Comparison of Nanosecond and Femtosecond Laser Mass Spectrometry (FLMS)

Kilic, Hamdi Sukur (1997) Comparison of Nanosecond and Femtosecond Laser Mass Spectrometry (FLMS). PhD thesis, University of Glasgow.

Abstract

The work presented in this thesis was carried out with the primary objective of developing a sensitive ultrafast laser based technique for detecting and identifying nitroaromatic compounds. In addition the mechanisms involved in the production of the fragmentation patterns in the mass spectrum were studied. The data presented in this thesis demonstrates the potential of femtosecond laser mass spectrometry (FLMS). The first chapter discusses the fundamental principles of the interaction between the laser and the atomic/molecular system, where single photon excitation and relaxation processes are described, considering on and off resonance conditions. The second chapter describes the multiphoton interaction, i.e., excitation, dissociation, ionisation and other radiative and radiationless processes. In Chapter 2, a description of the Glasgow rate equation model is also given. The third chapter gives the basic principles and a brief description of the experimental systems and techniques used. The nanosecond resonance enhanced multiphoton ionisation (REMPI) is briefly described in Chapter 2 and its application to NO from the dissociation of the NO2 molecule is presented in the early part of Chapter 4. In addition in Chapter 4, the femtosecond laser mass spectrometric investigation of the dissociative ionisation of the NO2 molecule is presented. The results using the femtosecond laser is compared with the results obtained using nanosecond laser mass spectrometry. A comparison of the theoretical results of the Glasgow rate equation model and the experimental results for femtosecond and nanosecond laser mass spectrometry is also carried out in Chapter 4. In Chapters 5 and 6, respectively, the nanosecond and femtosecond mass spectra of nitromethane and nitrobenzene are compared. Efficient production of the parent ion was obtained for both samples and the mass spectra are presented. Morover, the different dissociative ionisation and fragmentation mechanisms for both samples are identified. In both chapters, the laser intensity dependencies of the parent ion as well as the fragmentation patterns of the mass spectra are presented. In Chapter 7, the multiphoton dissociative ionisation processes in the three isomers of nitrotoluene, DNT and TNT are investigated and their nanosecond and femtosecond laser induced mass spectra are compared and the performance of femtosecond laser mass spectrometry is demonstrated. Although, a comparison of the mass spectra of three isomers of nitrotoluene and the mass spectra of the o-nitrotoluene, dinitrotoluene (DNT) and trinitrotoluene (TNT) is made, it is shown that the nanosecond laser mass spectra of the nitrotoluene isomers are similar and the distinction between them is very difficult. On the other hand, a comparison of the femtosecond laser mass spectra of the three isomers of nitrotoluene is made and the fingerprints of these three isomers are determined using three mass spectra. The differences between the mass spectra of the o-nitrotoluene, DNT and TNT are also discussed in detail. The dissociative ionisation and fragmentation patterns as well as their laser intensity dependencies are used to discuss the dissociation pathways in these molecules. The different dissociation- ionisation mechanisms are presented and identified with particular reference to the femtosecond regime. Finally, in Chapter 8 aspects of the data presented in this thesis are discussed briefly as well as future plans.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: K W D Ledingham
• Keywords: Applied physics, Optics
• Date of Award: 1997
• Depositing User: Enlighten Team
• Unique ID: glathesis:1997-75477
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 19:50
• Last Modified: 19 Nov 2019 19:50
• URI: https://theses.gla.ac.uk/id/eprint/75477