Gas-grain and gas-phase reactions dominate the formation of molecules in the interstellar medium ( ISM ) .
Gas-grain reactions require a substrate ( e.g .
a dust or ice grain ) on which the reaction is able to occur .
The formation of molecular hydrogen ( H _ { 2 } ) in the ISM is the prototypical example of a gas-grain reaction .
In these reactions , an atom of hydrogen will strike a surface , stick to it , and diffuse across it .
When it encounters another adsorbed hydrogen atom , the two can react to form molecular hydrogen and then be ejected from the surface by the energy released in the reaction .
We perform in-depth classical molecular dynamics ( MD ) simulations of hydrogen atoms interacting with an amorphous water-ice surface .
This study focuses on the first step in the formation process ; the sticking of the hydrogen atom to the substrate .
We find that careful attention must be paid in dealing with the ambiguities in defining a sticking event .
The technical definition of a sticking event will affect the computed sticking probabilities and coefficients .
Here , using our new definition of a sticking event , we report sticking probabilities and sticking coefficients for nine different incident kinetic energies of hydrogen atoms [ 5 K - 400 K ] across seven different temperatures of dust grains [ 10 K - 70 K ] .
We find that probabilities and coefficients vary both as a function of grain temperature and incident kinetic energy over the range of 0.99 - 0.22 .