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EPSRC Reference: EP/F015291/1
Title: Photoionisation of rotationally-cooled H2 and D2 beyond the Born-Oppenheimer approximation: angular distribution studies.
Principal Investigator: King, Professor GC
Other Investigators:
Researcher Co-Investigators:
Project Partners:
Department: Physics and Astronomy
Organisation: University of Manchester, The
Scheme: Overseas Travel Grants (OTGS)
Starts: 07 May 2007 Ends: 06 July 2007 Value (£): 13,194
EPSRC Research Topic Classifications:
Scattering & Spectroscopy
EPSRC Industrial Sector Classifications:
No relevance to Underpinning Sectors
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Summary on Grant Application Form
Molecular hydrogen is the simplest and most abundant neutral molecule in the Universe. Due to its relative simplicity it is the natural starting point for both experimental and theoretical studies. It is also gaining technological importance through its use in fuel cells for the environmentally-friendly production of energy. A fundamental process involving molecular hydrogen is photoionisation by ultra violet light where a photon ejects an electron from the molecule. However, despite the simplicity of the molecule it is observed that the photoionisation spectrum is not simple and smooth, as would be expected if the photon ionised the molecule in a direct manner. Instead it is dominated by a wealth of sharp structure which arises because it is much more likely for photoionisation to occur in a two-step process. Here a high-lying, neutral state of H2 is first excited by absorption of a photon. This state then decays to the molecular ion H2+ with the emission of a photoelectron in a process called autoionisation. Apart from being the dominant process, autoionisation is important because it involves interactions between electronic motion and the molecular motions of rotation and vibration that do not occur in direct photoionisation. The excitation and decay paths involved in this two-step process can be investigated by observing the energies and yields of the ejected photoelectrons. Furthermore, by measuring the angular behaviour of the emitted photoelectron it is possible to determine the angular momentum exchanged between the photoelectron and the molecular ion. In practice this picture becomes more complicated because more than one rotational level of the hydrogen molecule is populated at room temperature. The photoionisation spectra are then considerably more complex, and more importantly, it makes it much more difficult to connect experiment with theory. Previous experiments have generally been a sum over several rotational levels of the ground vibrational state of H2. This lack of rotationally-selected experimental data remains a road block to the theoretical development of this fundamental system. We have developed experimental techniques to remove this road block where we put more than 99% of the target H2 molecules into a single rotational level. We have built an electron spectrometer that allows us to identify individual rotational levels of the final ionic state and measure the angular behaviour of the photoelectrons. We also have access to an ultra violet light source with the necessary spectral resolution to isolate individual rotational levels of the intermediate neutral states. Brought together this enables a comprehensive study of photoionisation in a molecule for well defined rotational states in: the initial target state, the intermediate state and the final ion state. A final capability is that we can substitute H2 by the isotope D2. This slightly shifts the molecular energy levels and can reveal transitions that are otherwise hidden while the change in inter-nuclear separation modifies the dynamics of the photoionisation process. The proposed experiments would thus provide a complete and clear picture of photoionisation in this fundamental molecular system.
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Organisation Website: http://www.man.ac.uk