum, and population of electronic states at high (dynamic) pressures. In principle, shock pressures in excess of 100 GPa (1 Mbar) can be obtained; moreover, the absolute pressures are known for any given experiment to ~ 1%. The spectral range, however, is limited to about 350 to 700 nm by the response of photographic film. Thus spectroscopy under dynamic compression offers both advantages and disadvantages with respect to the gathering of spectral data in static high-pressure equipment [2,3]. In addition to the application of spectroscopic data in obtaining a fundamental understanding of the solid-state physics and chemistry of solids, high-pressure optical data have two important applications to the physics of the earth's interior, particularly with regard to the environment of the lower mantle of the earth. In the pressure regime above ~30 GPa, a major objective of both contemporary static [4,5] and dynamic [6,7] experimentation is the discovery and characterization of the crystal structures appropriate for the silicates and oxides of the earth's lower mantle. Because transition element absorption spectra are highly sensitive to the coordination number and geometry of the cation sites in oxides and silicates, the present techniques appear promising with regard to distinguishing among the different possible high-pressure phases [8]. The second geophysical application of spectral data for candidate mantle minerals and their crystal-chemical analogs is the determination of the dependence of photon opacity and index of refraction on wavelength, pressure, and temperature. The opacity and, to a lesser extent, the index of refraction (a subject not addressed by the present paper) control the radiative transport of heat in the temperature regime of the earth's lower mantle (> 3000 K) [9-12]. The above considerations have motivated both earlier exploratory studies of the optical properties of oxides under shock [13] and the development of the present refined apparatus for measuring mineral spectra under shock conditions. In this report, our apparatus is described in detail together with its application to the measurement of optical absorption spectra of Cr^(3+)-doped Al_2O_3 (ruby) under shock compression.

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