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

Low-energy electron-impact excitation of the ã^3 B_(1u) (π→π∗) state of ethylene

A two‐state close‐coupling calculation of electron‐impact excitation of ethylene to the ã ^3B_(1u) state (π→π*) is carried out using a version of the Schwinger multichannel method developed for distributed‐memory parallel computers. The calculated integral cross section shows a sharp rise at threshold, in agreement with available experimental data. The integral and differential cross sections are useful in understanding the mechanism of this excitation process and indicate the possible presence of a core‐excited shape resonance near threshold

Spin exchange in elastic e-O_2 collisions

Recent experiments using polarized electron beams have shown that spin exchange effects for elastic collisions with O_2 and NO are much smaller than for Na atoms. We report calculated spin-flip differential cross sections for elastic collisions of polarized electrons with O_2 in agreement with experiment. In general, we can attribute the large variation of the spin-flip differential cross sections to resonances and interference effects. Such features arising from interference, however, may be washed out in the molecular case due to the average over orientations. Calculations with oriented O_2 molecules show strong exchange effects as for Na

Collisions of polarized electrons with O_2

Recently several methods have been developed and applied to studies of electron‐molecule collisions at low impact energies. These approaches include the R‐matrix, Linear algebraic, complex Kohn, and Schwinger multichannel (SMC) methods. In principle, these methods can account for important physical effects in such collisions which arise from open and closed electronic channels. In this paper we will report some results of preliminary studies of collisions of low‐energy electrons with molecular oxygen, an open‐shell system. In contrast to closed‐shell systems such as H_2 and N_2, the electron polarization may change in collisions with these open‐shell targets due to spin‐exchange processes. We have determined these spin‐exchange effects for elastic e‐O_2 collisions and found them to be much smaller than those seen in alkali atoms such as Na. Our results compare well with those of recent measurements of these effects for O_2 by Hanne and his collaborators

Multichannel Schwinger variational cross sections for electron-impact excitation of the b 3Σu+ state in H_2

As a first application of the Schwinger multichannel theory, we have calculated integral and differential cross sections for electron-impact excitation of the transition X 1Σg+→b 3Σu+ in H2 for scattering energies from 13 to 30 eV at the two-state level. We find good agreement between our integral cross sections and the results obtained previously in a two-state close-coupling study. Our method does not rely on single-center expansions to calculate the body-frame scattering amplitude and is designed to be applicable to molecules of arbitrary geometry

Studies of electron–polyatomic-molecule collisions: Applications to e-CH4

We report the first application of the Schwinger multichannel formulation to low-energy electron collisions with a nonlinear polyatomic target. Integral and differential cross sections are obtained for e-CH4 collisions from 3 to 20 eV at the static-plus-exchange interaction level. In these studies the exchange potential is directly evaluated and not approximated by local models. An interesting feature of the small-angle differential cross section is ascribed to polarization effects and not reproduced at the static-plus-exchange level. Our differential cross sections are found to be in reasonable agreement with existing measurements at 7.5 eV and higher energies

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

Application of the Schwinger multichannel formulation to electron-impact excitation of the B 1Σu+ state of H2

In this paper we report cross sections for electron-impact excitation of the X 1Σg+→B 1Σu+ transition in H2 for collision energies of 15, 20, and 30 eV. For this dipole-allowed transition with its associated long-range potential, the contributions of the more strongly scattered low-angular-momentum partial waves to the cross section were obtained from a two-state Schwinger multichannel calculation, and a modified Born-closure scheme was used to include the contributions from the remaining weakly scattered partial waves. Agreement between the calculated differential cross sections and available experimental data is encouraging