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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.

Energetic oxygen ions are an important component of the plasma incident onto Titan's atmosphere. Therefore, we report measurements of electron capture and ionization collisions of N2 with incident O+ over the energy range 10-100 keV. Using time of flight coincidence counting techniques we also measured the collision-induced dissociation of N2 following ionization and electron capture. The electron capture and ionization cross sections were found to have comparable magnitudes. Capture collisions are dominated by non-dissociative processes with the dissociative processes providing contributions that are only slightly smaller. In contrast, ionization is entirely dominated by the dissociative processes. The energy distributions of the N+ and N atom fragments ejected by 20, 50, and 100 keV incident O+ projectiles have also been determined. These fragments carry considerable amounts of energy and if produce in the exobase region can readily escape. The cross sections measured here have been used with Cassini energetic ion and atmospheric density data to determine the ionization by and neutralization of energetic O+ penetrating Titan's N2 rich atmosphere. Neutralization by charge exchange is found not to occur efficiently above Titan's exobase, so energetic particles with large gyroradii penetrate the atmosphere primarily as ions. When the energetic O+ flux is large, we also show it is an important source of ionization and heating at depth into Titan's atmosphere and the fragments contribute to the net atmospheric loss rate.

The results of an investigation into the damage caused to dry plasmid DNA after irradiation by fast (keV) hydrogen atoms are presented. Agarose gel electrophoresis was used to assess single and double strand break yields as a function of dose in dry DNA samples deposited on a mica substrate. Damage levels were observed to increase with beam energy. Strand break yields demonstrated a considerable dependence on sample structure and the method of sample preparation. Additionally, the effect of high-Z nanoparticles on damage levels was investigated by irradiating DNA samples containing controlled amounts of gold nanoparticles. In contrast to previous (photonic) studies, no enhancement of strand break yields was observed with the particles showing a slight radioprotective effect. A model of DNA damage as a function of dose has been constructed in terms of the probability for the creation of single and double strand breaks, per unit ion flux. This model provides quantitative conclusions about the effects of both gold nanoparticles and the different buffers used in performing the assays and, in addition, infers the proportion of multiply damaged fragments.

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.

Measurements of electron capture and ionization of O2 molecules in collisions with H+ and O+ ions have been made over an energy range 10-100 keV. Cross sections for dissociative and nondissociative interactions have been separately determined using coincidence techniques. Nondissociative channels leading to O product formation are shown to be dominant for both the H+ and the O+ projectiles in the capture collisions and only for the H+ projectiles in the ionization collisions. Dissociative channels are dominant for ionizing collisions involving O+ projectiles. The energy distributions of the O+ fragment products from collisions involving H+ and O+ have also been measured for the first time using time-of-flight methods, and the results are compared with those from other related studies. These measurements have been used to describe the interaction of the energetic ions trapped in Jupiter's magnetosphere with the very thin oxygen atmosphere of the icy satellite Europa. It is shown that the ionization of oxygen molecules is dominated by charge exchange plus ion impact ionization processes rather than photoionization. In addition, dissociation is predominately induced through excitation of electrons into high-lying repulsive energy states (electronically) rather than arising from momentum transfer from knock-on collisions between colliding nuclei, which are the only processes included in current models. Future modeling will need to include both these processes.

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.

Cross sections for the formation of slow molecular and energetic fragment ions in the photoionization of hydrogen and deuterium have been determined using synchrotron radiation within the energy range 25–40 eV. Superexcited states have been shown to play an important role in the photoionization process but recently predicted resonance effects have not been observed.

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.

With modern undulators generating light of an arbitrary polarization state, experiments exploiting this feature in the soft X-ray region are becoming increasingly widespread. Circularly polarized light in the soft X-ray region is of particular interest to investigate of magnetic metals such as Fe, Co and Ni, and the rare earths. A versatile multilayer polarimeter has been designed and developed to characterize the polarization state of the soft X-ray beam. A W/B4C multilayer transmission phase retarder and reflection analyser has been used for polarimetry measurements on the beamline (I06) at Diamond Light Source. The design details of the polarimeter and preliminary polarimetry results are presented.

Using tunable vacuum-UV radiation from a synchrotron, negative ions are detected by quadrupolar mass spectrometry following photoexcitation of three gaseous halogenated methanes CH\(_3\)X (X = F,Cl,Br). The anions X\(^-\), H\(^-\), CX\(^-\), CHX\(^-\) and CH\(_2\)X\(^-\) are observed, and their ion yields recorded in the range 8-35 eV. The anions show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically described as AB + h\(v\) \(\rightarrow\) A\(^-\) + B\(^+\) (+ neutrals). Absolute cross sections for ion-pair formation are obtained by calibrating the signal intensities with those of F\(^-\) from both SF\(_6\) and CF\(_4\). The cross sections for formation of X\(^-\) + CH\(_3\)\(^+\) are much greater than for formation of CH\(_2\)X\(^-\) + H\(^+\). In common with many quadrupoles, the spectra of \(m\)/\(z\) 1 (H\(^-\)) anions show contributions from all anions, and only for CH\(_3\)Br is it possible to perform the necessary subtraction to obtain the true H\(^-\) spectrum. The anion cross sections are normalised to vacuum-UV absorption cross sections to obtain quantum yields for their production. The appearance energies of X\(^-\) and CH\(_2\)X\(^-\) are used to calculate upper limits to 298 K bond dissociation energies for D\(^o\) (H\(_3\)C-X) and D\(^o\) (XH\(_2\)C-H) which are consistent with literature values. The spectra suggest that most of the anions are formed indirectly by crossing of Rydberg states of the parent molecule onto an ion-pair continuum. The one exception is the lowest-energy peak of F\(^-\) from CH\(_3\)F at 13.4 eV, where its width and lack of structure suggest it may correspond to a direct ion-pair transition.

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.

Using agarose gel electrophoresis, we measured the effectiveness of high-Z metal particles of different sizes on SSB and DSB yields for plasmid DNA irradiated with 160 kVp X rays. For plasmid samples prepared in Tris-EDTA buffer, gold nanoparticles were shown to increase G'(SSB) typically by a factor of greater than 2 while G'(DSB) increased by a factor of less than 2. Similar dose-modifying effects were also observed using gold microspheres. Addition of 10(-1) M DMSO typically decreased damage yields by a factor of less than 0.5. Plasmid samples prepared in PBS showed significantly different damage yields compared to those prepared in Tris-EDTA (P0.001) with G'(SSB) and G'(DSB) increasing by factors of 100 and 48, respectively. Furthermore, addition of gold nanoparticles to samples prepared in PBS decreased G'(SSB) and G'(DSB) by factors of 0.2 and 0.3, respectively. The results show plasmid damage yields to be highly dependent on differences in particle size between the micro- and nanometer scale, atomic number (Z) of the particle, and scavenging capacity of preparation buffers. This study provides further evidence using a plasmid DNA model system for the potential of high-Z metal nanoparticles as local dose-modifying agents.

Time-of-flight-based mass analysis of charged water fragments have been used to measure the dissociative and the nondissociative reaction pathways of water formed during collisions with 15 to 100 keV and 500 to 3500 keV H+ projectiles and with 8 to 100 keV H-0 projectiles. The fragmentation pathways resulting from the ionization and the electron capture collisions with the incident H+ and H-0 projectiles, as well as collisions involving projectile electron loss by the incident H-0 projectiles, were separately recorded by detecting the target product ions in coincidence with either the ejected target electrons or the charge-analyzed projectiles. The fragmentation profile shows that at high collision energies the ionization of water arises mainly through outer shell processes. At lower energies valence electron capture and ionization dominate and transfer ionization leads to substantially different fragmentation patterns. H-0 and H+ projectiles are found to be equally efficient at ionizing the water molecule. These results are of particular interest to workers in astrophysics and those involved in cancer therapy with heavy particle ion beams.

Polar photodissociation of ${\mathrm{CF}}_{n}{\mathrm{Cl}}_{4\ensuremath{-}n}\phantom{\rule{0.2em}{0ex}}(n=0--2)$ has been studied using synchrotron radiation within the energy range 195--217 eV. The first observations of negative photoion fragments from these molecules after core excitation are reported. In addition to observing a number of previously known resonances two additional resonant states, just above the $\mathrm{Cl}\phantom{\rule{0.3em}{0ex}}2p$ ionization limit, are observed and play an important role in the polar photodissociation process. The difficulties in identifying these above threshold spin-split features using negative photoion spectroscopy are discussed.

counting techniques. In the case of proton impact the formation of N2 ions dominates for both electron capture and ionization channels at all energies, whereas for N + ions, the N2 formation dominates for electron capture and the dissociative processes for ionization channels. The energy distribution of the fragment products at 20 and 100 keV have also been measured for the first time using the TOF method. These cross sections are useful in the simulation of energetic ions and atoms interacting with Titan’s N2-rich atmosphere. Titan resides primarily within Saturn’s magnetosphere where H + and N + ions are the major ions present along its orbit. It is found that the neutralization of these ions by charge exchange does not occur efficiently above Titan’s exobase, so energetic particles with large gyroradii penetrate primarily as ions. The ionization rate and energy deposition in Titan’s atmosphere by the energetic H + ions observed by the Voyager spacecraft are explained with the help of the present measurements. INDEX TERMS: 0343 Atmospheric Composition and Structure: Planetary atmospheres (5405, 5407, 5409, 5704, 5705, 5707); 2116 Interplanetary Physics: Energetic particles, planetary; 2423 Ionosphere: Ionization mechanisms; 2151 Interplanetary Physics: Neutral particles

The dissociative photoionization of a single-enantiomer chiral molecule by circularly polarized synchrotron radiation was investigated, for the first time, in the gas phase. Photoion mass spectra were produced by the interaction of (+)-(S)-, (−)-(R)- and rac-2-amino-l-butanol with circularly polarized light. Comparison of these spectra places an upper bound of approximately 2% on circular dichroism in the dissociative photoionization of 2-amino-l-butanol at 21 eV, which may have consequences for the theory that the origin of biological homochirality was predominantly enantioselective photofragmentation by circularly polarized light. We have also identified and elucidated many of the difficulties of performing gas phase CD measurements in crossed beam experiments. Chirality 14:478–483, 2002. © 2002 Wiley-Liss, Inc.

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.

Ion pair production and their kinetic energy distribution in the charge transfer of 3–20 keV H+ and He+ ions with simple molecules has been studied. Multiple ionization fragmentation of SF6 by 1 MeV protons using the triple coincidence technique has allowed the different fragmentation channels to be identified unambiguously along with the kinetic energies released. The data is compared with that obtained with 54 and 695 eV photons using the PEPIPICO technique.

The kinetic energy spectra of fragment protons and deuterons produced in the dissociative photoionization of H2, D2, CH4 and CD4 by VUV photons within the energy range 12–50 eV have been measured. Cross sections for negative ion production have also been determined. It is shown that, in addition to direct photoionization, intermediate superexcited states play an important role in these photoionization processes.

Publisher Summary This chapter discusses the dissociative ionization of simple molecules by fast ions. The process of dissociative ionization in heavy particle collisions has until recently been the subject of relatively little study despite its fundamental nature and its significance in determining the ionization, thermal balance and chemistry in interstellar clouds, planetary atmospheres, and comet tails. Therefore, many important features of even the most basic processes involving hydrogen molecules are still not completely understood. In recent years, three main groups have commenced detailed experimental investigations of different facets of the dissociative ionization of simple molecules in heavy particle collisions. Virtually all investigations to date have, mainly for experimental convenience, studied the dissociative ionization of homonuclear diatomic molecules by the beams of positive ions. Clearly future experiments will have to address the special problems of heteronuclear and polyatomic molecules and other heavy particle beams.

Using synchrotron radiation negative ions have been detected by mass spectrometry following vacuum-UV photoexcitation of trifluorochloromethane (CF\(_3\)Cl), trifluorobromomethane (CF\(_3\)Br) and trifluoroiodomethane (CF\(_3\)I). The anions F\(^-\), X\(^-\), F\(_2^-\), FX\(^-\), CF\(^-\), CF\(_2^-\) and CF\(_3^-\) were observed from all three molecules, where X = Cl, Br or I, and their ion yields recorded in the range 8-35 eV. With the exception of Br\(^-\) and I\(^-\), the anions observed show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Dissociative electron attachment, following photoionization of CF\(_3\)Br and CF\(_3\)I as the source of low-energy electrons, is shown to dominate the observed Br\(^-\) and I\(^-\) signals, respectively. Cross sections for ion-pair formation are put on to an absolute scale by calibrating the signal strengths with those of F\(^-\) from both SF\(_6\) and CF\(_4\). These anion cross sections are normalized to vacuum-UV absorption cross sections, where available, and the resulting quantum yields are reported. Anion appearance energies are used to calculate upper limits to 298 K bond dissociation energies for \(D^0\)(CF\(_3\)-X) which are consistent with literature values. We report new data for \(D^0\)(CF\(_2\)I\(^-\)-F) ≤ 2.7 ± 0.2 eV and \(\Delta_fH^0_{298}\) (CF\(_2\)I\(^+\)) ≤ (598 ± 22) kJ mol\(^{-1}\). No ion-pair formation is observed below the ionization energy of the parent molecule for CF\(_3\)Cl and CF\(_3\)Br, and only weak signals (in both I\(^-\) and F\(^-\)) are detected for CF\(_3\)I. These observations suggest neutral photodissociation is the dominant exit channel to Rydberg state photoexcitation at these lower energies.

Ionization of xenon atoms in single highly excited states ‖nf≳ (where 25⩽n⩽40) by collisions with CCl4, CCl3F, and SF6 has been investigated. Absolute rate constants for Xe+ production are reported together with the identities of the major negatively charged species produced in the collisions. Cross sections determined from these rate constants are also given. The data lend support to the theoretical model, which views such collisions as being dominated by the interaction between the target molecules and the excited electron, with the Xe+ ion core playing a minor role.

The production of highly excited hydrogen atoms in proton collisions with the inert gases, hydrogen and nitrogen has been investigated within the energy range 4-20 keV. Cross sections for the formation of atoms in states with principal quantum number, n, between 24 and 40 have been determined using the field ionisation technique and found to scale as n-3. A comparison with optical data for n

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.

Very large and fragile atoms may be produced by exciting normal atoms with light or by collisions with other atomic particles. Atoms as large as 10−6 m are now routinely produced in the laboratory and their properties studied. In this review some of the simpler experimental methods available for the production and detection of such atoms are described including tunable dye laser-excitation and field ionization. A few recent experiments which illustrate the collision properties and the effects of electric and magnetic fields are also described. The relevance of highly excited atoms in other areas of research including radioastronomy and isotope separation are discussed.

Charge transfer between H2+ and Ar, O2+ and Xe has been investigated as a function of the vibrational quantum number of the molecular ion within the energy range 20-1000 eV. The reverse reactions have also been studied and the ratio of the cross sections for the two spin-orbit levels of the ground-state rare-gas ion, sigma (2P32/)/ sigma (2P12/) determined. The results are interpreted in terms of a simple theory which incorporates the appropriate energy defects and Franck-Condon factors.

A photoion-photoelectron coincidence technique for studying the effect of projectile ion internal energy on atomic collision cross sections is described in detail. The technique is used to assess the effect of spin-orbit coupling on symmetric charge transfer in the rare gases argon, krypton and xenon within the collision energy range 5-1000 eV. The ratio of the cross section for the two spin-orbit levels of the ground-state ion, sigma (2P12/)/ sigma (2P32/) was found to be relatively insensitive to collision energy and just measurably less than unity for argon, 0.90+or-0.05 for krypton and 0.80+or-0.05 for xenon. These results extend and confirm previous data obtained by a non-coincidence photoionisation technique and also agree well with current theoretical predictions.

Charge transfer between H2+ ions in specific vibrational states and argon atoms has been investigated. H2+ ions were produced by photoionisation and state analysis was achieved by the registration of charge transfer events in coincidence with energy-analysed photoelectrons. Results at 20 eV are interpreted with the aid of a simple theory which incorporates the effects of changes in different vibrational levels and the 2P3/2,1/2 states of Ar+. The implications of these results for the variation of vibrational distribution in molecular beam experiments are discussed.

The ionization and dissociation of CO and H2 by fast 3He2+ ions have been studied experimentally. Cross sections for secondary ion formation have been measured and interpreted in terms of predissociative and Auger processes.

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.

Theoretical cross sections for the ionisation of higly excited xenon atoms in collision with the polar molecules NH3, H2S, SO2 and HCl are reported. Calculations were performed using a simple model based on the Born approximation's description of a free electron colliding with the target molecule. A comparison of the results with recent experimental data is encouraging.

Thermal beams of xenon atoms in single high Rydberg states have been produced by photoexcitation of Xe($sup 3$P$sub 0$) metastable atoms. The highly excited states which were populated in this way were identified as np1/2$sub 1$ with 11less than or equal tonless than or equal to16 and nf3/2$sub 1$ with 8less than or equal tonless than or equal to40. The field ionization characteristics and radiative lifetimes of these atoms have been studied.

The energy and angular distribution of secondary oxygen ions produced in He+-O2 collisions has been investigated within the energy range 3-25 keV. Dissociative charge transfer processes dominate and involve many exoergic and endoergic channels. Cross sections for capture into the c 4 Sigma u- and B 2 Sigma g- predissociating states of O2+ have been separately determined. Angular distributions of the fragment ions exhibit anisotropies characteristic of momentum transfer and short predissociation lifetimes.

The energy and angular distributions of fragment protons produced in H+-H2 collisions have been investigated within the energy range 5-25 keV. The observed energetic protons arise from at least four repulsive states, the 2p sigma u, 2p pi u and 2s sigma g states of H2+ and the unbound H+H+ state. No significant contribution from autoionisation is observed. Analysis of the angular distributions has been performed to distinguish the separate degenerate channels available to the ejected electron. The results show a remarkable similarity to photoionisation data.