Barnett, Sarah Mary; (1992) Translational spectroscopy of the metastable products of electron impact dissociative excitation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In molecular dissociation processes the fragments may be produced with kinetic energies exceeding the thermal energy of the parent gas molecules. The fragments will therefore be highly chemically reactive. The fragments may also be produced in electronically excited states, including those forbidden in electric dipole transitions. These states, metastable states, must decay through magnetic dipole or electric quadrupole transitions and are long-lived. The metastable fragments are therefore both fast-moving and long-lived and may travel large distances before decaying. Metastable dissociation fragments are thus important in the energy transfer processes of complex systems such as planetary atmospheres, lasers, gas discharges and plasmas. The Martian emission spectrum is dominated by the electric dipole forbidden transition CO(a3π-XlΣ+), the Cameron bands, in which the upper state is populated predominantly by electron impact dissociative excitation of carbon dioxide. In this work, metastable fragments produced by electron impact dissociation of nitrous oxide, carbon monoxide, carbon dioxide and carbonyl sulphide have been studied by the technique of translational spectroscopy. A low work function metastable surface detector was used to study the low lying metastable states of molecular nitrogen, N2(A3 Σ+u), and carbon monoxide, CO(a3 π). A channel electron multiplier was used for the selective detection of metastable O(5S) atoms. The kinetic energy distributions of the product fragments were derived from the time of flight spectra obtained and from these the nature of the dissociation process could be investigated.

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