The composition and distribution of dust along the line of sight toward the Galactic center

We discuss the composition of dust and ice along the line of sight to the G alactic center (GC) based on analysis of mid-infrared spectra (2.4-13 mu m) from the Short Wavelength Spectrometer on the Infrared Space Observatory ( ISO). We have analyzed dust absorption features arising in the molecular cl oud material and the diffuse interstellar medium along the lines of sight t oward Sgr A* and the Quintuplet sources, GCS 3 and GCS 4. It is evident fro m the depth of the 3.0 mu m H2O and the 4.27 mu m CO2 ice features that the re is more molecular cloud material along the line of sight toward Sgr AM t han toward GCS 3 and GCS 4. In fact, Sgr A* has a rich infrared ice spectru m with evidence for the presence of solid CH4, NH3, and possibly HCOOH. Hyd rocarbon dust in the diffuse interstellar medium along the line of sight to the GC is characterized by absorption features centered at 3.4, 6.85, and 7.25 mu m. Ground-based studies have identified the 3.4 mu m feature with a liphatic hydrocarbons, and ISO has given us the first meaningful observatio ns of the corresponding modes at longer wavelengths. The integrated strengt hs of these three features suggest that hydrogenated amorphous carbon is th eir carrier. We attribute an absorption feature centered at 3.28 mu m in th e GCS 3 spectrum to the C-H stretch in aromatic hydrocarbons. This feature is not detected, and its C-C stretch counterpart appears to be weaker, in t he Sgr A* spectrum. A key question now is whether or not aromatics are a wi despread component of the diffuse interstellar medium, analogous to aliphat ic hydrocarbons.