BINDING ENERGY OF MOLECULES ON WATER ICE: LABORATORY MEASUREMENTS AND MODELING

Abstract

We measured the binding energy of N 2 , CO, O 2 , CH 4 , and CO 2 on non-porous (compact) amorphous solid water (np-ASW), of N 2 and CO on porous ASW, and of NH 3 on crystalline water ice. We were able to measure binding energies down to a fraction of 1% of a layer, thus making these measurements more appropriate for astrochemistry than the existing values. We found that CO 2 forms clusters on the np-ASW surface even at very low coverages. The binding energies of N 2 , CO, O 2 , and CH 4 decrease with coverage in the submonolayer regime. Their values at the low coverage limit are much higher than what is commonly used in gas-grain models. An empirical formula was used to describe the coverage dependence of the binding energies. We used the newly determined binding energy distributions in a simulation of gas-grain chemistry for cold cloud and hot-core models. We found that owing to the higher value of binding energy in the submonolayer regime, a fraction of all these ices remains for much longer and up to higher temperatures on the grain surface compared to the single value energies currently used in the astrochemical models.