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An Experimental Study of Ionising Collisions Between Metastable Helium and Various Atoms and Molecules.

Sanderson, Joseph Henry; (1991) An Experimental Study of Ionising Collisions Between Metastable Helium and Various Atoms and Molecules. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

This thesis presents a study of chemi-ionisation reactions involving the two metastable states of helium He(23S) and He(21S) with Ar, CO2, N2O, CH4, C2H2, and NH3, also of He(23S) with SF6. The experimental techniques developed to measure both the relative total ionisation cross sections and the branching ratios for the different reactions is described, and a theoretical analysis is carried out in the case of collisions between helium atoms in both metastable states and argon. The relative variation of total ionisation cross section with velocity is carried out using the time-of-flight technique in a computer controlled experiment. The metastable atom source produces a wide velocity distribution (1 - 20 kms-1 or 0.02 - 8 eV). A newly-designed quench lamp enables the removal of fast He(2*S) metastable atoms from the beam. In every case except that of Ar + He(21S) in the range 1 - 5 kms-1 measurements with helium atoms in the He(21S) state are the first to be made. The measurements made for He(21S) with N2O and SF6 are also completely new. To make measurements using He(21S) the quench lamp is pulsed giving time-of-flight distributions with and without He(21S) information. Measurements with only the He(23S) state are subtracted from measurements with both states in order to determine the contribution from the He(21S) state. The computer control is essential to store this information simultaneously, but the process is by necessity slow and involves an appreciable amount of time in which data is not being recorded. To measure branching ratios for the different chemi-ionisation reactions as a function of collision velocity, a time-of-flight mass spectrometer is used to separate the different product ions. The pulsed quench lamp is used again in conjunction with computer control to record metastable atom time-of-flight distributions and ionisation time-of-flight spectra so that the variation of the ion branches with velocity can be seen. As with the relative ionisation cross sections, the He(21S) data is new except in the case of argon. The He(23S) data is also new in the case of N2O and SF6. Various interaction potentials for Ar+He(23S) and Ar+He(21S) have been used to calculate theoretical ionisation cross sections for comparison with the present experimental data. The potentials have been modified to find the best fit to the present experimental data, and the general applicability of the new potentials has been assessed.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: An Experimental Study of Ionising Collisions Between Metastable Helium and Various Atoms and Molecules.
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest
Keywords: Pure sciences; Chemi-ionisation reactions
URI: https://discovery.ucl.ac.uk/id/eprint/10106553
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