M. A. Mangan

Limitations of the axial recoil approximation in measurements of molecular dissociation

The axial recoil approximation holds that when a diatomic molecular ion is formed in a dissociative state, the atoms produced in the dissociation process will move outward along the straight line defined by the internuclear axis of the molecule. Analysis of experiments measuring the angular distribution of Auger electrons emitted by N2 following K-shell ionization of N2 molecules shows that the axial recoil approximation is not strictly true. Significant corrections must be made for the rotation of the molecule during the time of dissociation. Smaller corrections must be made for the thermal distribution of the translational velocities of the target molecules, and for instrumental effects. In the analysis of the N2 data, the corrections have the effect of smoothing the predicted angular distribution functions. The amount of the smoothing depends primarily on the temperature of the target gas and the shape of the potential-energy curve for the N22+ final state involved in the Auger transition.

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 .

Excitation of the 2pu state of H2+ as a function of energy loss

The relative cross section for the excitation of the 2pu state of H2+ by 400 eV electrons as a function of energy loss has been measured and calculated. Electrons scattered at 18° with energy losses ranging from 30 to 72 eV were detected in coincidence with H+ ions of 7.7 eV. The H+ ions were detected at 72° relative to the beam direction. Because of the rapid dissociation of the excited state, it follows that the detected ions were produced by target H2 molecules with internuclear axes oriented at 72°. The relative cross sections were calculated using prolate spheroidal coordinates and fitted to the data with one parameter. The general shape of the energy-loss curve is in agreement with the measurements.

The angular distribution of Auger transitions in N2

Abstract The angular distributions of Auger electrons emitted by nitrogen molecules are reported. The distributions are measured relative to the orientation of the internuclear axis of the molecule. The initial inner-shell vacancies are produced by bombardment with 1634 eV electrons. The ensuing Auger transitions are followed promptly by the dissociation of the doubly charged molecular ions. The experiment measures the counting rate for the coincident detection of an N+ fragment and an Auger electron of the appropriate energy. Since rotation times are long compared to dissociation times, the axial recoil approximation holds, and detection of the N+ fragment serves to determine the orientation of the target molecule. The data will be discussed in terms of a two-center model in which prolate spheroidal functions are employed.

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.

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.