R. L. Ezell

Angular correlation of the N+ ions produced in the dissociative double ionization of nitrogen

The double ionization of N2 by He+ projectiles was studied by measuring the angular correlation between the two N+ ions emitted in the dissociation of N2+2 molecular ions. If there were no recoil velocity imposed on the N2+2 ion in the initial ionizing collision, and thermal effects were neglected, the pair of N+ ions would have equal and opposite velocity vectors in the laboratory frame of reference. Measuring the coincidence yield of pairs of N+ ions as a function of the angle between their velocity vectors permits an estimate to be made of the component of momentum transferred to the N2+2 parent ions in the beam direction. The results presented in this report show the recoil velocity to be considerably less than the mean thermal velocity of N2 molecules at room temperature. We also report mesurements of the relative cross section for N+ production from N2+2 as a function of the orientation of the N2 target molecules relative to the projectile beam axis.

Molecular orientation dependence for projectile-H2 collisions

Abstract The cross sections for double ionization, ionization plus excitation and double excitation of H 2 for equivelocity electrons and protons on H 2 have been measured as a function of orientation of the internuclear axis. The projectile energies are 1.0 and 2.0 MeV/amu. The data can be fit to an expression of the form σ ( θ ) = σ 0 (1 + A cos 2 θ + B cos 4 θ ) where σ 0 is the value of the cross section at 90°. The excited states of H + 2 include the 2pσ u , 2pπ u and 2sσ g .

Two-electron processes in molecular collisions

Abstract Orientation effects are reported for the ionization plus excitation of H 2 by electron bombardment. The yield of H + -fragment ions as a function of angle relative to the beam direction are reported for angles from 18° to 90δ and collision energies of 408, 545, and 1089 eV. Comparisons are made between experiment and theoretical predictions based on dipole interactions between projectile and target electrons.

Double ionization and ionization plus excitation of H2 by fast projectiles

Abstract The double-ionization cross sections of H 2 by electrons, protons, and He 2+ in the range of 125–3500 keV/amu have been measured. At 750 keV/amu the cross section can be expressed in the Born series σ ++ = 1.05 Z 2 − 0.69 Z 3 + 0.74 Z 4 (in units of 10 −20 cm 2 ) where Z is the projectile charge. The relative cross sections for excitation of dissociative states of H 2 + by electron bombardment have been measured.

Proton induced dissociative ionization of acetylene, ethylene and ethane

Abstract Measurements of relative total yields are reported for 2 to 10 eV positively charged fragments produced in the bombardment of acetylene, ethylene and ethane by 1.0 MeV protons and by 0.5 MeV deuterons. Yields for D + projectiles are higher than those for H + projectiles by more than an order of magnitude. Kinetic energy spectra and relative yields observed for H + n C + and CH + n ions are presented. The C 2 H 6 and C 2 H 4 spectra are dominated by sharply defined H + n peaks at about 4 eV and by C + and CH + n at about 2.5 eV. Additional structure is observed for C 2 H 2 spectra. Results are compared to available data from electron bombardment.

Two-electron processes in projectile-H2 collisions

Molecular hydrogen is a two-electron system that has been studied extensively such that its excited states,its potential curves and its ionization potential are well known.1 For the doubly excited states of H2, a number of potential curves have been calculated, 2 but they have not been subjected to the same level of experimental scrutiny as have the other H2 potential curves. The properties of its molecular ion H2+ are also well known, and its potential cruves have been tabulated for use in numerical calculations, s, 4 Molecular hydrogen has the unique property that for any process involving the excitation or ionization of both electrons, a final state is formed that will dissociate into two energetic fragments. One of these fragments is charged and can be easily detected. It is this property that enables the study of two-electron processes in projectile-H~ collisions. Figure 1 illustrates the interactions included in the model used to describe the experimental results. The Goldstone diagrams of Fig. l(a-c) depict those events which occur by a single projectile interaction and an electron-electron or electron-hole interaction. The direction of time is from the bottom of each figure to the top. Particles travel forward in time and holes are shown as propagating backwards. The interactions drawn with an X and a dashed line signify interactions between the projectile and the target electron. Figure l(d) corresponds to an uncorrelated double collision event.

Single and double ionization ofH2byHe2

Cross sections for the nondissociative single ionization and double ionization of H{sub 2} by He{sup 2+} projectiles in the range of 125--750 keV/amu are reported. The results are compared to impact-parameter calculations and to measurements with He{sup +} projectiles. The direct double-collision process is found to be the important contributor to double ionization in this range of energies. The single-ionization cross section, when compared to proton bombardment, does not scale as {ital Z}{sup 2}.