Laser Ionisation Spectroscopy of Alkalis: Applications to Resonance Ionisation Mass Spectrometry

Smyth, Michael Henry Charles (1988) Laser Ionisation Spectroscopy of Alkalis: Applications to Resonance Ionisation Mass Spectrometry. PhD thesis, University of Glasgow.

Abstract

Resonance ionisation spectroscopy (RIS) at Glasgow University began as a result of the need to calibrate large gas-filled multiwire proportional counters (MWPCs) currently being built at CERN, specifically the ALEPH time projection chamber. From this work the direction shifted towards the development of laser ionisation as an analytical tool, with the design of two resonance ionisation time of flight mass spectrometers. The two instruments have slightly different remits. One is particularly suited to surface analysis, the other to trace element detection. The work outlined in this thesis was intended to help in the design of these time of flight mass spectrometers, by highlighting difficulties likely to be encountered in the resonant ionisation and detection of small numbers of atoms. In order to prove the potential of resonance ionisation, and also to gain experience in the experimental techniques applicable to resonance ionisation mass spectrometry, initial experiments were carried out on elemental caesium and rubidium in a simple proportional counter. Chapter 1 outlines the basic theory behind the resonance ionisation technique, and shows its wide applicability to elemental ionisation and detection. A brief historical outline of previous experimental and theoretical work on resonance ionisation traces the development of RIS as an analytical tool, leading to the design and construction of a resonance ionisation time of flight mass spectrometer at Glasgow. Chapter 2 is a brief description of some of the theoretical aspects of resonance ionisation. A simple population rate equation model is used to derive expressions for the ion yields for a two level atom as a function of atomic and laser parameters. A semi-classical model of the atom-radiation interaction is given, leading to the model of Rabi oscillations between electronic states in an intense laser field. Transitions involving more than one photon are qualitatively described. The laser systems used for resonance ionisation are described in chapter 3, along with the ion detectors used. Descriptions of the proportional counter, and quadrupole and time of flight mass spectrometers are given. Chapter 3 concludes with a discussion of the reasoning behind the decision to use caesium and rubidium for the initial experiments with these detectors. Chapter 4 begins with a brief survey of previous work on the resonance ionisation of alkali metals. The electronic structure of atomic and molecular caesium and rubidium is summarised, and energy level diagrams for these systems are presented. Experimental work conducted at Glasgow to investigate the background ionisation in proportional counters is reported in chapter 5. These results were deemed important in that they suggested that, at wavelengths below 300 nm, the ionisation of organic impurities in proportional counters, or any ionisation spectrometer, could swamp the resonant ionisation signal of interest, particularly at trace concentration levels. The ionisation of these impurities might therefore be a limiting factor to the sensitivity of resonance ionisation at these UV wavelengths. Two impurities were identified in the proportional counter, phenol and toluene. The origin of phenol was traced to plastic piping used to introduce the buffer gas to the proportional counter. The origin of toluene was not determined. Chapter 6 reports on the resonance ionisation spectroscopy of caesium and rubidium. Early work concentrated on using a specially designed proportional counter, which was both robust and free from contaminants. One and two photon transitions were investigated. The collisional enhancement of the ionisation of photoexcited Rydberg levels was investigated using a simple model of the process. The proportional counter was also incorporated into a quadrupole mass spectrometer for an early attempt at the resonant ionisation mass spectroscopy of atomic and molecular rubidium. With the completion of the construction of the time of flight mass spectrometer, the experimental work switched to this instrument. Preliminary results are presented in chapter 7. These have mainly been obtained to date, (due to technical difficulties with the ion gun), with a fairly simple technique of pulsed laser ablation/ionisation of a sample, ions being formed in the ablation process itself and by the nonresonant ionisation of ablated neutrals. Not surprisingly the selectivity of this process is limited although resonant transitions can be distinguished. A brief calculation of the projected sensitivity of the instrument, when operating in its normal mode of pulsed ion bombardment with resonant ionisation, is also presented, and ways in which the sensitivity may be increased are explored. The conclusion draws together the results from the work with the proportional counter/quadrupole mass spectrometer, and suggests future experiments, both spectroscopic and analytical, which could be carried out in this instrument, with the addition of a low temperature oven to atomise samples. Experiments could be done to investigate the collisional ionisation of highly excited states, search for autoionisation states in multielectron atoms, investigate the potential of field ionisation as a substitute for photoionisation and also determine the validity of population rate equations to describe resonance ionisation. Experiments to determine the sensitivity of the time of flight mass spectrometer will shortly be conducted. This instrument promises to revolutionise the detection of trace elements, particularly in surface analysis.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Applied physics, Optics, Nuclear physics and radiation
• Date of Award: 1988
• Depositing User: Enlighten Team
• Unique ID: glathesis:1988-77708
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Jan 2020 11:53
• Last Modified: 14 Jan 2020 11:53
• URI: https://theses.gla.ac.uk/id/eprint/77708