Variation of the sticking of methanol on low-temperature surfaces as a possible obstacle to freeze out in dark clouds

Abstract

ABSTRACT Sticking of gas-phase methanol on different cold surfaces – gold, 13CO, and amorphous solid water (ASW) ice – was studied as a function of surface temperature (7–40 K). In an ultrahigh-vacuum system, reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption methods were simultaneously used to measure methanol sticking efficiency. Methanol band strengths obtained by RAIRS vary greatly depending on the type of the surface. Nevertheless, both methods indicate that the sticking of methanol on different surfaces varies with surface temperature. The sticking efficiency decreases by 30${{\ \rm per\ cent}}$ as the surface temperature goes from 7 to 16 K, then gradually increases until the temperature is 40 K, reaching approximately the initial value found at 7 K. The sticking of methanol differs slightly from one surface to another. At low temperature, it has the lowest values on gold, intermediate values on water ice, and the highest values are found on CO ice, although these differences are smaller than those observed with temperature variation. There exists probably a turning point during the structural organization of methanol ice at 16 K, which makes the capture of methanol from the gas phase less efficient. We wonder if this observation could explain the surprising high abundance of gaseous methanol observed in dense interstellar cores, where it should accrete on grains. In this regard, a 30${{\ \rm per\ cent}}$ reduction of the sticking is not sufficient in itself but transposed to astrophysical conditions dominated by cold gas (∼15 K), which could reduce the sticking efficiency by two orders of magnitude.