Sticking of molecules on non-porous amorphous water ice

TL;DR

This study measures the sticking coefficients of various molecules on non-porous amorphous water ice, revealing limitations of traditional measurement methods and proposing a new formula to improve modeling of interstellar chemistry.

Contribution

It provides new measurements of molecular sticking coefficients on np-ASW and introduces a more accurate measurement method and a general formula for use in astrochemical models.

Findings

Standard methods underestimate trapping events.

A new formula relates sticking coefficient to temperature and binding energy.

Impacts gas-grain chemistry simulations in the interstellar medium.

Abstract

Accurate modeling of physical and chemical processes in the interstellar medium requires detailed knowledge of how atoms and molecule adsorb on dust grains. However, the sticking coefficient, a number between 0 and 1 that measures the first step in the interaction of a particle with a surface, is usually assumed in simulations of ISM environments to be either 0.5 or 1. Here we report on the determination of the sticking coefficient of H$_2$, D$_2$, N$_2$, O$_2$, CO, CH$_4$, and CO$_2$ on non-porous amorphous solid water (np-ASW). The sticking coefficient was measured over a wide range of surface temperatures using a highly collimated molecular beam. We showed that the standard way of measuring the sticking coefficient --- the King-Wells method --- leads to the underestimation of trapping events in which there is incomplete energy accommodation of the molecule on the surface. Surface scattering experiments with the use of a pulsed molecular beam are used instead to measure the sticking coefficient. Based on the values of the measured sticking coefficient we suggest a useful general formula of the sticking coefficient as a function of grain temperature and molecule-surface binding energy. We use this formula in a simulation of ISM gas-grain chemistry to find the effect of sticking on the abundance of key molecules both on grains and in the gas-phase.