The Application of Resonance Enhanced Multiphoton Ionisation Mass Spectrometry to the Detection of Aromatic Molecules

[摘要] The work presented in this thesis was carried out with the primary objective of developing a sensitive laser based procedure for detecting and identifying aromatic type compounds, in particular nitrobenzene and nitrotoluene. The first chapter discusses the basic underlying principles of Resonance Ionisation Spectroscopy (RIS) as applied to atoms, and of Resonance Enhanced Multiphoton Ionisation Spectroscopy (REMPI) of molecules. Several different laser ionisation schemes are discussed which demonstrate the flexibility of the technique. The purpose of Chapter 2, where the theoretical principles of resonance enhanced multiphoton ionisation spectroscopy are introduced, is to provide the reader with the necessary information required for an understanding of the technique. A semiclassical approach to the interaction of light with atoms and molecules is presented, as well as a discussion of photophysical processes which occur in the excited states of molecules. In addition, a population rate equation model restricted to a three level ionisation scheme (two bound states and the ionisation continuum) allows determination of the necessary laser parameters to efficiently implement the REMPI process. The experimental apparatus is described in detail in Chapter 3. Chapter 4 deals specifically with a course of preliminary experiments which were performed in a high vacuum chamber with quadrupole filtered ion detection. The results of Resonant 2-Photon Ionisation studies of toluene and aniline vapours are presented. In addition, the experimental results obtained when investigating the possibility of detecting and identifying individual molecular species in a mixture of molecules (at atmospheric pressure) in a simple home-made ionisation chamber are presented. This work was carried out with Dr. A. Marshall, and the interpretation of the results and analysis was carried out jointly. Chapter 5 describes the introduction of time-of-flight mass spectrometry to aid the study of REMPI processes in nitroaromatic molecules. The results of a comprehensive series of experiments concerning REMPI of nitrobenzene and mono-nitrotoluene molecules in the wavelength range 245-250nm are discussed in some detail. One of the most important results of this work was the observation of a characteristic wavelength dependent signature of the NO+ ion (m/z=30). The author was solely responsible for designing, constructing and commissioning the TOF mass spectrometer system. The work of Chapter 6 discusses the results of a series of experimental investigations which were conducted in an attempt to identify the dissociation processes which prevail during the interaction between intense ultraviolet laser fields and nitroaromatic vapours. The wavelength dependent ionisation spectra (in the range 233-260nm) of the NO+ ion produced from high purity samples of nitric oxide and nitrogen dioxide are compared with the NO+ spectra from nitroaromatics (Chapter 5) and several conclusions are made which confirms the manner in which this particular class of molecules break up when irradiated with UV laser light. This work was done in collaboration with Dr. A. Marshall and the results and analysis were carried out jointly. Chapter 7 discusses the wavelength dependent production of carbon and hydrogen ions from nitroaromatic molecules. Several sharp transitions in the carbon ion spectrum have been identified as single and multiple photon resonances in neutral atomic carbon. The 1s → 2s two-photon resonance in hydrogen has also been observed. In all, six singlet transitions and one triplet transition in carbon have been identified. These have not previously been observed as far as the author is aware. Finally, Chapter 8 discusses briefly the project as a whole and discusses several experiments which are planned for the future.

[发布机构] University:University of Glasgow

[关键词] Molecular physics, Applied physics, Computational physics