A study of the 2.5–25 μm spectrum of H 2 O ice

We present new absorbance spectra of the 3‐, 6‐ and 12‐μm features of amorphous and crystalline H 2 O ice obtained between 10 and 140 K. Three sets of measurements have been made. In series I, the ice film was initially deposited on to a CsI substrate at 10 K and successive spectra were then obtained at intermediate temperatures as the ice was warmed up to 140 K. The second set, series II, comprises spectra for ice films deposited and measured at temperatures between 10 and 140 K. In the third set of measurements, series III, spectra were obtained for an ice film deposited at 140 K and then at intermediate temperatures as the film was cooled down to 10 K. The series I and II results show that the ice undergoes an amorphous‐to‐crystalline phase transition in the 110–120 K range. The 3‐ and 12‐μm bands have similar trends in full width at half‐maximum (FWHM) and opposite peak wavelength shifts. The temperature behaviour of the 6‐μm band is different, as no clear phase transition temperature can be discerned from its FWHM and peak wavelength position data. In the series III spectra the peak wavelength positions and FWHM of the three bands remain relatively constant, thus demonstrating the stability of the crystalline phase against thermal cycling. A comparison between the laboratory results and astronomical spectra suggests that the identification of the librational band of H 2 O ice in OH 231.8 + 4.2 may be incorrect.