Crossed Beam Technique Research Articles

Dissociative excitation of the singly charged lead ion in electron collisions with PbCl2 molecules

Inelastic collisions of slow electrons with lead dichloride molecules yielding excited lead ions in a single encounter event were studied by the extended crossed-beam technique. At an incident electron energy of 100 eV, 67 cross sections for dissociative excitation of PbII spectral lines were measured. Three optical excitation functions were determined in the electron energy range 0–100 eV. The obtained results are compared with data on excitation cross sections of PbII in electron-atom collisions.

A crossed-beam study of the reaction C( [formula omitted]D)+H 2(X [formula omitted], v=0) [formula omitted]CH(X [formula omitted], v′)+H( [formula omitted]S)

Product angular and time-of-flight distributions have been measured for the first time for the prototypical insertion reaction C( 1 D)+H 2 using the crossed-beam technique with mass spectrometric detection at the collision energy of 1.86 kcal mol −1. Center-of-mass angular and kinetic energy distributions have been derived for CH(X 2 Π , v′=0)+H products and compared with those of statistical calculations based on phase-space theory.

Light-ionic fragment production in dissociative electron–molecular-ion collisions: Detection ofD+andD2+fromNDn+(n=2–4)andODn+(n=2,3)

Absolute cross sections are reported for the electron impact dissociative excitation of the deuterated ions ${\mathrm{ND}}_{n}^{+}$ $(n=2\ensuremath{-}4)$ and ${\mathrm{OD}}_{n}^{+}$ $(n=2,3)$ from threshold up to 70 eV using a crossed beams technique. The method focuses on measuring light dissociation-product ions from a heavy target. For all studied targets, ${\mathrm{D}}^{+}$ formation is the dominant channel, with absolute cross section values in the range of $(1.3\ensuremath{-}2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{\mathrm{cm}}^{2},$ once the initial rise from the threshold is past, continuing to 70 eV. The present measurements for dissociation of ${\mathrm{OD}}_{2}^{+}$ and ${\mathrm{OD}}_{3}^{+}$ yielding ${\mathrm{D}}^{+}$ are compared with recent results obtained in the storage ring experiments, these cross sections are found to be in a reasonable agreement within the quoted uncertainties over the part of the energy range where they are expected to be the same. The cross sections for obtaining ${\mathrm{D}}_{2}^{+}$ are about an order of magnitude smaller than the cross sections for ${\mathrm{D}}^{+}$ formation from the same targets.

Enzyme immunoassay using a crossed-beam thermal lens technique

An enzyme immunoassay based on the use of crossed-beam thermal lens detection is described. In this assay, poly- N-isopropylacrylamide, a water-soluble, thermally precipitating synthetic polymer, was used as a carrier to minimize non-specific binding. The enzyme substrate of the horseradish peroxidase that was employed was 3,3′,5,5′-tetramethylbenzidine. The color development of the enzyme–substrate reaction was stopped by SDS and Na 2SO 3 to achieve a stable blue solution. The background reduction and stabilization made it possible to use a crossed-beam thermal lens technique as the measurement method. This method was demonstrated to be applicable by determination of hepatitis B surface antigen in human serum. A detection limit of 0.15 ng/ml was obtained. This was more sensitive than that of the commercially available ELISA method.

Inelastic Collisions of Slow Electrons with Cadmium(II) Iodide Molecules

Inelastic collisions of electrons with cadmium diiodide molecules were studied by the extended crossed-beam technique. Cross sections for dissociative excitation of spectral lines of the excited atom and the singly charged ion of cadmium were measured at an incident electron energy of 30 eV. The obtained results are compared with data on direct excitation cross sections in e–Cd collisions.

Processes involving electron capture and multiple ionization in collisions of fastH+andHe2+ions with lead atoms

A crossed-beam technique incorporating time-of-flight analysis and coincidence counting of the collision products has been used to study ${\mathrm{Pb}}^{q+}$ formation with q up to 8 in collisions between ground-state Pb atoms and ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{2+}$ ions within the range 50--600 keV ${\mathrm{amu}}^{\mathrm{\ensuremath{-}}1}$. The separate cross sections for simple charge transfer, transfer ionization, and pure ionization leading to the formation of ${\mathrm{Pb}}^{q+}$ ions have been obtained and the relative importance of these processes has been established. Accurate measurements and rigorous theoretical descriptions of these multielectron processes in such heavy atoms are difficult and data are still very limited. The present measurements have been designed to extend our previous studies of multiple ionization of a few selected heavy metal atoms and to provide a further check on the extent to which the main collision processes can be described quantitatively in terms of simple models based on an independent electron description. In our previous work with Fe, Cu, and Ga atoms using the same experimental approach, we were able to describe the formation of multiply charged ions through both transfer ionization and pure ionization with a high degree of success using an independent electron model. However, the present results for Pb show that the success of this simple approach is much more limited for these much heavier atoms.

Electron-impact ionization of Fe 5+ ions

Employing the dynamic crossed-beams technique, absolute cross sections for single, double, and triple ionization of Fe 5+ ions by electron impact have been measured from the threshold up to 1 keV, partly up to 5 keV. The data for single ionization are in very good agreement with configuration-average distorted-wave (CADW) calculations if excitation-autoionization processes are taken into account. The results for double and triple ionization are in poor agreement with semiempirical predictor formulae. Using a high resolution energy-scan technique, we have studied narrow resonances in the double ionization cross section in the energy range from 560–940 eV. Besides contributions from excitation-double-autoionization processes, a remarkable number of structures arising from resonant-excitation-triple-autoionization processes could be observed.

Electron impact double ionization of multiply charged iron ions

Using the crossed-beams technique, cross sections for electron impact double ionization of Feq+ ions in charge states q = 1 - 6 (except 2) are measured in the energy range from the respective ionization thresholds up to 1 keV, partly up to 5 keV. The data for Fe4+, Fe5+ and Fe6+ show significant contributions from inner-shell processes. Using a high-resolution energy-scan technique, we study narrow resonances in the cross sections for Fe4+ and Fe5+. Besides contributions from excitation-autoionization processes, a remarkable number of structures arising from resonant-excitation triple-autoionization processes are observed. The results are compared with the scaling laws of Fisher et al and the semiempirical formula published by Shevelko and Tawara, respectively.

Electron impact single ionization of multiply charged iron ions

Employing the animated crossed-beams technique, absolute cross sections for electron impact single ionization of the iron isonuclear sequence are measured for charge states q = 1-6 for electron energies from threshold up to 1 keV, as well as for the intermediate charge states q = 9,10 up to an electron energy of 5 keV. The cross sections observed for Feq+ ions in charge states q = 1 - 4, 9 and 10 show a significant ionization signal below the respective ground state threshold resulting from ions in excited, long-lived metastable states in the parent ion beam. In the case of Fe5+ and Fe6+ no metastable components are found. The experimental results are in good agreement with the theoretical predictions if excitation-autoionization processes are taken into account. Our measured cross sections are also in good agreement with experimental results of other groups.

Electron Impact Ionization of Gallium Ions

Electron impact ionization cross sections σq,q+n for gallium ions have been measured for the reaction e + Gaq+ → Ga(q+n)+ + (n + 1)e for single (n = 1, q = 1, 3, 8) and triple (n = 3, q = 3, 4) ionization. The measurements have been performed using the crossed-beams technique in an energy range from the respective ionization threshold up to 1 keV, partly up to 6 keV.The cross sections for single ionization of ions in the charge state q = 1 and 3 show contributions below the ground state threshold caused by ionization of ions in excited, long-lived states in the parent ion beam. The cross sections are compared with the semiempirical Lotz formula.The cross section for Ga4+ triple ionization shows contributions from inner-shell processes. Semiempirical formulae are in good agreement with the experimental data only at energies below the cross section maximum.

Electron Capture in Collisions between Multiply Charged Ions

A crossed-beams technique has been used to measure total cross sections for charge transfer in 3He2+ + Xq+ collisions (Xq+ = N4+, O5+) at barycentric energies between 8 keV and 200 keV. The cross section for charge transfer in He2+ + N4+ collisions is calculated within the molecular model and shows good agreement with the experimental data.

An experimental study of electronic excitation and electron capture processes in Rb(52S1/2) collision with Cs+ and Li+ ions

The collisional systems Rb–Cs+ and Rb–Li+ (with all species in the ground entrance channel) have been studied using the crossed beams technique combined with decay fluorescence measurements. In the former system, only the emissions from the excited atoms Rb(52P1/2,3/2) and Cs(72P1/2,3/2) have been observed, while for the latter the electron exchange channels Li(22P1/2,3/2) and Li(32P1/2,3/2) were present, all of them giving signal strengths much lower than those shown by the same processes for other alkali atom–alkali ion collisional pairs.The collision energy–excitation cross section dependence for all four channels has been determined, as well as the branching ratio between J-states for those involving Cs ions as projectiles. A qualitative interpretation of the observed inelastic processes is performed using abinitio one-electron calculations.

Experimental Investigations of Single-Electron Detachment Processes from H− ions Colliding with MeV/u, Highly Charged Ions

Single electron detachment processes from negative hydrogen ions under collisions with MeV/u highly charged ions have been investigated using the so-called crossed-beams technique. The preliminary results of the single-electron detachment cross sections obtained is found to be in crude agreement with some empirical and theoretical estimations.

Elastic scattering of electrons by polyatomic molecules in the energy range 300-1300 eV: CO, , , H and

Differential cross sections (DCS) for the elastic scattering of electrons have been measured by the crossed-beam technique for CO, , , H and H in the angular range - and at impact energies of 300, 500, 700, 900, 1100 and 1300 eV. The measurements were made with a view to test the validity of the independent atom model (IAM) for polyatomic molecules in this energy region. A comparison of experimental results with the IAM calculations indicates that the model is adequate for the linear molecules CO and in the energy region 900-1300 eV. The Born approximation for the evaluation of the atomic scattering amplitudes was found to be valid only at 1300 eV for all the molecules except for H. A survey of the previous experimental and theoretical data based on different models shows that proper molecular wavefunctions have to be used for the evaluation of short-range potentials and that valence-bond distortion, absorption and non-adiabatic effects in the polarization potential have to be considered.

Beam loss in storage rings caused by ion–ion collisions

By means of the crossed-beams technique we have measured absolute cross sections for electron capture in collisions between two Hg + ions and between two Ta + ions at center-of-mass energies ranging from 1 to 45 keV. Based on these data particle loss cross sections for beam intensity loss due to intrabeam ion–ion collisions are estimated. By application of the beam temperature concept the expected loss in the storage rings of the HIBALL-II scenario is roughly estimated.

Electron-impact multiple ionization of singly and multiply charged titanium ions

Using the crossed-beams technique the following absolute cross sections have been measured for the reaction : <PRE/> double ionization triple ionization quadruple ionization . Measurements for high charge states and ionization degrees, respectively, show significant contributions of inner-shell processes which partly dominate the respective cross section. The results are compared with the scaling laws of Fisher et al and the semiempirical formula published by Shevelko and Tawara, respectively.

Electron-impact single ionization of low-charged titanium ions

The electron-impact single ionization of low-charged Ti ions was studied both experimentally and theoretically. Absolute cross sections for ions in charge states q = 1 to 6 except 3 were measured using a crossed-beams technique as well as calculated with the configuration-average distorted-wave method, including both direct ionization and excitation-autoionization contributions. For charge states q = 4, 5 and 6, the measurements are in good agreement with the calculations considering that large components of the incident ion beam were in metastable configurations. The cross sections for single ionization of and were determined in part with a high-resolution energy-scan method. Besides contributions of excitation-autoionization mechanisms, a remarkable number of structures arising from resonant-excitation double-autoionization (REDA) processes could be observed.

Electron-impact dissociative excitation ofCDn+(n=2−5): Detection of light fragment ionsD+andD2+

Absolute cross sections are reported for the electron-impact dissociative excitation of the deuterated hydrocarbon ions ${\mathrm{CD}}_{n}^{+}$ $(n=2\ensuremath{-}5)$ from threshold to 70 eV using a crossed beams technique. The method focuses on detecting and measuring light dissociation-product ions from a heavy target. The ion targets are typical of those formed in the discharge ion source, and some may be in excited electronic and/or vibrational states. A common feature of the studied targets was a dominant ${\mathrm{D}}^{+}$ formation cross section with almost the same absolute value of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ for all $n$ from 25 to 70 eV. The present measurements are compared with the available data, and for ${\mathrm{D}}^{+}$ from ${\mathrm{CD}}_{2}^{+}$ and ${\mathrm{CD}}_{5}^{+}$ the data agree well with recent results obtained with the storage-ring technique.

Electron-impact ionization of Mg

A crossed beam technique is used to measure ionization cross sections of metallic atoms. Relative values of cross sections of single, double and triple ionization of magnesium have been successfully measured over the 0-700 eV range with an estimated accuracy of 11%, 19% and 25%, respectively. Absolute values of cross sections have been obtained by normalization to a theoretical value at high electron energy. The theoretical value is calculated by using the generalized oscillator strength approximation. Results are compared to previously published values and, for single ionization in particular, a comparison with theoretical cross sections is performed.

Electron impact ionization of the hydrogen-like ions B4+, C5+, N6+ and O7+

For the first time absolute cross sections for electron impact ionization of the hydrogen-like ions B4+, C5+, N6+ and O7+ at electron energies from below threshold up to about 6 keV have been measured using the crossed-beams technique. All measured cross sections are in very good agreement with available distorted-wave exchange calculations and the semiempirical Lotz formula. The behaviour of the classically scaled cross sections along the hydrogen isoelectronic sequence is discussed. A recently developed scaling technique allows us to predict all non-relativistic cross sections along the isoelectronic sequence on an absolute scale.