The Formation of H₂ and Other Simple Molecules on Interstellar Grains

Summary

Molecular hydrogen is formed on interstellar grains by two main processes. In the first, or Langmuir-Hinshelwood, mechanism, hydrogen atoms land on a grain and diffuse over the surface by either tunneling or hopping until they find each other. In the second, or Eley-Rideal, mechanism, hydrogen atoms landing on grains are fixed in position. Reaction occurs only when a gaseous hydrogen atom lands atop an adsorbed one. Based on new experimental results concerning the rate of diffusion of H atoms on interstellar-like surfaces, it is clear that the rate is significantly slower than estimated in the past. The range of temperatures over which diffusive formation of H₂ occurs is correspondingly reduced although sites of strong binding can raise the upper temperature limit. The surface formation of molecules heavier than hydrogen is still not well understood.

Introduction

It is almost certain that H₂ and a variety of other molecules are formed on the surfaces of low-temperature interstellar dust particles. On these surfaces, binding sites for adsorbates exist interspersed among regions of higher potential. On a grain of typical radius 0.1 µ there are roughly 10⁶ such binding sites, onto which neutral gas-phase molecules stick with high efficiency. The binding energy, or energy required for desorption (E_D), depends on the surface and on the adsorbate. For example, the binding energy of H atoms on olivine (a silicate-type material) has just been measured to be 372 K by Katz et al. (1999), who also measured the binding energy of H on amorphous carbon to be 658 K.