CONDENSATION AND VAPORIZATION STUDIES OF CH3OH AND NH3 ICES - MAJOR IMPLICATIONS FOR ASTROCHEMISTRY

In an extension of previously reported work on ices containing H2O, CO , CO2, SO2, H2S, and H-2, we present measurements of the physical and infrared spectral properties of ices containing CH3OH and NH3. The con densation and sublimation behavior of these ice systems is discussed a nd surface binding energies are presented for all of these molecules. The surface binding energies can be used to calculate the residence ti mes of the molecules on grain surfaces as a function of temperature. I t is demonstrated that many of the molecules used to generate radio ma ps of and probe conditions in dense clouds, for example CO and NH3, wi ll be significantly depleted from the gas phase by condensation onto d ust grains. Attempts to derive total column densities solely from radi o maps that do not take condensation effects into account may vastly u nderestimate the true column densities of any given species. Simple CO condensation onto and vaporization off of grains appears to be capabl e of explaining the observed depletion of gas phase CO in cold, dense molecular cores. This is not the case for NH3, however, where thermal considerations alone predict that all of the NH3 should be condensed o nto grains. The fact that some gas phase NH3 is observed indicates tha t additional desorption processes must, be involved. The surface bindi ng energies of CH3OH, in conjunction with this molecule's observed beh avior during warm up in H2O-rich ices, is shown to provide an explanat ion of the large excess of CH3OH seen in many warm, dense molecular co res. The near-infrared spectrum and associated integrated band strengt hs of CH3OH-containing ice are given, as are middle infrared absorptio n band strengths for both CH3OH and NH3.