D G Thompson

Exchange approximations in the scattering of slow electrons by polyatomic molecules

The Hara (1967) and asymptotically adjusted versions of the free-electron-gas exchange (FEGE) approximation are compared with the orthogonalisation technique for electron scattering by CH4 and H2O. In general both FEGE approximations are more attractive than the orthogonalisation approximation but there is evidence that the asymptotically adjusted version overestimates the exchange effects at low scattering energies.

The elastic scattering of electrons by nitrogen, neon, phosphorus and argon atoms

Results are presented for the elastic scattering of electrons by nitrogen and phosphorus atoms in the energy range 0-1 ryd, and by neon and argon atoms in the energy range 0-4 ryd. Calculations have been carried out in a single state approximation but include both exchange and polarization effects. The polarization potential has been determined using the polarized orbital method of Temkin. Results are in good agreement with experiment for neon and argon but it is found that the single state approximation is not sufficient for a correct representation of the nitrogen 3P resonance.

Elastic scattering of slow electrons by CH4 and H2O using a local exchange potential and new polarisation potential

A parameter-free polarisation potential is proposed for the elastic scattering of electrons by polyatomic molecules, based on the method of Pople and Schofield and the work of Temkin. Cross sections are obtained for CH4 and H2O using this polarisation potential and a Hara local exchange potential. Agreement with experiment is good.

Low-energy electron scattering by H2S molecules: elastic, rotational and vibrational excitation

Cross sections for elastic scattering, rotational excitation and vibrational excitation are reported for e-H2S scattering in the energy range 0.5-10 eV. Calculations have been carried out using the adiabatic fixed-nuclei approach. For vibration the symmetric stretch and bending modes have been considered within the nuclear impulse approximation. The fixed-nuclei scattering amplitudes have been determined for an electron-molecule interaction comprising: (i) a static potential derived from molecular ground-state near-Hartree-Fock functions; (ii) an exchange term in the free electron gas Hara-type approximation; (iii) a parameter-free polarisation potential. The convergence problems associated with scattering by a polar molecule are treated by the angular transformation approximation of Collins and Norcross (1978). The elastic and rotationally summed cross sections exhibit three distinct scattering regions: a threshold peak dominated by the A1 scattering state; an enhancement at about 2 eV due to the B2 scattering state-it is possible to extract the resonance parameters Er=2.22 eV and Gamma =1.31 eV; and a broader enhancement around 6-8 eV mainly due to A1 scattering. The B2 resonance is also responsible for a large enhancement in the vibrational cross sections at 2-3 eV. The shape of the rotationally summed and vibrational results are in good agreement with the experimental data of Rohr (1978).

Vibrational excitation of symmetric and bending modes of H2O by slow electron impact

Electron impact vibrational excitation cross sections (differential, integral and momentum transfer) for the symmetric (100) and the bending (010) modes of the H2O molecule are calculated in the fixed-nuclei and the impulse approximations from threshold to 10 eV. The normal coordinate dependent K-matrix elements are augmented with the laboratory frame first Born matrix elements for higher angular momenta in order to obtain converged cross sections via a closure formula. A sharp threshold peak and a broad resonance structure is seen around 6-8 eV in the integral cross section in agreement with the recent observations: while the A1 scattering state plays a not unimportant role, the major contribution to this broad feature is due to the B2 state. All results compare well with the crossed-beam experimental results of Seng and Linder (1974).

The scattering of slow positrons by CH4 and NH3

The fixed-nuclei approximation is used to investigate the collisions between slow positrons and methane molecules. A static potential and various models for the polarisation potential are used to calculate total elastic cross sections. There is fair agreement between recent experimental measurements and the results from these models. The authors also evaluate Zeff, the effective number of annihilation electrons of the molecule, taking into account the effect of distortion in the molecular orbitals due to the incoming positrons. They obtain, e.g. for CH4, 99.5 compared with an experimental value of 140-155. They argue that there is no need to postulate the formation of positron-CH4 complexes to explain the high value of Zeff. They also calculate Zeff for NH3 where the experimental value is 1300. A calculation ignoring charge-cloud distortion gives 241.

Enantio-selective collisions between unpolarized electrons and chiral molecules

A new enantio-selective process is considered: left- and right-handed molecules can scatter unpolarized electrons differently if true chirality is defined by the experimental geometry. This effect is caused by Coulomb forces resulting in significant asymmetries. The theoretical basis of this phenomenon is analysed within the framework of Barrons concept of chirality. Our conclusions are illustrated by numerical calculations for elastic electron collision from H2S2.

The scattering of low energy electrons by H2O and H2S

We present results for the scattering of low energy electrons (<15 eV) by H2O and H2S. We use a single-centre formalism and include exchange exactly and polarization through a parameter-free model potential. The exchange equations are solved using a linear algebraic method. We have calculated rotationally summed differential cross sections for both H2O and H2S; comparison with experiment is good. We also give results for rotational state-to-state transitions in H2O, calculated within the adiabatic approximation.

Low-energy electron-CH4 collisions using exact exchange plus parameter-free polarisation potential

Results are reported for the rotationally elastic, inelastic and summed cross sections (differential, integral, momentum transfer and energy loss)from electron-CH4 scattering in the 0.1-20 eV energy region. Electron exchange is treated exactly and distortion effects are included using the parameter-free model polarisation potential. Agreement with available experimental measurements is good The results are also compared with other theoretical treatments. The authors also report the value of the scattering length from the model.

Momentum transfer cross sections for the low-energy electron scattering by NH3 molecules

The fixed-nuclei and the adiabatic-rotation approximations are used to calculate rotationally elastic, inelastic and summed momentum transfer cross sections for the electron-NH3 scattering in the range 0.01-10 eV. A recently proposed model-potential approach of Jain and Thompson (1983), in which ab initio parameter-free polarisation potential is used along with the exact static and the appropriate exchange potentials, is employed for this purpose. Results are also obtained with the phenomenological empirical polarisation potential. The results with the new polarisation potential are in better agreement with the available experimental data.