Information on circumstellar dust particles pertains to basic questions of origin and process in the galaxy. A first order problem is identification of "stardust" through comparison of astronomical infrared (IR) spectra to laboratory data on solids. Although the concept is straightforward, implementation is problematic. For example, dust features in the IR spectra of cold, carbon stars have been interpreted as hexagonal-rhombohedral SiC. This result disagrees with identification of presolar SiC grains in meteorites as exclusively the cubic polymorph. The discrepancy is fundamental because such stars are major dust producers. The comparison of data from thin film, or raw dispersion spectra, for Of equal importance to the database is the understanding the physical processes producing absorption vs. scattering features in IR spectra. Mie scattering theory will be modified to account for the presence of bulk absorption processes, and to test the model through comparison with reflection, thin film, and dispersion data from selected phases (e.g., SiC, SiO2, olivine and gamet). The size of grains in the dispersions will be limited to sizes appropriate for surface effects, <0.05 micron. The revised theory should allow calculation of spectra with variable amounts of scattering and absorption components from laboratory absorption data. The results of the project will be useful in the fields of astronomy, planetary science, and remote sensing. It will also be important to mineralogy, in that physical properties such as ordering or concentrations of hydrous species are currently extracted from dispersion spectra without accounting for the component of scattering inevitably present and without considering that particles with sizes more than three times larger than the skin depth can falsify peak profiles. This research effort is a new, interdisciplinary direction involving an international collaboration. Strong links with active research areas in the department will help to rebuild a graduate program. The Office of Multidisciplinary Activities in the Directorate of Mathematical and Physical Sciences and the Division of International Programs are providing the funds for this award. ***
