Islam, F.; (2010) The formation of molecular hydrogen in the interstellar medium. Doctoral thesis , UCL (University College London).

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Abstract

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.

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