Pierre Defrance

Merged electron-ion beam experiments. V. Dissociative recombination of OH+, H2O+, H3O+ and D3O+

For pt.IV see ibid., vol.14, no.8, p.1353-671 (1981). Absolute cross sections for dissociative recombination of OH+, H2O+, H3O+ and D3O+ have been measured using a merged electron-ion beam apparatus through an energy interval from 0.01 to 1 eV. The recombination cross section for the diatomic OH+ is small, one of the smallest measured by the authors for a diatomic ion. The cross sections for the polyatomics H2O+, H3O+ and D3O+ are large and nearly identical. Like most other polyatomic ions they exhibit nearly an Ecm-1 energy dependence below approximately 0.1 eV. At higher energies the slope of the cross section versus electron energy is much steeper with an energy dependence near Ecm-2. Cross sections for D3O+ are less than those for H3O+ indicating the presence of a small isotope effect.

Merged electron-ion beam experiments. IV. Dissociative recombination for the methane group CH+,...,CH5+

The dissociative recombination of the methane-derived group, CH+,...,CH5+, has been measured over the centre-of-mass energy interval 0.03 to 2 eV. Assuming a Maxwellian distribution of the electron velocities, the derived rate coefficients for CH+,...,CH5+ at 100K are, respectively, 5, 8.7, 12, 12.8 and 12.8*10-7 cm3 s-1. The recombination cross section for the diatomic ion CH+ is large. This has far reaching consequences for the theories of molecular formation in interstellar clouds. The cross sections for the polyatomics CH2+,...,CH5+ are also large and roughly equal in magnitude. Like CH+ they exhibit a close to E-1 energy dependence below 0.1 eV. For CH3+, CH4+ and CH5+ the slope of the cross sections increases above 0.1 eV.

Electron impact dissociation and ionization of N-2(+)

Absolute cross sections for electron impact ionization, dissociative excitation (DE) and dissociative ionization of N-2(+) ions are measured in the energy range from threshold to 2500 eV. The animated crossed electron-ion beam method has been employed. The individual contributions of ionization products (N-2(2+)) and dissociation fragments (N+), which have both identical mass-to-charge ratio and average velocity, are deduced from the analysis of product velocity distributions. Particular attention was paid to determining the transmission efficiency for dissociation fragments, since their collection was incomplete during the measurements. Threshold energies and kinetic energy released to dissociation fragments are measured. The role of states contributing to different reactions is discussed. For DE, the present results are found to be much smaller than the results of Peterson et al (1998). For ionization (single and dissociative), a satisfactory agreement with their result is obtained as well as with the prediction of Kim et al (2000) obtained in the binary-encounter Bethe approximation.

Double ionization of neutral atoms, positive and negative ions by electron impact

New experimental data on double-ionization cross sections of neutral atoms, positive and negative ions by electron impact are reported. These and other relevant data are compared with recent multiple-ionization semiempirical formulae extended for the case of two-electron ionization. Theory gives analytical dependence of the double-ionization cross sections on the main parameters of the collision system in a wide energy range. Far the He atom, He-like ions and H- ion high-energy asymptotic behaviour of the double-ionization cross sections are considered on the basis of a quantum mechanical treatment. Suggested cross section scaling for fast collisions, employing a target-effective nuclear charge, is in good agreement with experimental data.

Electron-impact ionization of neon ions (q=4-8)

Absolute cross section measurements for electron-impact single ionization of Ne-q+ (q = 4-8) ions and for double ionization of Ne5+ and Ne6+ are reported. The animated crossed-beams method has been applied. The measurements cover the energy range from threshold to several keV. The Ne4+ and Ne6+ ion beams are found to contain an important population of ions formed in metastable states. For single ionization, results are in overall good agreement with direct ionization theoretical predictions, when available. Excitation-autoionization is observed in the ionization threshold region for Ne4+ and in the K-L transition energy region for Ne6+ and Ne7+. Double ionization of Ne5+ and Ne6+ is found to be dominated by K-shell ionization followed by autoionization.

Total cross sections and kinetic energy release for the electron impact dissociation of H-2(+) and D-2(+)

Absolute total cross sections have been measured for electron impact dissociative excitation and dissociative ionization of H-2(+) and D-2(+) in the energy range 5-3000 eV. The vibrational population of the primary H-2(+) beam has been analysed by dissociative charge exchange on a potassium target, and is in good agreement with the measurements of von Busch and Dunn (1972 Phys. Rev. A 5 1726). Kinetic energy release (KER) distributions have been extracted from momentum analysis of the released protons and deuterons at selected impact energies. A model calculation has been performed to interpret the different spectra. Below 100 eV, the distributions exhibit a sharp peak in the range 0-1 eV that is attributed to the dissociative excitation of high vibrational levels to the 2psigma(u) repulsive state in the vicinity of their outer turning point. This observation is consistent with the measured vibrational population extending up to upsilon = 13, as confirmed by the appearance threshold of the dissociative ionization (DI) channel. The KER distributions exhibit a second contribution peaking between 1 and 5 eV, resulting from the admixture of the (1ssigma(g) --> 2psigma(u)), (1ssigma(g) --> 2ppi(u)) and (1ssigma(g) --> 2ssigma(g)) electronic transitions. A distinctive hump is also present around 9 eV, that coincides both with the maximum of the DI contribution, and with the high-energy shoulder of the 2ppi(u) and 2ssigma(g) contributions. The present measurements are in qualitative agreement with the previous results of Caudano and Delfosse, and are fairly well reproduced by our first-order model.

Absolute cross sections and kinetic energy release distributions for electron impact ionization and dissociation of CD

Absolute cross sections for electron impact dissociative excitation and ionization of CD+ leading to the formation of ionic products ( D+, C+, C2+ and C3+) are reported in the energy range from their respective thresholds to 2.5 keV. Around the maximum, cross sections are found to be ( 10.5 +/- 1.0) x 10(-17) cm(2), ( 20.6 +/- 3.5) x 10(-17) cm(2), ( 1.20 +/- 0.11) x 10(-17) cm(2) and ( 8.2 +/- 1.5) x 10(-20) cm(2) for D+, C+, C2+ and C3+, respectively. In the very low- energy region, dissociative excitation leading to the C+ formation dominates over the D+ one. The cross section for dissociative ionization ( C+ + D+ formation) is found to be ( 6.9 +/- 1.3) x 10(-17) cm(2) around 105 eV and the corresponding threshold energy is ( 22.1 +/- 0.5) eV. The animated crossedbeams method is used and the analysis of ionic product velocity distributions allows the determination of the kinetic energy release distributions. They are seen to extend from 0 to 15 eV both for C+ and for D+, and up to 40 eV both for C2+ and for C3+. For singly charged products, the comparison of the present energy thresholds and kinetic energy release with published data allows the identification of states contributing to the observed processes. In particular, contributions from primary ions formed in the a(3)Pi metastable state are perceptible. At fixed electron energy, the cross sections for the various ionization channels are seen to reduce exponentially with the potential energy of each dissociated ion pair. Anisotropies are estimated to be in the range 8 +/- 2% for both C+ and D+. The total CD+ single ionization cross section calculated by application of the Deutsch - Mark formalism is found to be in good agreement with experimental results.

A crossed-beam experiment for electron impact ionization and dissociation of molecular ions: its application to CO+

A crossed electron-ion beam experimental set-up has been upgraded for the study of electron impact ionization and dissociation of molecular ions by means of ionic product detection. Both the experimental set-up and the data analysis procedures are described in detail for the estimation of ( i) absolute cross sections, ( ii) kinetic energy release distributions ( KERD) and ( iii) anisotropies of angular distributions. Absolute cross sections are obtained separately for dissociative excitation ( DE) and for dissociative ionization ( DI). A double focusing magnetic field analyser is used for the observation of product velocity distributions, in the laboratory frame, at selected electron energies. The KERD in the centre of mass frame is calculated from the measured velocity distribution as well as the anisotropy of the angular distribution with respect to the initial orientation of the molecular ions. Results are reported for dissociative ionization and dissociative excitation of CO+ to C+ and O+ fragments in the energy range from about 5 eV to 2.5 keV. Absolute cross sections for DE at maximum, i. e. for an electron energy around 35 eV, are found to be ( 9.69 +/- 2.08) x 10(-17) cm(2) and ( 6.24 +/- 1.33) x 10(-17) cm(2), for C+ and O+, respectively, and the corresponding threshold energies are found to be ( 8.5 +/- 0.5) eV and ( 14.8 +/- 0.5) eV. The DE process leading to C+ production is seen to dominate at low electron energies. For DI, the absolute cross section is found to be ( 12.56 +/- 2.38) x 10(-17) cm(2) around 125 eV and the corresponding threshold energy is ( 27.7 +/- 0.5) eV. KERDs, which extend from 0 to 24 eV both for C+ and O+, exhibit very different shapes at low electron energy but similar ones above 100 eV, confirming the role observed respectively for DE and DI. The groups of states contributing to the different processes are identified by comparing present energies thresholds values and the KERDs with theoretical values. Anisotropies are estimated to be in the range 3-6% for both C+ and O+.

Electron impact ionization of N4+ and N5+

Abstract A new crossed beams apparatus for measuring absolute electron impact ionization cross-sections of multiply charged ions is described. First measurements are also reported which deal with N 4+ and N 5+ . The results are in good agreements with those previously obtained by the Oak Ridge group. For N 5+ a non-negligible signal is observed below the ground state ionization threshold. It is attributed to the presence of ions in metastable states. From the semi-empirical Lotz formula, the population of ions in those states is estimated to be approximately 3%.

Absolute cross sections for electron impact ionization and dissociation of O-2(+)

Absolute cross sections for electron impact single ionization, dissociative excitation and dissociative ionization of O-2(+) were measured in the energy range from threshold to about 2.5 keV by means of a crossed beam set-up. The analysis of product velocity distributions has been applied in order to (i) separate the three processes and (ii) determine the kinetic energy release distribution of ionic fragments. Dissociation is seen to dominantly follow the ionization and the excitation processes, the maximum of the cross sections being found to be (2.20 +/- 0.09) x 10(-17) cm(2), (5.3 +/- 1.0) x 10(-17) cm(2) and (22.0 +/- 4.6) x 10(-17) cm(2) for simple ionization, dissociative ionization and dissociative excitation, respectively. The total kinetic energy released to the fragments is seen to extend up to about 20 eV.