Hydrogen Recombination on Astrophysically Relevant Surfaces

Publication Numbers: 25,29,36.

The formation of molecular hydrogen in the insterstellar
medium is a process of fundamental importance.
It was recognized long ago that H$_2$ cannot form in the gas phase
efficiently enough to account for its abundance.
It was thus proposed that dust grains act as catalysts and a theoretical
framework was developed for the calculation of the hydrogen
recombination rate.

For a number of years, my colleagues
G. Vidali and V. Pirronello
have been working on experimental studies of
hydrogen recombination on astrophysically relevant surfaces
in order to examine this proposal.
The aim of these experiments is to examine the
efficiency of such surfaces as catalysts for hydrogen
recombination and to find out
whether such processes may account for the abundance of
H$_2$ in interstellar clouds.

The desorption spectra of
HD (which was measured instead of H$_2$ to reduce noise)
indicated that the kinetics was of the second order at low
coverages of H and D.
This lead to an expression for the formation
rate of H$_2$ in clouds which is
quadratic in the density of H atoms,
from the commonly used expression of
Hollenbach and Salpeter which is linear in the density.

I became involved in this project after realizing that the same
rate-equation analysis used in our studies of thin film
growth is required in order to
interpret the experiments, and to
bridge the huge gap between the laboratory and the astrophysical
time scales.
Using a rate equation model, we were
able to find an analytical solution of the problem under steady state
conditions, and showed that both
the linear and quadratic expressions
are obtained as two limits
of the same model.
Later, we developed a more complete rate equation model
for diffusion and recombination of hydrogen on surfaces.
Using our model for quantitative analysis of the experimental
results, we obtained excellent fits to the experimental
data which led to firm predictions for the values of the relevant
activation energies. Using computer simulations we managed
to connect between the experimental time scales (minutes)
and the astrophysical time scales (millions of years), and to
predict the recombination efficiency of the experimentally used