Crossed Beam Technique Research Articles

Ionisation and electron capture in collisions of H+ and He2+ ions with carbon monoxide

A crossed-beam technique employing time-of-flight analysis and coincidence counting of the collision products has been used to identify and obtain cross sections for the main channels in the collisions of 10-98 keV amu-1 H+ and 6.7-65 keV amu-1 He2+ ions with carbon monoxide. Separate cross sections for non-dissociative and dissociative electron capture, transfer ionisation and ionisation involving the production of CO+, C+, O+, CO2+, C2+ and O2+ ions have been determined for the first time. While non-dissociative electron capture and ionisation are dominant in the energy ranges considered, transfer ionisation is also important, particularly in the case of He2+ impact.

Crossed-beam investigations of state-resolved collision dynamics of simple radicals

Abstract A critical survey is given of new developments in the study of collision dynamics of diatomic radicals. The impetus for this survey comes from new experimental results on near state-to-state scattering dynamics of radicals with simple collision partners that have been obtained using the crossed-beam technique. These results, on both inelastic and reactive scattering, provide a unique opportunity to compare experimental observations with formal quantum scattering theory. Photodissociation is also briefly discussed because of its relevance to our fundamental understanding of fine-structure effects associated with radicals arising in all three processes. A few basic concepts pertaining to the dynamics of open-shell systems are outlined. The emphasis is on the underlying physical principles rather than on detailed theoretical treatments. To illustrate the strong interaction between theory and experiment a few case studies of systems investigated under crossed-beam conditions are highlighted. These new results also prompt an appraisal of what an experimentalist should be most anxious to measure and indicate specific areas in which further theoretical developments can be most profitable in the future.

Ionisation and electron capture in collisions of slow H+ and He2+ ions with hydrogen

A crossed-beam technique previously used in the authors' laboratory for studies of ionisation of H2 by H+ and He2+ impact in the ranges 38-1500 and 31-550 keV amu-1, respectively, has been adapted to permit measurements to be extended down to 10 keV amu-1. Both ionisation and electron capture have been studied using time-of-flight mass spectroscopy together with electron-slow-ion and slow-ion-fast-ion coincidence counting of the collision products. Separate cross sections for non-dissociative and dissociative electron capture and ionisation channels have been determined. Some significant discrepancies between the authors' results and previous measurements due to Afrosimov et al. (1969) are noted. At their lowest impact energies electron capture substantially exceeds ionisation. In addition, for He2+ impact, unlike protons, dissociative ionisation and transfer ionisation cross sections substantially exceed those for non-dissociative ionisation.

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.

Observation of fine-structure transitions in argon charge transfer at low energies using a crossed-beam technique

The crossed-beam method was used to investigate the charge–transfer reaction of Ar+(2P3/2) with Ar(1S0) as a function of collision energy over the range of 2–100 eV (center of mass). Both the exactly resonant channel and the endoergic fine-structure transition with ΔJ=1 were detected and the cross section ratio σ3/2→1/2/(σ3/2→1/2+σ3/2→3/2) was found first to increase with increasing collision energy to a maximum value of about 0.5 at 30 eV and then decrease at higher energy. The relative abundance of the fine-structure transition channel at low energy is much higher than that predicted by the accepted general theory for these processes. It is suggested that a curve-crossing mechanism not previously considered and the perturbation of trajectories at low energies resulting from the strongly bound Ar+2 intermediate may be responsible for the unexpected observations.

Electron Impact Ionization of C+, N+and P+Ions

Absolute cross sections for electron impact ionization of C + , N + and P + ions have been measured for electron energies from threshold up to 1000 eV, using the crossed beam technique. The measured cross sections for both C + and N + ions are in good agreement with the semiempirical Lotz formula over the whole energy range investigated, and also with those previously reported except for near threshold. The results for P + ions show a more rapid rise from the threshold and larger values around the peak than those calculated with the Lotz formula.

Beam intensity losses in the storage rings of the heavy-ion inertial fusion scenario HIBALL II due to ion-ion collisions

Employing the crossed-beams technique we have measured absolute cross sections both for electron capture and for ionization in collisions between two B

Electron Impact Ionization of O+, S+and S2+Ions

Absolute cross sections for electron impact single ionization of O + , S + and S 2+ ions have been measured at an electron impact energy range from below threshold to 1000 eV, using the crossed beam technique. The measured cross sections for O + ions are reproduced quite well with the semiempirical Lotz formula over the whole energy range investigated. The measured cross sections for S + and S 2+ ions show a more rapid rise from the threshold to their peak than those expected from the Lotz formula. For S 2+ ions, another small bump beginning at 120 eV is observed, where no level exists that may contribute to any direct or indirect ionization processes.

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.

Ionisation of Ti+, Ti2+ and Ar2+ by electron impact

Cross sections for the single ionisation of Ti+, Ti2+ and Ar2+ by electron impact have been measured in the energy range from the thresholds to 2000, 2000 and 1250 eV respectively using the fast crossed beams technique. The cross sections for Ti2+ and Ar2+ are compared with previously measured cross sections and with those given by the semi-empirical formula of Burgess and Chidichimo (1983). The cross section for Ti+ has not been measured previously and is compared with those given by the Lotz formula and the McGuire scaled plane-wave Born approximation.

Transfer ionization in H++H

Absolute cross sections for transfer ionization in ${\mathrm{H}}^{+}$+${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ collisions have been measured in the center-of-mass energy range 50 eV--40 keV, with the use of merged- and crossed-beam techniques, together with coincident detection of the products. A mechanism is proposed for the process in the low-energy region. The cross section, calculated accordingly, is found to be in satisfactory agreement with the experimental data.

Cross sections for electron capture and loss. I. H+ and H- impact on H and H2.

The electron-capture cross sections for H/sup +/ impact on H and H/sub 2/ and the electron-loss cross sections for H/sup -/ impact on H and H/sub 2/ have been measured for projectile energies between 2.0 and 0.063 keV. Relative cross-section values for all these reactions were measured directly, and placed on an absolute scale by adopting a standard value of 6.95 x 10/sup -16/ cm/sup 2/ for the electron-capture cross section for H/sup +/+H/sub 2/ collisions at 2.0-keV H/sup +/ energy. A crossed-beam technique was used, and the reaction-product H atoms were measured by a secondary-electron-emission detector. The techniques used to make the measurements are described, and the results are compared with other experimental and theoretical data.

Cross sections for electron capture and loss. II. H impact on H and H2.

The electron-capture and electron-loss reactions for H impact on H and H/sub 2/ have been examined for projectile energies between 2.0 and about 0.1 keV. Relative cross sections for these reactions were measured directly, and the data for H targets were placed on an absolute scale by normalizing to the available results for H/sub 2/ targets. For the electron-capture (ion-pair formation) reaction, some new data are also reported here for H/sub 2/ targets. The crossed-beam techniques used to accomplish the measurements are described, and the results are compared with other experimental and theoretical data where possible.

Spatially resolved saturated absorption measurements of OH in methane-air flames

A cross-beam saturated absorption spectroscopy technique, utilizing a single pulsed dye laser, has been developed for local concentration measurements in flames. With a differential detection of the probe and the reference laser beam intensities, a significant improvement of the technique has been achieved. In this work the basic theory of the method is discussed. Its use in combustion studies is demonstrated by presenting OH concentration profiles in two premixed laminar methane-air flames.

Absolute cross-section measurements for electron-impact ionization of Al+, Cd+, and Hg+

Absolute cross sections for single ionization of Al/sup +/, Cd/sup +/, and Hg/sup +/ by electron impact have been measured for electron energies from threshold to 2 keV using the crossed-beams technique. The Al/sup +/ cross section shows a peak value of (74 +- 6) x 10/sup -18/ cm/sup 2/ at about 46 eV. In addition, for Al/sup +/ there is significant structure around 80 eV, an energy corresponding to the 2p/sup 5/3s/sup 2/nl autoionizing states of Al/sup +/. The Cd/sup +/ and Hg/sup +/ cross sections show no major structures and have peak values of (147 +- 6) x 10/sup -18/ cm/sup 2/ and (169 +- 5) x 10/sup -18/ cm/sup 2/, respectively, both at an electron energy of roughly 100 eV. Comparisons are made to semiempirical and theoretical predictions and to other existing experimental data.

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.

Electron impact multiple ionisation of multiply charged krypton ions

Absolute cross sections for double ionisation of Krq+ ions (q=1,. . ., 4) and for triple ionisation of Krq+ ions (q=1, 2) in single collisions with incident electrons have been measured by using a dynamic crossed-beams technique. Electron energies ranged from below threshold up to 1000 eV for Kr2+ and up to 700 eV for the other charge states. With increasing q the cross sections for double ionisation are determined by an increasing relative contribution of single ionisation of the 3d subshell and subsequent autoionisation. The authors' data for double ionisation of Kt4+ are in good agreement with measurements of Pindzola et al. (1984). For electron energies about 300 eV triple ionisation appears to be dominated by ionisation-double-autoionisation contributions from the 3p and 3s subshells. Between threshold and about 200 eV, however, the observed cross sections sigma q,q+3 seem to be due to direct multiple-electron processes.

Electron-ion collisions: Dielectronic recombination and ionization

Inelastic electron-ion collisions have been studied by using crossed-beam techniques. The effects of state mixing by extrinsic fields in the collision region were investigated for the dielectronic recombination process Mg +(3s) + e → Mg(3p, nl) → Mg(3s, nl) + hν. By field ionization of the Rydberg atoms produced, cross sections σ( n) have been measured. The observed large changes of σ( n) with alteration of the extrinsic field provided the first incontrovertible experimental evidence that dielectronic recombination can be changed by external fields. Absolute cross sections for electron impact single, double and triple ionization of singly and multiply charged krypton ions were measured. The energy ranged from below threshold up to 1000 eV. The influence of metastable states on the cross sections for single ionization is discussed. Contributions of inner-shell ionization with subsequent auto-ionization to double and triple ionization are identified. Connections between the three principal inelastic processes in electron-ion collisions, i.e. recombination, excitation and ionization, are pointed out. The influence of these processes on plasma-modeling calculations is briefly discussed.

Ionisation and charge transfer in collisions of H+and He2+with potassium

A crossed-beam coincidence technique incorporating time-of-flight spectroscopy of the collision products has been used to study charge transfer and ionisation in collisions of H+ and He2+ with potassium atoms within the range 38-2070 keV amu. Separate cross sections for pure ionisation, transfer ionisation and electron capture leading to the formation of up to four-times ionised potassium have been determined and compared with the available theoretical estimates.

Cross-beam polarization in flames with a pulsed dye laser

We report the first application of a cross-beam polarization spectrometry technique with a pulsed dye laser in flames. Saturation, curve of growth, and dependence of the polarization signal on the crossing angle between the probe and pump beams have been experimentally investigated. The results indicate that polarization spectroscopy is a promising technique in the area of combustion diagnostics.