Title

Laboratory synthesis of molecular hydrogen on surfaces of interstellar dust grain analogues

Date of Award

1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Advisor(s)

Gianfranco Vidali

Keywords

hydrogen, desorption, recombination

Subject Categories

Condensed Matter Physics | Physics

Abstract

Molecular hydrogen is by far the most abundant molecule in space. H$\sb2$ formation in the interstellar medium (ISM) is a fundamental process in astrophysics. The radiative association of two hydrogen atoms is a process too rare to be efficient because it involves forbidden roto-vibrational transitions, and gas-phase three-body reactions are rare in the diffuse ISM to explain H$\sb2$ abundance. It has been recognized that H$\sb2$ recombination occurs on surfaces of dust grains, where the grains act as the third body in the H + H reaction.

This thesis reports on laboratory measurements of molecular hydrogen formation and recombination on surfaces of astrophysical interest. It also describes how atomic/molecular beam and surface science techniques can be used to study physical processes leading to the formation of hydrogen molecules at surfaces under conditions relevant to those encountered in the interstellar medium.

Flash desorption experiments have been conducted to yield desorption energies, order of desorption kinetics and recombination efficiency (defined as the sticking probability S times the probability of recombination upon H-H encounter, $\gamma$) over a wide range of coverage. Significant recombination occurs only at the lowest temperatures ($<$20 K).

The recombination rates are obtained as functions of surface temperature and exposure time to H and D atom beams. Our measurements give lower values for the recombination efficiency than model-based estimates. We propose that our results can be reconciled with average estimates of the recombination rate from astronomical observations, if the actual surface of an average grain is rougher, and its area bigger, than the one considered in models. On the basis of our experimental evidence, we recognize that there are two main regimes of H coverage that are of astrophysical importance; for each of them we provide an expression giving the production rate of molecular hydrogen in interstellar clouds.

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