Laboratory Studies of Molecular Hydrogen Formation on Surfaces of Astrophysical Interest

Summary

We review laboratory studies of the formation of molecular hydrogen on surfaces and in conditions of astrophysical interest. Theoretical analysis and predictions are given on how experimental results can shed light on actual physico-chemical processes occuring in the interstellar medium. Preliminary measurements of H atom sticking are also shown.

Introduction

Molecular hydrogen is the most abundant molecule in the Universe. In space it plays two main roles that render it of incomparable importance:

1. – once formed it becomes a very efficient coolant that increases the rate of collapse of interstellar clouds contributing to shape galaxies and to regulate their dynamics;

2. – once ionized by UV photons or by cosmic rays H₂ enters and triggers all reactions schemes that form most of molecular species in the gas phase.

Molecular hydrogen has always been a source of puzzles for astrophysicists in spite of its simplicity. The mechanism of its formation in the extreme conditions encountered in the interstellar medium is certainly one of them. When a sufficient abundance of electrons and ions exists, ion-atom reactions may be quite effective in producing H₂ in the gas phase (Stancil & Dalgarno 1998; when, on the contrary, physical conditions allow almost only the presence of neutral hydrogen atoms, the radiative association of two of them is highly inefficient and cannot explain the observed abundances. The main reason is that the time-scale for the release of a consistent fraction of the formation energy (4.5 eV) through the emission of a photon via forbidden transitions is too long and the proto-molecule that is formed in a repulsive state almost invariably breaks up.