Studies in Photoionization

This thesis is concerned with a study of the fragmentations of some small ions produced by photoionization. The study of unimolecular reactions of this kind is of fundamental interest, providing a much clearer insight into the processes occurring than do conventional kinetic studies where results are complicated by collisions between molecules. Such studies are also of interest in connection with the mechanisms and energetics of the ion dissociation processes responsible for the fragmentation patterns in mass spectroscopy.
A general discussion is given of the processes occurring within a molecule after the absorbtion of a photon, with specific reference to the study of the energy distribution of the electrons produced in photoionization and the energetics of the fragmentation of the residual photoions. Direct and statistical theories of dissociation are considered; in particular the well established Quasi Equilibrium Theory (QET). This theory assumes that the rate of dissociation of an ion is slow relative to the rate of internal energy equilibration and describes each fragmentation pathway as a motion along a separable 'reaction coordinate'. This theory is in many ways similar to the statistical theories of unimolecular reactions. It is difficult to devise a direct and unambiguous test of the extent to which the QET is obeyed in particular cases.
The release of kinetic energy in ion dissociations was first noted in relation to mass spectral appearance energies - partly accounting for the so called 'kinetic shift' of appearance energies from their thermodynamic limits. A major aim of the work presented here was to relate this release of kinetic energy to the mechanism of fragmentation.
A discussion of experimental techniques is introduced by a general consideration of the types of photon source and energy analyzer which may be used. The experimental work was carried out with an analyzer built originally as a photoelectron spectrometer (with a view to various possible avenues of development). For ion kinetic energy measurements various modifications were found to be necessary, particularly to achieve satisfactory mass resolution of the various ionic fragments from their time of flight through the instrument. Initial experiments were made by pulsing the photon source but extensive investigations and modifications led to a new technique in which the ions were continuously produced and their admission to the energy analyzer was pulsed. Electronic equipment was developed to allow the recording of the data on a multi-channel analyzer. Eventually a reasonable mass resolving power was obtained and the kinetic energy resolution for ions improved to approximately 15 meV.
Much consideration was given to the interpretation of the ion energy distribution data obtained. A detailed investigation of the fragmentation of the O2+ ion was employed to test these methods as this dissociation is fairly well understood from spectroscopic information. Equations were developed to enable the observed energy distributions to be related to the kinetic energy releases occurring in centre of mass coordinates and computer programs written to handle the data. For the O2+ ion it was found that the observed shape of the O+ fragment energy distribution could be completely accounted for on the basis of the known potential energy curves for the O2+ ion.
An experimental study was then made of a group of fluorine compounds; for some of these the investigations required an initial study of their photoelectron spectra in order to obtain information about the excited states of the ions. For CCl3F, CCl2F2 and CClF3 the photoelectron spectra and their assignments are in broad agreement with results published elsewhere after this work was completed. In the case of CClF3 a band not reported elsewhere was observed. The photoelectron spectrum of C2F6 was also obtained; it does not appear to have been previously reported. The energy distribution of fragment ions produced in the photoionization of all these compounds as well as those of CF4 and SF6 were recorded and analyzed using both the helium and neon resonance lines (21.22 eV and 16.85 eV respectively).
Little evidence was found to support the idea that the pattern of ionic decomposition is necessarily determined by the bonding character of the electron lost in ionization. A high reverse activation energy appears to apply to processes in which fluroine is lost from these molecules. The loss of a chlorine atom however seems to be a less rapid process with little kinetic energy release.
In an investigation of the fragmentation of NO2+ the ions NO+, O+ and N+ were observed. Consideration of the predissociations of the various states of NO2+ showed that dissociation from the 1A2 state of the ion could only be via a spin-forbidden process. High kinetic energy releases were observed in many cases, reflecting the large excess energies possessed by the ions over their dissociation limits. In general the experimental energy distributions were shown to be consistent with the excess energies involved. In the case of the O+ fragment there appears to be structure in the distribution of energy release which may reflect the vibrational levels of the parent molecule ion.