Metals and Alloys Group: Optical Properties of Metals at Extreme Conditions



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		Figure 1. Calculated (a) and measured (b) optical absorption spectra for solid and liquid Al at various temperatures. Below the melt temperature (= 938 K), a peak in the spectra near 1 eV is clearly visible. Above the melt, we predict no peak in this energy range at ambient pressure. This prediction agrees with early experimental work on hot solid and liquid Al (shaded diamonds: Miller, 1969; lines: Mathewson and Meyers, 1972), but disagrees with a more recent experimental study (open circles: Krishnan and Nordine, 1993). We are currently exploring the optical properties of Al at temperatures and densities appropriate for shock melting in an effort to determine the extent to which the optical constants are a sensitive indicator of phase in these conditions. This will complement experimental gas-gun work of J.R. Patterson, J.H. Nguyen, and N.C. Holmes currently underway on Al. See L.X. Benedict, J.E. Klepeis, and F.H. Streitz, Phys. Rev. B vol.71, 064103 (2005) for more details.
		Optical Properties of Metals at Extreme Conditions Lorin Benedict, John Klepeis, and Lin Yang Methods: FP-LMTO, Planewave Pseudopotential Collaborators: Shock Physics Group There is currently a critical need for diagnostics capable of obtaining information in real time from state-of-the-art experiments that probe materials at extreme pressures and temperatures. At a minimum, one wishes to distinguish the state of the material (liquid versus solid) during the course of the experiment. Ideally, we would also be able to differentiate between multiple solid phases and determine the temperature of the material. Real-time in situ measurements of optical properties have the potential to address all of these important issues by providing structure- and temperature-dependent spectroscopic fingerprints. This diagnostic capability would be very portable and thus could equally well be utilized in high-power laser experiments at NIF and other facilities, in dynamic compression experiments using gas guns, or in studies at the pulsed-power Z-machine at Sandia. The real-time nature of this diagnostic would enable the determination of structural quantities as they are changing in an experiment. In order to use optical properties as a real-time diagnostic we must develop the capability to calculate optical quantities with high reliability and correlate them with materials properties of interest. The primary challenge is in the computation of the absorption spectra of liquid metals and hot (partially disordered) solids. Prominent features in the absorption spectra of solids often involve transitions between delocalized band states. Such states involve many atoms and therefore the accurate computation of these spectral features is only possible by including large numbers of atoms in the computation. The loss of spatial periodicity in a liquid or hot solid makes these calculations particularly difficult. However, it is precisely this loss of periodicity that causes otherwise sharp spectral features to broaden and change shape, giving the optical spectra the structure- and temperature-dependence we seek. Comparison with experimental data is a critical component in developing a robust diagnostic and therefore we plan close collaborations with parallel experimental efforts. In particular, Jeff Nguyen has already developed the capability for measuring emissivity as a function of time at single wavelengths in gas gun experiments and is currently working to enable broad band measurements.
		RECENT PUBLICATIONS
		L. X. Benedict, J. E. Klepeis, and F. H. Streitz, Calculation of optical absorption in Al across the solid-to-liquid transition, Phys. Rev. B 71, 064103 (2005).

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		Maintained by Robert E. Rudd -- Last updated on 9 March 2007. UCRL-WEB-229431