Perry, James Samuel Anthony; (2001) State-selective reactions of cosmic dust analogues at cryogenic temperatures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Molecular hydrogen (H2) is the most abundant molecule in interstellar space. It is crucial for initiating all of the chemistry in the Interstellar Medium (ISM) and consequently plays an important role in star formation. However, the amount of H2 believed to exist in the ISM cannot be accounted for by formation via gas-phase reactions alone. The current, widely accepted, theory is that H2 forms on the surface of cosmic dust grains. These grains are thought to be composed of amorphous forms of carbon or silicates with temperatures of around 10 K. This thesis describes the development of a new experiment that has been constructed to study H2 formation on the surface of cosmic dust analogues and presents the initial experimental results. The experiment simulates, through ultra-high vacuum and the use of cryogenics, the conditions of the ISM where cosmic dust grains and H2 molecules exist. During the experiment, a beam of atomic hydrogen is aimed at a cosmic dust analogue target. H2 formed on the target's surface is ionised using a laser spectroscopy technique known as Resonance Enhanced Multiphoton Ionisation (REMPI) and detected using time-of-flight mass spectrometry. The sensitivity of REMPI is such that H2 molecules can be ionised from selective internal energy states. This allows the rovibrational populations of the H2 molecules desorbing from the cosmic dust targets to be determined, providing information on the energy budget of the H2 formation process in the ISM. Preliminary results from the experiment show that H2 molecules formed on a cold diamond-like-carbon surface have a significant non-thermal population of excited vibrational and rotational energy states.

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