Title

Adsorption on and desorption from surfaces of astrophysical interest studied by atomic beam scattering

Date of Award

1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Science Teaching

Advisor(s)

Gianfranco Vidali

Keywords

Adsorption, Desorption, Surfaces, Atomic beam scattering, Astrophysics

Subject Categories

Cellular and Molecular Physiology

Abstract

Molecular hydrogen formation in the interstellar medium is one of the most relevant, perhaps the most relevant, problem of astrophysics. Being the most abundant molecule, molecular hydrogen enters or triggers virtually all molecule-formation schemes in space. Many of these reactions occur in the gas phase with participation of ions. Catalytic reactions involving hydrogen and heavier species take place on surfaces of dust particles in the extreme conditions of several astrophysical environments. Up to now, most investigations of surface catalysis have been carried out only on materials and in temperature ranges that are not of great relevance to space applications.

This thesis describes how surface science tools can be employed to study physical processes leading to formation of molecules at surfaces under astrophysically relevant conditions. An experiment has been carried out in our laboratory to investigate key reactions between gas-phase particles and atoms/molecules adsorbed on surfaces of dust grain analogues (graphite, silicate, etc.) by using molecular beam methods and surface science techniques. An apparatus to study key reactions on low temperature surfaces is described in detail, as well as measurement procedures and some results of astrophysical relevance.

We report on experimental investigations of HD production on low temperature surfaces of silicates under conditions relevant to hydrogen recombination on dust grain surfaces in the interstellar medium. Following adsorption of H and D at thermal energies on a silicate surface (olivine) in the 5-15 K range, flash desorption experiments were conducted to yield desorption energies and order of desorption kinetics. We find that significant recombination occurs only at the lowest temperatures.

On the basis of our experimental evidence, our measurements give lower values for the recombination efficiency (sticking probability S times the probability of recombination upon H-H encounter, $\gamma$) than model-based estimates. We propose that our results can be reconciled with average estimates of the recombination rate $({1\over2}n\sb{H}n\sb{g}v\sb{H}AS\sb\gamma$) from astronomical observations, if the actual surface of average grain is rougher, and its area bigger, than the one considered in models.

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