A Jain

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.

Electron scattering with methane molecules at 20–500 eV

We report quantum mechanical calculations of the differential, momentum transfer, and integral cross sections for electron‐CH4 elastic scattering in the energy range 20–500 eV. The e−‐CH4 interaction term is composed of three local spherical potentials, namely, the static, the exchange, and the polarization. The static potential is generated without additional approximations from near‐Hartree–Fock methane wave functions; the exchange interaction is included in the free‐electron‐gas‐exchange (FEGE) model; and for polarization, we employ a parameter‐free ab initio potential of Jain and Thompson. All the cross sections are in good accord with recent absolute experiments.

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.

Elastic scattering of electrons and positrons by CH4 at 25–800 eV

The differential and integral cross sections for the e±‐CH4 elastic scattering are reported in the energy range 25–800 eV. A simple model is used: assuming methane molecules to be spherical, simple analytic form of the static potential is derived by employing spherical wave function; this static potential, along with approximate polarization potential and a local exchange potential, is treated exactly to calculate cross sections via partial‐wave analysis. The integrated cross sections for electrons and positrons start approaching each other above 500 eV. The positron‐methane cross sections are found to be smaller everywhere and without any forward peak than the corresponding electron cross sections. The differential cross sections for the electron scattering are in very good agreement with the available experimental data.

Elastic scattering of intermediate- and high-energy electrons by N2 and CO in the two-potential coherent approach

Elastic differential, integral and momentum transfer cross sections are reported for the scattering of electrons by N2 and CO in the energy range 40-800 eV. The two-potential approach of Hayashin and Kuchitsu (1976) is employed, in which the total interaction potential is represented as a sum of the short- and the long-range potentials. It is found that the interference cross section term arising due to the coherent sum of the scattering amplitudes from the short- and the long-range parts of the interaction potential, is stronger than the pure long-range potential cross section term, of course, in the small angular region (<or=40 degrees ). The author also studies the multiple scattering within the molecule and finds that in the multiple scattering series the single-double (SD) term dominates over the pure double-double (DD) term in the whole energy region considered. However, there is evidence (below 100 eV) that the DD term is compatible with the SD term in the middle angles and exhibits some peaking behaviour at about 90 degrees . To evaluate elastic scattering amplitudes for each potential, a partial-wave analysis was performed. Results are discussed and compared with the recent absolute measurements. All the authors results are in very good accord with the observed data.

Elastic scattering of intermediate and high energy electrons with N2 and CO molecules

The elastic scattering of intermediate and high energy electrons with N2 and CO molecules is investigated in a two-potential coherent approach in which the intramolecular scattering effects are also considered. The short-range static potentials centred at both the atoms are derived from target molecular wavefunctions. Results are compared with recent measurements and the corresponding calculations using Yukawa-type atomic scattering factors. The new results are in better accord with experiment.