THE SIH VIBRATIONAL STRETCH AS AN INDICATOR OF THE CHEMICAL-STATE OF INTERSTELLAR GRAINS

We have measured the infrared spectra of amorphous silicate grains and films formed in a variety of laboratory experiments. We have found that the frequency of the SiH fundamental stretch is extremely sensitive to the chemical environment of the grain in which the silicon is bound and varies from 2270 cm-1 in oxidized grains to 2110 cm-1 in a reducing environment. We propose that features previously observed in W33A could be due to -SiH groups in amorphous silica (2190 cm-1), partially reduced silica (2165 cm-1), silicon nitride (2140 cm-1) or slightly oxidized silicon carbide (approximately 2120 cm-1). We predict that observations at 2270 cm-1 will reveal a feature due to -SiH groups in highly oxidized interstellar and circumstellar grains. Such a feature might also appear in the spectra of comets.