Study of H[sub 2] formation on the surface of interstellar dust grains at high temperature using kinetic Monte Carlo method

Abstract

We studied the formation of H2 on the interstellar dust grain using continuous-time random-walk (CTRW) Monte Carlo technique. Both physisorption and chemisorption sites were considered for the adsorption of H atoms on the surface of olivine. In our model, an incoming H atom was first given a physisorption site to adsorb to the surface and then it was allowed to move to a chemisorption site depending on the probability of reaction. We considered hydrogen atom mobility due to both thermal hopping and quantum mechanical tunneling. We considered a wide range of temperature ranging from 5 K to 825 K representing different interstellar regions, we also considered various values of incoming H flux which corresponds to different hydrogen number density (0.1 cm−3 to 100 cm−3). We also used rough surface with multiple binding sites. It is found that tunneling dominates the surface chemistry at low temperature for all the cases, but as the temperature increases, the thermal hopping starts dominating. Surface with chemisorption sites can produce H2 at very high temperature also (as high as 700 K depending upon the depth of the chemisorption well) which is not possible when only physisorption sites are considered.