R Browning

Dissociative photoionization of H2 and D2 through the 1sσg ionic state

The dissociative photoionization of H2 and D2 has been investigated experimentally and theoretically from just below threshold to 30.5 eV (405 AA). Experimentally the ratios of the number of dissociated to undissociated photoions H+/H2+, D+/D2+ have been determined using a mass spectrometric system essentially free from discrimination against ion species. These ratios are found to rise to plateau values of 0.0208+or-0.0017 and 0.0082+or-0.0008 respectively. Over most of the energy range investigated, the photoions are formed solely through the 1s sigma g ionic state, permitting approximate calculations to be made. A comparison between the calculated and experimental results indicates the need for improved calculations. Significant differences are found on comparing the photoionization data reported with the corresponding values obtained by particle impact.

Charge transfer between H2+ and AR: the effect of molecular vibration studied using photoelectron-photoion coincidence spectroscopy

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.

Dissociative photoionisation of H2: proton kinetic energy spectra

The kinetic energy spectra of the protons produced in the dissociative photoionisation of H2 and directed predominantly along the electric vector of a photon beam have been obtained at certain wavelengths between 19.8 and 40.8 eV. The time-of-flight method described employed a specially constructed wide aperture analyser with multichannel analysis to extract the very weak signals available. The system has been used to measure the energy distributions of the protons formed through the ground state of H2+ and at higher energies, where two-electron excited states of H2 and other states of H2+ also contribute to proton production. Protons identified with the lowest 1 Sigma u+ state (predominantly 2p sigma u2s sigma g) exhibit a peak in their kinetic energy distribution which appears to move to higher energies as the photon energy is increased. The contributions from direct and autoionising processes are assessed and related to other published results including electron impact data.

Negative photoion spectroscopy of SF6 in the inner valence and S 2p energy regions

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.

Fragmentation of molecular gases by 5-45 keV protons

Collisions involving ionization and fragmentation by 5-45 kev protons during passage through thin targets of H2, N2, O2, CO and CH4 have been investigated by the mass spectroscopic analysis of the secondary ionic products of single collisions. Particular care was taken to ensure that all these ions were collected and recorded with high efficiency in spite of the wide spread in both their energies of formation and initial directions. Measured fragmentation cross sections are as much as five times larger than previously published values. It is shown that, although the energy variation of cross sections for the formation of undissociated molecular ions may be in general accord with the predictions of the simple adiabatic maximum rule, the application of this rule to dissociative processes involving the production of singly or doubly ionized fragments may give entirely misleading results. The measurements show that in all the cases investigated a significant fraction of the total secondary ion yield arises through collisions involving fragmentation.

The effect of molecular vibration on charge transfer between H2+ and H2

Charge transfer between H2+ ions in specific vibrational states and H2 molecules has been investigated within the energy range 8-1000 eV using photoelectron-photoion coincidence spectroscopy. Results up to the sixth vibrational level show that cross sections vary significantly with vibrational energy in a complicated manner which is not in accord with multistate impact parameter theory.

Dissociative photoionization of H2 at 26.9 eV

Protons with kinetic energies up to 4.4 eV have been observed in the dissociative photoionization of H2 using Ne II 26.9 eV line radiation. The most energetic protons must result from the dissociative autoionization of doubly excited H2, and it seems most probable that the lowest state of 1 Sigma u+ symmetry is involved. It is estimated from the integrated kinetic energy spectrum that at 26.9 eV the photoionization cross section for the production of 2 eV to 4.4 eV protons from H2 is 0.012-0.004+0.007 Mb.

Ionization and charge transfer in fast H+-He+ collisions using an intersecting beam technique

A fast intersecting beam technique has been used to obtain total cross sections sigma (He2+) for He2+ formation from the combined processes H++He+ to H++He2++e (ionization) and H++He+ to H+He2+ (charge transfer) in the centre-of-mass energy range 72-402 keV. At these energies, ionization is believed to provide the main contribution to the measured cross sections which are shown to be in good agreement with cross sections for ionization predicted theoretically by scaling data for H+-H collisions. Values of sigma (He2+) also show the expected tendency to converge to corresponding cross sections for ionization by electron impact at high equivalent velocities.

Symmetric charge transfer in argon, krypton and xenon: the effect of spin-orbit coupling studied using photoelectron-photoion coincidence spectroscopy

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.

Electron loss from fast metastable and ground state helium atoms in passage through gaseous targets

A beam attenuation technique developed previously in this laboratory has been used to obtain cross sections sigma 01 and sigma 0*1 for fast ground state and metastable helium atoms in helium, neon, argon, krypton and nitrogen at impact energies within the range 60-350 keV. Values have also been obtained for targets of hydrogen and helium in the range 10-40 keV. Measured cross sections sigma 0*1 are shown to be in good agreement with values calculated recently by Bates et al. for targets of H2, Ne, Ar and Kr using a classical impulse approximation. The observed energy dependence of sigma 01 in the case of targets of hydrogen and helium is also found to be in good agreement with that predicted by the recent calculations of Bell et al. based on the first Born approximation.