Junfang Gao

Theoretical calculation of fully differential cross sections for electron-impact ionization of hydrogen molecules

We have recently proposed the orientation averaged molecular orbital (OAMO) approximation for calculating fully differential cross sections (FDCS) for electron-impact ionization of molecules averaged over all molecular orientations. Orientation averaged FDCS were calculated for electron-impact ionization of nitrogen molecules using the distorted wave impulse approximation (DWIA) and the molecular three-body distorted wave (M3DW) approximation. In this paper, we use the same methods to examine the FDCS for ionization of hydrogen molecules. It is found that the DWIA yields reasonable results for high-energy incident electrons. While the DWIA breaks down for low-energy electrons, the M3DW gives reasonable results down to incident-electron energies around 35 eV.

Coplanar symmetric and asymmetric electron impact ionization studies from the 1b1 state of H2O at low to intermediate impact energies

(e, 2e) ionization differential cross sections are presented for incident electron energies ranging from 15 eV to 95 eV above the ionization threshold of the 1b1 molecular state of H2O. Experimental results and theoretical analysis were derived for three energies in a coplanar symmetric geometry, and for three energies in an asymmetric geometry. The experimental data show a wide variation in the cross section over this range of energies, whereas the theoretical analysis carried out using a sophisticated molecular DWBA model, which includes the final state post collision interaction (PCI), shows best agreement at lower energies. The experimental techniques used to collect the data are described here as well as an improved theoretical approach using elastic scattering cross sections to evaluate the accuracy of the distorted waves utilized in the calculation of the ionization cross sections.

(e, 2e) ionization measurements from the 3σg and 2σu* states of N2—comparison between experiment and theoretical predictions of the effects of exchange, polarization and interference

Gao et al (2005 Phys. Rev. A 72 032721) have predicted a Youngs type interference effect in the fully differential cross sections for ionization of the 3σg state of N2 for highly asymmetric collisions with one electron detector fixed at very small scattering angles (1° or 10°). The purpose of this work was to look for this interference effect at a larger scattering angle. (e, 2e) ionization measurements have been conducted from the 3σg and 2σu* states of N2 in a coplanar asymmetric geometry, where one electron emerges in the forward direction and the correlated electron is measured as a function of scattering angle. Both final-state electrons have an energy of 30 eV, and the forward scattering angle was θa = 22° relative to the incident beam direction. The theoretical prediction is that there should be a strong interference peak near 180°. The measurements were carried out from the 3σg state over a range of scattering angles from θb ~ 10° to θb ~ 170° using a magnetic angle changing spectrometer. The present experimental results for 3σg find a normal binary peak plus another peak at back angles in the vicinity of 180°. Consequently, this work supports the possibility of a strong Youngs type interference effect for small fixed scattering angles.

Theoretical Treatment of Electron-Impact Ionization of Molecules

There is currently no reliable theory for calculating the fully differential cross section (FDCS) for low energy electron‐impact ionization of molecules. All of the existing experimental FDCS data represent averages over all molecular orientations and this can be an important theoretical complication for calculations that are computer intensive. We have found that using an averaged molecular orbital is an accurate approximation for ionization of ground states. In this paper, we will describe the approximation, discuss its expected range of validity and show some FDCS results using the approximation for ionization of H2 and N2.