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Title: Laser ablation ionisation mass analysis of metal and superconductor samples.
Author: Zheng, Rong-Er
Awarding Body: Glasgow Caledonian University
Current Institution: Glasgow Caledonian University
Date of Award: 1995
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In the last few years, Laser Ablation Ionisation Mass Analysis (LAIMA) has taken on a considerably increased importance, especially in the low and medium ablation flux regimes, both in the area of analytical chemistry and the materials preparation. This technique has been well established by the Laser Ionisation Studies (LIS) Group at University of Glasgow, which is the collaborating institution. It has been used extensively to characterise the technique of laser ablation for atomising solid samples leading to chemical analysis of major and minor impurities in metals and to help elucidate the physical processes involved in pulsed laser deposition of thin films and matrix assisted laser desorption ionisation (MALDI). In this thesis, two recent versions of this technique, Resonant Laser Ablation (RLA) and Post Ablation Ionisation (PAI), have been applied to the analysis of trace elements in metal samples and to the characterisation of laser ablated YBa2Cu3Oyp lumes. The thesis begins with an overview of LAIMA, prior to a detailed review of relevant studies,i ncluding: (i) theb asicc onceptso f the lasera blationb asedt echniquef or solid sample analysis, (ii) comparison of several different approaches for laser ablation combined with mass spectrometry and (iii) the possible photo-ionisation mechanisms involved in LAIMA. ResonantL aser Ablation (RLA) is a relatively new laser-basedte chniquef or solid sample analysis, in which only one UV-tuneable laser is used for both the atomisation and the resonanpt hotoionisationA. detailedr eviewo n its developingh istoryi s giveni n Chapter 2 to show where the idea has come from, how it works and how far it has gone to date. The review centreso n characterisinagn d understandintgh e mechanismisn volved in the RLA process. Several proven and promising applications of this technique are also discussedto showt hat RLA is a promisingt echniquefo r surfacea nalysis. R. Zheng 1995 /Summary viii Chapter 3 deals with hydrodynamic modelling and characterisation of laser induced neutral atom liberation either for analytical applications or for thin film deposition, with the emphasis on the effects of hydrodynamic models on the velocity distributions of ablated material together with diagnostic studies of pulsed laser ablation of hightransition- temperature superconductors, such as YBa2Cu3Oy. Some basic concepts of pulsed laser deposition (PLD), a promising method for producing thin films, are also introduced in this chapter. The apparatus, which was used in this work, is housed in the LIS laboratories at the University of Glasgow and consists of commercially available and purpose-built components which together form three principal systems, viz. laser sources, time-offlight mass spectrometer and data acquisition system. A detailed description of these systems is given in Chapter 4. One part of the author's contribution was to the on-going experimental development programme and involved in partial development of a UV fibrebased delivery system for pulsed laser beams. When fully developed, such a system would be able to achieve two desirable aims in RLA work: vibrationless transport of laser pulses to the target surface within a high vacuum ablation chamber and, of course, greater safety of operation. The system has yet to be well established for reasons which will be briefly discussed in this chapter. The relevant theory and characterisation of fibres are given in Appendix A The bulk of author's contribution, within the general group effort, was to perform the experiments which are described in Chapters 5 and 6. A detailed description of the investigation of Mn using a 450 incidence RLA arrangement is given in Chapter 5. This initially involved the useo f a pure Mn samplet o assessth e resonante nhancementa nd the effect of the laser power. The work was then extended to the application of RLA to trace analysis and to obtaining a better understanding matrix effects in RLA. Over two orders of magnitude of enhancement of ion yield were obtained using a pure Mn sample at relatively lower laser fluence (58 pJ). To assess the matrix effects in trace analysis, several different matrices, Al, Cu and Zn, were chosen for these investigations. It was found that the Mn ion yield was matrix dependence with the detection limits <0.2 ppm for Mn in aluminium and 50 ppm for Mn in zinc. A comparison between two RLA approaches and a detailed discussion of matrix effects are given in Chapter 5. R. Zheng 1995 /Summary ix The work presented in Chapter 6, was specifically carried out to measure the average velocities of the different species in a bulk YBa2Cu3Oy superconducting target to determinew hether they possessa common most probable velocity. This is an important factor in pulsed laser deposition of thin films and it is also important in quantification of the surface analysis of multicomponent samples using PAI. The non-resonant PAI approach was used to permit the simultaneous investigation of number of atoms and molecules. It has been shown that the velocities of Cu, Ba, and BaO were similar to one another but those of Y and YO were considerably slower for both of the ablation wavelengths examined ( 355 and 532 nm) and also at two different distances from the target surface (2 and 3 mm). At the shorter wavelength and higher ablation laser fluxes, the differences between the two groups of velocities seemed to decrease. This has been attributed to increased numbers of collisions, but this point must be verified over a much greater range of flux values before any firm conclusions can be made. The data has been fitted with the present theoretical laser sputtering models. It would appear, however, that none of them quite predicts this behaviour. Finally, Chapter 7 contains a general discussion of the work and suggests possible future developments, including several further experiments, which may lead a better understanding the mechanisms involved in RLA and determine the future direction of characterising laser ablated plumes and developing appropriate new instrumentation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Optics & masers & lasers Optics Atoms Molecules