EXAFS AND XANES - NEW ASTROPHYSICAL TOOLS TO STUDY THE SOLID-STATE STRUCTURE OF INTERSTELLAR GRAINS

I show that new information about the compositions and crystal structu res of interstellar grains can be derived from measurements of extende d X-ray absorption fine structure (EXAFS) above the absorption edges i n the spectra of highly extincted and bright X-ray stars. It will be a lso possible to probe the long-range crystal structures of solid grain s by analyzing extended X-ray absorption-edge profiles modified by X-r ay absorption near edge structure (XANES). I show that recently develo ped high-resolution (E/Delta E approximate to 1000) X-ray spectrometer s (e.g., the High-Energy Transmission Grating Spectrometer on the Adva nced X-Ray Astrophysics Facility [AXAF] or the Reflection Grating Spec trometer on the X-Ray Multi-Mirror satellite [XMM]) will be able to me asure these features.