NEAR-THRESHOLD ROTATIONAL-EXCITATION IN ELECTRON POLAR-MOLECULE SCATTERING

Variational R-matrix calculations have been performed for electron scattering by the HF molecule. Calculations over a grid of internuclear distances are used to compute the vibronic R-matrix, using a new adiabatic phase matrix method described here. This body-frame vibronic R-matrix is converted to the rovibronic R-matrix by a modified frame transformation at the R-matrix boundary. The external rovibronic close-coupling equations are solved by analytic methods. This methodology makes it feasible to carry out full first-principles calculations including molecular rotation and vibration, although only vibrationally elastic results are reported in the present paper. The threshold behaviour of the body-frame results can be described in terms of dipole eigenfunctions. Validity and range of the analytic formula proposed by Clark are discussed quantitatively. Results in the laboratory frame show the influence of rotation on vibrationally elastic e-HF collisions. The computed cross sections at small energies, close to the rotational thresholds, show rather sharp peaks that are dominated by j --> j +/- 1 excitations. Analysis here shows that these peaks are multichannel threshold structures due to virtual states and that they are accompanied by threshold cusp structures. Scaling laws valid at rotational channel thresholds in the presence of the strong long-range dipole interaction are discussed. In the energy range of the rotational thresholds the differential cross section has a prominant forward peak. Good agreement is found with recent experimental results of Radle et al. The present results are compared with approximations of various levels of sophistication.