Cross sections for rotational excitation of CH_4 by 3–20-eV electrons

We report calculated differential, integral, and momentum-transfer cross sections for rotational excitation of CH_4 by electron impact in the 3–20-eV energy range. These cross sections were derived from fixed-nuclei scattering amplitudes obtained using the Schwinger multichannel method. Our results represent the first rotational excitation cross sections obtained for a polyatomic molecule using entirely ab initio procedures. The cross sections agree well with those of earlier model-potential calculations. A comparison of these calculated cross sections with available experimental data is in general encouraging, but some discrepancies remain

Electronic excitation of oriented molecules by low-energy electrons: An application to H2

We report inelastic differential cross sections for electronic excitation of an oriented molecule by low-energy electrons. Specifically we look at the dependence of these cross sections for the X1Σg+→b3Σu+ transition in H2 on both incident and scattered angles as well as on impact energy. These electron scattering cross sections exhibit a pronounced dependence on the incident and scattered angles, which suggests that related electron-energy-loss spectroscopy studies can be a useful probe of adsorbate-substrate structure

Schwinger multichannel method: A study of a Feshbach resonance in e-H_2 collisions

We present results of the application of the Schwinger multichannel method to study the role of Feshbach resonances on the elastic and electronic excitation cross sections in e-H_2 collisions. Using a ^2Σ_g^+2 resonance associated with the a ^3Σ^+_g and E^1Σ^+_g states of H_2 as an example, we quantitatively illustrate the dramatic influence of open channels on the decay of such Feshbach resonances in electron-molecule collisions

Studies of electron-molecule collisions: Applications to e-H2O

We report elastic differential and momentum transfer cross sections for the elastic scattering of electrons by H2O for collision energies from 2 to 20 eV. These fixed-nuclei static-exchange cross sections were obtained using the Schwinger variational approach. In these studies the exchange potential is directly evaluated and not approximated by local models. The calculated differential cross sections, obtained with a basis set expansion of the scattering wave function, agree well with available experimental data at intermediate and larger angles. As used here, the results cannot adequately describe the divergent cross sections at small angles. An interesting feature of the calculated cross sections, particularly at 15 and 20 eV, is their significant backward peaking. This peaking occurs in the experimentally inaccessible region beyond a scattering angle of 120°. The implication of this feature for the determination of momentum transfer cross sections is described

Applications of the Schwinger multichannel method to electron-molecule collisions

We discuss some recent developments in the implementation of the Schwinger multichannel method for electron-molecule collision calculations. The evaluation of matrix elements involving the operator VG^(+)_PV, previously accomplished by insertion of a Gaussian basis on either side of G^(+)_P, is now done by direct numerical quadrature. This approach avoids the necessity of very large Gaussian basis sets, allowing the size of the basis to reflect only the dynamical requirements of the scattering wave function. We find that the reduction in the required basis size results in improved efficiency, in spite of the additional numerical effort of performing the quadrature. Trial applications to electron-CH4 scattering in the static-exchange approximation and to electronic excitation of H_2 illustrate the excellent convergence characteristics of the procedure

A theoretical study on the photoionization of the valence orbitals of phosphine

We report a theoretical study on the photoionization of phosphine in the static-exchange level and frozen core approximation, using the method of continued fractions. The main subject of the present study is to investigate in which extent the Hartree-Fock description of the target applied to molecular photoionization is valid. Also, the role played by multichannel coupling is analysed. Our study shows that single-channel Hartree-Fock calculations can provide reliable results except for photon energies near the photoionization threshold