J Geddes

Formation of H(2p) and H(2s) atoms in collisions of 2-26 keV protons with hydrogen atoms

A modulated crossed-beam technique has been used to determine cross sections for the four reactions: H++H(1s) to H(2s or 2p)+H+ (electron capture) and H++H(1s) to H++H(2s or 2p) (excitation) within the energy range 2.3-26 keV. Results have been based on measurements, using a Doppler-shift identification technique, of the Lyman alpha radiation emitted from the beam intersection region both in the absence and in the presence of an electric field. While cross sections for excitation of the 2s state have not been reported previously, cross sections for capture into the 2s state agree well with those measured by Bayfield (1969a) using a furnace target technique. The H(2p) formation cross sections are also shown to be in reasonable accord with values based on the crossed-beam measurements of Stebbings et al. (1965).

A new microwave discharge source for reactive atom beams

A new atom beam source has been developed in which two special slotted line radiators have been used to couple 2.45 GHz microwave power to a simple pyrex glass discharge tube with a relatively large plasma volume. For hydrogen, oxygen and chlorine dissociation, fractions of up to about 0.95, 0.54 and 0.95, respectively, have been obtained with corresponding atom beam densities at the exit of 4*1013, 3.5*1012 and 1.3*1012 cm-3. Atom beam fluxes of up to 0.34, 0.016 and 0.012 SCCM have been obtained for H, O and Cl respectively. The source is simple, cheap to construct, and has proved stable and reliable in operation.

A microwave discharge atom beam source of high intensity

A 2.45 GHz microwave discharge atom beam source of compact design has been developed. A standard extended quarter wavelength cavity design has been used in conjunction with a simple discharge tube and cooling arrangement. The source, while primarily designed for the production of atomic hydrogen, has also been tested with oxygen and nitrogen. The characteristics of the microwave source are compared with those of a Slevin type RF source and the influence of different cleaning procedures on performance have been investigated. Unlike the Slevin source the performance of the microwave source is not critically dependent on the cleaning procedure or gas purity. For hydrogen, both sources could provide a dissociation fraction of about 90% but the beam intensity obtainable from the microwave source (>1014 atoms cm-3) at the exit of a 1 mm capillary tube was significantly greater than that from the Slevin source. For oxygen, where the performance of the Slevin source was poor, the microwave source provided a dissociation fraction of up to about 60% and a beam density of about 1013 atoms cm-3.

Electron capture, loss and excitation in collisions of H+, H(1s), H(2s) and H- in atomic oxygen

Electron capture, electron loss and excitation cross sections have been determined for 2.5-25 keV H(1s) and H(2s) atoms and H+ and H- ions in both O and O2. An iridium tube furnace has been used to provide a target of highly dissociated oxygen. The cross section for the total destruction of H(2s) atoms in O and O2 has also been determined. In general, the cross sections in atomic oxygen exhibit an energy dependence similar to that of the corresponding cross sections in O2, but are smaller in absolute magnitude.

Balmer α emission in H2-H2 collisions

The production of Balmer Halpha radiation in passage of 10-100 keV H2 molecules through H2 has been investigated. Cross sections for total Halpha emission from excited atoms have been determined, together with cross sections for the formation of H(3s) atoms.

Balmer α emission in collisions of H, H+, H2+ and H3+ with N2, O2 and H2O

The production of H alpha radiation in the passage of H, H+, H2+ and H3+ ions through N2, O2 and H2O has been investigated within the energy range 3-100 keV. For the H2O target Doppler-shifted and unshifted H alpha radiation were separately recorded. Cross sections have been determined for (a) the production of fast H(3s) atoms, (b) total H alpha emission from the fast projectile and (c) H alpha emission arising from dissociative excitation of H2O.

New low-energy measurements and calculation of ionization in collisions

In response to the strong current theoretical interest in models of low-energy ionization in collisions, a crossed-beam coincidence counting technique previously used in this laboratory to obtain cross sections in the range 9 - has been successfully adapted to extend the measurements downwards in energy from 9 to . In addition, we have extended the triple-centre calculations of McLaughlin B M, Winter T G and McCann J F 1997 J. Phys. B: At. Mol. Opt. Phys. 30 1043, down to where agreement with experiment is found to be satisfactory. Our new measurements and calculation are at variance with relative cross sections measured by Pieksma M, Ovchinnikov S Y, van Eck J, Westerveld W B and Niehaus A 1994 Phys. Rev. Lett. 73 46 in the range 1 - , but are in good accord with their calculations based on the hidden-crossings model including only S and T promotion. However, our measurements and calculation do not support the more recent calculations by Pieksma M, Ovchinnikov S Y and Macek J H 1998 J. Phys. B: At. Mol. Opt. Phys. 31 1267 in which a contribution from a radial decoupling mechanism is also included in the model.

Importance of dissociative excitation by slow He2+ ions in one-electron capture collisions with H2

Translational energy spectroscopy has been used to study one-electron capture by 0.5-2.0 keV amu-1 He2+ ions in collision with H2. The main excited state product channels have been identified and the relative cross sections determined. These measurements provide the first direct evidence of the relative importance of dissociative excitation channels in electron capture (with He+ formed in the ground state and H atoms mainly in the n=2 states) which dominate the entire energy range. Non-dissociative electron capture into the n=2 and n=3 states of He+, which is also observed, increases from 1% of the total electron capture cross section at 0.5 keV amu-1 to about 25% at 2 keV amu-1.

MULTIPLE IONIZATION OF LEAD BY ELECTRON IMPACT

A pulsed crossed-beam method incorporating time-of-flight spectroscopy, previously developed in this laboratory, has been used to obtain relative cross sections for n = 1- to 6-fold ionization of ground-state Pb atoms by electrons at energies ranging from near threshold to 3000 eV. These results greatly extend the energy range of previous measurements which, in any case, exhibit serious discrepancies. Rather than rely on any previous measurements on Pb, the present data have been normalized to the previously measured cross section for electron impact ionization of Ga at 200 eV. At our high-energy limit of 3000 eV, our measured cross sections decrease by much less than an order of magnitude with increasing n and and are also converging rapidly at high velocities, indicating large contributions from Auger processes. Observed near-threshold structure in the cross section curve is consistent with known autoionization transitions, and while there is also some indication of structure in cross section curves for multiple ionization, the large number of possible levels and experimental uncertainties preclude a detailed analysis. Recent theoretical predictions of multiple ionization based on approximate scaling procedures have been shown to be only partially successful in describing the present data.

The photodissociative ionization of hydrogen and deuterium in the VUV via states

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.