H2 is the most abundant molecule in the interstellar medium and forms on the
surface of interstellar dust grains. Laboratory studies have been conducted of HD
formation on a dust grain analogue, which is a highly-oriented pyrolytic graphite
surface held at 15 K, under ultra-high vacuum. The molecules desorb from the
surface in a distribution of ro-vibrational states, which are probed using Resonance
Enhanced Multi-Photon Ionization Spectroscopy. HD in a particular ro-vibrational
state is ionized using laser photons detected by a time-of-flight mass spectrometer.
The HD+ ion yields are then data processed to obtain the relative rotational
populations of HD formed within one vibrational level and an average rotational
temperature can be found. In this thesis, HD formed in vibrational states v = 3 – 7
have been studied. This carries on from previous studies of HD and H2 in the v = 1
and 2 states. Within each vibrational level, the most populated rotational state was
found to be J = 1 or 2. The most populated vibrational state was found to be v = 4.
The HD experimental results were extrapolated to give the relative ro-vibrational
population distribution of nascent H2, which provides a new model for the
formation pumping of H2. This new formation pumping model has been
implemented into a radiative transfer code, written by Casu and Cecchi-Pestellini,
which takes into account formation, radiative and collisional pumping mechanisms
to calculate the total population distribution of H2 in an interstellar cloud and to
generate H2 spectra. The sensitivity of the H2 spectra to the physical conditions of
interstellar dark clouds, such as cloud density and temperature, has been
investigated. H2 spectra generated using the new experimentally-derived formation
pumping model has also been compared to H2 spectra generated using other
established, theoretically-derived formation pumping models
