Ionisation Studies on Some Small Molecules

The work of this thesis targeted the applications of a well established laser spectroscopic technique-REMPI; particuarly a laser-based procedure for the trace detection of environmentally sensitive molecules NOx (x=l,2) and carbon monoxide (CO). The thesis also explores a new laser mass spectrometry technique-femtosecond mass spectroscopic trace analysis. The thesis consists of seven chapters which cover the fundamental concepts and principles (chapter 1), theories (chapter 2), instrumentation (chapter 3), the main experimental results of this work (chapter 4, 5 and 6) and future plans (chapter 7). Chapter 1 introduces the fundamentals and some related basic concepts, photodissociation and predissociation, introduction to laser spectroscopy analysis and ionisation of atoms and molecules in intense laser fields. This chapter will help the reader to understand later chapters. The principle of mass spectrometry has been included in chapter 3 and 4 and therefore not mentioned in this chapter. Chapter 2 deals with some theoretical work. Firstly a brief review of the historical background of multiphoton ionisation has been given. Thereafter the basic ionisation schemes of atoms and molecules has been described and the results of perturbation theory were quoted and the rate equations of MPI deduced. Chapter 3 describes the instrumentation, including: - conventional laser light sources, laser pulse energy detector, ion detector, LeCroy 9304 175 MHz digital oscilloscope, time-of-flight (TOF) mass spectrometer and ultrafast laser system. Chapter 4 has reviewed the technologies of NOx detection, introduced the REMPI spectroscopy of NOx, described the experimental methodology used in this thesis and reported the results of trace analysis of NOx at atmospheric pressure conditions using a simple ionisation chamber. The wavelength dependence of the ion signal around 226 nm and 382 nm have been investigated. The sensitivity of the ion signal size for different gases and wavelengths have been determined and the related limits of detection (LODs) have been deduced. In chapter 5, a brief description of the CO molecule and a short review about the REMPI spectroscopy of CO has been given. An optical spectroscopic fingerprint at 230 nm has been presented and the characteristics of the spectroscopic structure has been explained. The results of the sensitive detection using this fingerprint have been reported. The result includes the measurements of LODs for CO in the air and CO in nitrogen and the level of CO in a real urban air sample. A combined analysis using both optical and mass spectroscopy is very helpful to explain the unknown peaks of the optical spectra. A spectrum showing a gas mixture of NO2, CO and air in the wavelength region of 224-232 nm is given. This indicates that using one simple gas container (ionisation chamber) and the same dye, three gases (NO2, CO and O2) can be detected. Chapter 6 concerns the ionisation of small molecules in intense laser fields in the femtosecond time regime. In particular the results for NO2 and CO2 ionisation have been analysed and additionally the ionisation of NO (fragmented from NO2) and CO have been also analysed. The influence on ion yields of laser wavelengths, photon polarisation and power densities have been investigated. It turns out that for NO2, there is a competition between the ID (ionisation followed by dissociation) and DI (dissociation followed by ionisation) processes which depends on laser wavelengths and other factors. However for CO2 molecules, the ID process is always dominant in this thesis indicating that the lifetime of the dissociative energy levels of this molecule are at least less than 100 fs. Chapter 7 proposes the future work which should be carried out and some ideas which might be verified. Therefore this chapter is an exciting one. All the data in this thesis were analysed by the author. The data for the chapter 4 and 5 were taken at our home laboratories by the author and the data for the chapter 6 were taken at the Rutherford Appleton Laboratory (RAL) in collaboration with my Glasgow collegues and with considerable assistance from RAL collegues