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Using synchrotron radiation in the range 12-35 eV, negative ions are detected by mass spectrometry following vacuum-UV photoexcitation of methane. Ion yields for H\(^-\), CH\(^-\) and CH\(_2^-\) are recorded, the spectra of CH\(^-\) and CH\(_2^-\) for the first time. All ions display a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Cross sections for ion-pair formation are put onto an absolute scale by calibrating the signal strengths with those of F\(^-\) from SF\(_6\) and CF\(_4\). Following normalisation to total vacuum-UV absorption cross sections, quantum yields for anion production are reported. There is a major discrepancy in the H\(^-\) cross section with an earlier measurement, which remains unresolved. The anions arise from both direct and indirect ion-pair mechanisms. For a generic polyatomic molecule AB, the former is defined as AB \(\rightarrow\) A\(^-\) + B\(^+\) (+ neutrals), the latter as the predissociative crossing of an initially-excited Rydberg state of AB by an ion-pair state. In a separate experiment, the threshold photoelectron spectrum of the second valence band of CH\(_4\), ionisation to CH\(_4^+\) A \(^2\)A\(_1\) at 22.4 eV, is recorded with an instrumental resolution of 0.004 eV; many of the Rydberg states observed in indirect ion-pair formation converge to this state. The widths of the peaks are lifetime limited, increasing with increasing \(v\) in the \(v_1\) (a\(_1\)) vibrational ladder. They are the first direct measurement of an upper value to the dissociation rate of these levels into fragment ions.

Photoion-pair formation in H 2 O and D 2 O has been studied using synchrotron radiation within the energy range 15-50 eV. Appearance potentials for the formation of O - , OH - and H - have been determined and coincide with their thermochemical thresholds. Above threshold it is shown that superexcited states play an important role in the polar photodissociation process. Substantial isotope effects have been observed.

Polar photodissociation of a set of bromo-chloro-alkanes in the vicinity of the Br 3d core edge has been observed for the first time. It is shown that negative photoion spectroscopy is a powerful tool for investigating the various decay mechanisms of core-excited molecules. Analysis of these results indicates that the observed polar photodissociation arises from two competing spectator Auger decay processes in which the molecule can dissociate either before or after the core hole relaxation.

The molecular and dissociative photoionization of C 2 H 2 and C 2 H 4 has been studied using synchrotron radiation within the energy range 11–30 eV. Positive and negative photoion mass spectra have been obtained. Appearance thresholds for the formation of fragment ions have been determined and are compared with the thermochemical values. Above threshold it is shown that superexcited states, involving intravalence transitions and shape resonances which can decay through ion pair formation, play an important role in the photodissociative ionization process. The positive photoion yield curves display significant hitherto unobserved structures, which are unrelated to those present in the photoelectron spectra.

Negative fragment ion formation from SF6 has been studied using synchrotron radiation within the energy range 20-205 eV. The fragment ions F-, F2-, S- and SF- have been observed. It is shown that shape resonance and unoccupied valence orbitals play an important role in negative ion formation after photoexcitation of inner valence and S 2p electrons. The negative ion yields after photoexcitation of a S 2p core electron show evidence of localization of the shape resonance orbitals around the central S atom. The yields of F- and F2- in the outer valence and inner valence energy regions not only confirm that ion pair formation occurs via predissociating Rydberg states but also demonstrate, for the first time, the importance of intravalence transitions.

The molecular and dissociative photoionization of C2H6 has been studied using synchrotron radiation within the energy range 11-40 eV. Positive and negative photoion mass spectra and threshold photoelectron spectra have been obtained. These show that superexcited states, which can decay through ion pair formation, play an important role in the photodissociative ionization process. The positive photoion yield curves display significant and hitherto unobserved structures, which are unrelated to those present in the photoelectron spectra.

A new method for determining the degree of circular and linear polarization of synchrotron radiation in the vacuum ultraviolet is described and has been demonstrated to be experimentally feasible. It consists of transforming short wavelength VUV radiation into the longer visible region using atomic resonance absorption in a gas and observing the polarization characteristics of the fluorescent light.

The dissociative photoionization of CH4 and CD4 has been studied using synchrotron radiation within the energy range of 12-60 eV. The mass spectra of secondary CHn+/CDn+ ions have been found to display significant isotope effects and to depend on the target gas temperature. The kinetic energy spectra of the fragment protons and deuterons have also been determined and it is shown that, in addition to direct photoionization, intermediate, superexcited states play an important role in the photodissociative ionization process.

The appearance energies and kinetic energy spectra of energetic fragment protons and deuterons produced in the dissociative photoionization of and have been measured using synchrotron radiation within the energy range 20-45 eV. Measurements were made both parallel and perpendicular to the E vector of the radiation and dissociation mechanisms and pathways for several of the inner valence states involved have been determined.

The kinetic energy spectra of the fragment protons and deuterons produced in the dissociative photoionization of and by 29.5 - 60 eV photons have been measured perpendicular to the E vector of the radiation. It is shown that the superexcited state and the repulsive state play a dominant role in the photoionization process and autoionization state widths have been determined.

The kinetic energy spectra of fragment protons and deuterons produced in the dissociative photoionization of H2 and D2 by 37 eV photons have been measured along the E vector of the radiation. It is shown that the 1Σu+(Q2) superexcited state plays a dominant role in the photoionization process. An analysis of the results provides an experimental estimate of (a) the autoionization lifetime of this state as a function of internuclear separation and (b) the branching ratio for the different autoionization pathways.

The kinetic energy spectra of the fragment protons and deuterons produced in the dissociative photoionization of H2 and D2 by 27.5-32.0 eV photons have been measured. It is shown that the results can provide an experimental estimate of the autoionizing state width of the 1Sigmau(1) superexcited state as a function of internuclear separation R. The experimentally determined state widths are significantly larger than theoretical predictions especially at large R. Furthermore there is an unexpectedly large isotope effect.

The kinetic energy spectra of the fragment protons and deuterons produced in the dissociative photoionization of H2 and D2 by 30.5 eV photons have been measured. It is shown that the ratio of the proton to deuteron spectra can provide an experimental estimate of the autoionizing lifetime of the 1 Sigma u(1) superexcited state as a function of internuclear separation. The experimentally determined lifetimes are significantly smaller than theoretical predictions.

Cross sections for single and multiple electron capture in collisions of C4+ with H2, N2 and O2 have been measured within the energy range 0.33-666 eV amu-1. Where comparison is possible, the results are in good agreement with previous measurements at higher energies. Below 100 eV amu-1 there are significant differences between experiment and the currently available theories.

The production of highly excited (24

The energy distribution of protons and pairs of fragment protons produced in 15 keV He+-H2 collisions has been investigated experimentally. The results are compared with earlier measurements involving H+ projectiles and show some surprising differences. A possible explanation is proposed which assumes that two-electron excited states can play an important role in both processes, especially in the double-ionization process where they are formed in the first step of a two-step collision mechanism.

The energy and angular distributions of pairs of fragment N+ ions produced in H+-N2 collisions have been investigated within the energy range 5-25 keV. Six dissociation channels have been observed and the precursor electronic states of N:+ tentatively identified. Total cross sections for the formation of three of these states have been separately determined. Angular distributions of the fragment ion pairs exhibit strong energy-depen- dent non-dipolar anisotropies, characteristic of momentum transfer and short predissoci- ation lifetimes.

Photoexcitation of long lived xenon 5d[3/2]2 atoms to 5p5, nf', 4 ≤ n ≤ 8, autoionizing states has been studied using a crossed-laser fast-atom beam method. Autoionizing features have been analysed using Beutler-Fano lineshapes to provide accurate field-free level positions, lifetimes and line shape parameters. The behaviour of the autoionizing line profiles in an electric field (≤1.2kV/cm) has been studied.

Cross sections for charge transfer between ground state (4S) O+ ions in H2 have been measured within the energy range 0.1-1.0 keV. A novel photoionization source which eliminates problems associated with metastable ions was used and the results resolve the large discrepancies between previous experiments.

Collisional dissociation of 1 keV H2+ ions in specific vibrational states in hydrogen and helium has been investigated. The H2+ ions were produced by photoionisation and state analysis was achieved using a new photoion-photoelectron coincidence apparatus which is described in detail. Results are in accord with theoretical predictions and compatible with non-state-selective experiments.

Near-resonant charge transfer collisions involving Ar+ ions in H2, N2 and CO and Kr+ ions in CO have been measured as a function of the internal energy state of the primary ions within the translational energy range 10 eV to 1.5 keV. The ratios of the cross sections for the two spin-orbit levels of the ground-state rare-gas ions sigma (2P32/)/ sigma (2P12/) have been determined. The results are in accord with recent semiclassical multistate computations using ab initio potential energy surfaces. The validity of simple models incorporating appropriate energy defects and the Franck-Condon principle is discussed.

The energy and angular distribution of pairs of fragment protons produced in H+-H2 collisions has been investigated in the energy range 5-30 keV. It is shown that the energy distribution is a simple transformation of the ground-state wavefunction of H2 and an experimental determination of the ground-state probability density distribution is presented. Total and differential double ionisation cross sections are found to be in accord with earlier measurements.