Adsorption energies of carbon, nitrogen, and oxygen atoms on the low-temperature amorphous water ice: A systematic estimation from quantum chemistry calculations

Abstract

We propose a new simple computational model to estimate adsorption energies of atoms and molecules to low-temperature amorphous water ice, and we present the adsorption energies of carbon (3P), nitrogen (4S), and oxygen (3P) atoms based on quantum chemistry calculations. The adsorption energies were estimated to be 14100 +- 420 K for carbon, 400 +- 30 K for nitrogen, and 1440 +-160 K for oxygen. The adsorption energy of oxygen is well consistent with experimentally reported value. We found that the binding of a nitrogen atom is purely physisorption, while that of a carbon atom is chemisorption in which a chemical bond to an O atom of a water molecule is formed. That of an oxygen atom has a dual character both physisorption and chemisorption. The chemisorption of atomic carbon also implies a possibility of further chemical reactions to produce molecules bearing a C-O bond, while it may hinder the formation of methane on water ice via sequential hydrogenation of carbon atoms. These would be of a large impact to the chemical evolution of carbon species in interstellar environments. We also investigated effects of the newly calculated adsorption energies onto chemical compositions of cold dense molecular clouds with the aid of gas-ice astrochemical simulations. We found that abundances of major nitrogen-bearing molecules, such as N2 and NH3, are significantly altered by applying the calculated adsorption energy, because nitrogen atoms can thermally diffuse on surfaces even at 10 K.