H B Gilbody

Pulsed crossed-beam study of the ionisation of atomic hydrogen by electron impact

A pulsed crossed-beam technique incorporating time of flight spectroscopy has been successfully developed and applied to measurements of the electron impact ionisation cross sections of atomic hydrogen in the range 14.6-4000 eV. The method has been derived from a crossed-beam coincidence technique. By substituting a pulsed proton beam for the pulsed electron beam in the present work, measured electron impact ionisation cross sections have been normalised by reference to known equivelocity proton impact cross sections for both ionisation and charge transfer. The results resolve the discrepancy between earlier less accurate cross sections measured using the modulated cross-beam technique. They provide a reliable check on the range of validity of theoretical predictions over a wide energy range.

Single and double ionisation of helium by electron impact

A pulsed crossed-beam technique incorporating time-of-flight spectroscopy which was recently developed in this laboratory has been applied to measurements of the cross sections for single and double ionisation of helium. Measurements over the unusually wide energy range from near threshold to 10000 eV provide valuable checks on previous measurements based on different experimental approaches and an assessment of the range of validity of a number of theoretical predictions.

One-electron capture and loss by fast multiply charged boron and carbon ions in H and H2

Cross sections for one-electron capture by fast boron and carbon ranging from singly to fully ionised in both H and H2 have been measured within the energy range 100-2500 keV. At the higher velocities capture cross sections in H for the different initial charge states q>or=2 scale according to qn where n=2.4 and 2.5 for boron and carbon ions respectively. The present and previous capture cross sections for completely stripped ions are consistent with a q3 scaling rule at high velocities. Cross sections for one-electron loss by boron and carbon ions with charge states up to 2 and 3 respectively have also been determined. These considerably exceed the corresponding capture cross sections at the highest velocities considered.

Single and double ionization of atomic oxygen by electron impact

A pulsed crossed beam technique incorporating time-of-flight spectroscopy (previously developed in this laboratory) has been used to study the single and double ionization of ground-state oxygen atoms produced by an iridium tube furnace source. Relative cross sections sigma 1 and sigma 2 for single and double ionization have been determined in the respective energy ranges 14.1-2000 eV and 90-2000 eV which are wider than any previous measurements. These values have been normalized by reference to the absolute values of sigma 1 previously measured by Brook et al at energies (50-997 eV) where the possible influence of an unknown admixture of long-lived excited atoms in their experiment should be small. However at energies below about 25 eV our values of al become much smaller than the values of Brook at al. Above 40 eV, the present ratios sigma 2/ sigma 1 are in satisfactory agreement with other measurements which extend to 400 eV. Our values of al have been compared with selected theoretical predictions. Some evidence of structure in the energy dependence of sigma 1 in the region of the cross section maximum is consistent with predicted contributions from inner-shell 2s ionization and from autoionization in addition to outer 2p electron removal.

Experimental study of the ionisation of atomic hydrogen by fast H+ and He2+ ions

Cross sections for the ionisation of ground state hydrogen atoms by 38-1500 keV H+ and 125-2200 keV He2+ ions have been determined using a crossed-beam technique. Collision products produced in the intersection of the primary ion beam with a highly dissociated thermal energy beam of hydrogen are identified by time-of-flight spectroscopy and counted using a coincidence technique. Cross sections sigma (H+) for proton impact, which involve much smaller uncertainties and cover a wider energy range than previous measurements, now permit an accurate assessment of the validity of the various theoretical predictions. Cross sections sigma (He2+) for He2+ impact, the first such measurements, are also compared with theory and measured cross section ratios sigma (He2+)/ sigma (H+) are considered in terms of the Z2 scaling predicted by the Born approximation.

Electron capture and He+(2s) formation in fast He2+-H and He+-H collisions

Cross sections for the accidentally resonant charge-transfer process He2++H(1s) to He+(2s)+H+ and total cross sections for the process He2++H(1s) to He+( Sigma n,l)+H+ involving capture into all final bound states of He+ have been measured for 4-343 keV 3He2+ ions. A tungsten tube furnace was used to provide a target of highly dissociated hydrogen. The results greatly extend the energy range of previous measurements and provide data of increased accuracy thereby permitting a better assessment of the range of validity of theoretical descriptions of He2+-H collisions. Corresponding cross sections for capture in H2 have also been determined. Cross sections for the direct excitation process He+(1s)+H to He+(2s)+H( Sigma ) have also been measured for the first time within the same energy range.

Electron capture and ionisation in collisions of slow H+ and He2+ ions with helium

A crossed-beam technique previously developed and used in this laboratory for studies of ionisation and electron capture in H+-He and He2+-He collisions down to 64 keV amu-1 and 50 keV amu-1 has been adapted to extend measurements down to 9 keV amu-1 and 6 keV amu-1 respectively. The method, which incorporates time-of-flight spectroscopy together with electron-ion and ion-ion coincidence counting of the collision products, provides high precision cross sections for individual collision channels. Some serious discrepancies are obtained with previous cross sections for single ionisation measured by Afrosimov et al. (1969, 75) and the important role of transfer ionisation at low energies is confirmed. The need for better theoretical descriptions of low-energy ionisation is demonstrated.

Ionisation of atomic hydrogen by 9-75 keV protons

A crossed beam method incorporating time-of-flight analysis and coincidence counting of the collision products was used previously in this laboratory to study H+-H ionisation in the range 38-1500 keV. The same approach has now been used to extend measurements down to 9.4 keV. The authors measured cross sections involve only small uncertainties and, for the first time, permit a detailed assessment of a number of theoretical descriptions of ionisation in slow collisions. Comparison is made with the modulated cross beam measurements of Fite et al.(1960), the only previous measurements which extend down to keV energies. While the agreement is good at 40 keV, the authors measured cross section at 9.4 keV is only about 27% of the value based on the data of Fite et al. This large discrepancy is consistent with an error arising from neglect of secondary electron emission in their experiment.

Multiple ionization of copper by electron impact

A pulsed cross beam technique previously developed in this laboratory has been used to study the electron impact ionization of copper. Previous measurements have been very limited in scope and exhibit large discrepancies. Relative cross sections sigma n for the formation of 1 to 5 times ionized copper have been measured with high accuracy within the range 7.8-2100 eV. Individual cross sections have been obtained by normalization to absolute values of sigma 2 obtained by Freund et al (1990) at energies below 200 eV using a fast crossed beam technique. Weak structures in sigma 1 can be attributed to Auger decay processes following the creation of 3s subshell and L shell vacancies but there is a lack of other pronounced structures in sigma n for n>1 where many close-lying subshell vacancies are involved. At 2100 eV cross sections sigma 5 are less than three orders of magnitude smaller than sigma 1.

State-selective electron capture by C2+, C3+, N2+ and Ar2+ ions in rare gases

Energy loss/gain spectroscopy is used to study state-selective one-electron capture by 1.6-6 keV C2+ in He, Ne and Ar, by 0.8-8 keV N2+ in He and Ne, by 0.14-5 keV Ar2+ in He and Ne, and by 3-18 keV C3+ in He. The relative contributions of the various collision product channels are determined. Moderately exothermic processes involving curve crossings at moderate internuclear separations are shown to be dominant and a strong influence of metastable ions in the primary beam is observed for some reactions. Serious discrepancies with some previous measurements are noted. The main collision channels observed are in accord with the Wigner total electron spin conservation rule.