MOLECULAR ICES AS TEMPERATURE INDICATORS FOR INTERSTELLAR DUST - THE 44-MU-M AND 62-MU-M LATTICE FEATURES OF H2O ICE

We present new laboratory spectra of the 44- and 62-mu m lattice-mode absorption features in amorphous and crystalline H2O ice. Spectra of ice films prepared by three quite different methods are presented. In the first series of measurements, Series I, spectra were obtained for H2O-ice films initially deposited at a temperature of 10 K, and then at intermediate temperatures as they were warmed up to 150 K. In the second series of measurements, Series II, spectra were obtained for ice films deposited directly at temperatures between 10 and 150 K. In the third series of measurements, Series III, spectra were obtained for an ice Nm initially deposited at 140 K, and then at intermediate temperature steps as the film was cooled down to 10 K. The major characteristic of the first two series is a distinct change in spectral shape between 110 and 120 K, due to a change in the ice from an amorphous structure at low temperatures to a crystalline structure above 110 K. Although these two series of measurements yield very similar spectra, the main difference between them is that above 110 K the ratio of the strengths of the 44- and 62-mu m features is significantly larger for H2O ice deposited directly at a given temperature than for H2O ice deposited at 10 K and then warmed up to that temperature. This may prove to be a distinguishing feature between ice mantles formed in outflowing material in circumstellar envelopes and ice mantles formed in molecular clouds. Finally, we discuss how the temperature-dependent characteristics of these absorption features may be used to constrain the temperatures of interstellar dust grains.