AB-INITIO COMPLEX KOHN CALCULATIONS OF DISSOCIATIVE EXCITATION OF METHANE - CLOSE-COUPLING CONVERGENCE STUDIES

Results of close-coupling calculations using the complex Kohn method are reported for excitation of the methane molecule into electronically dissociative states. The excited states in question are of Rydberg nature and are classified by analogy with the isoelectronic Ne atom as 2p --> 3s, 2p --> 3p, and 2p --> 3d transitions, i.e., single excitations of the valence 2p electron into the n = 3 Rydberg manifold. On the basis of optical selection rules, one expects large excitation probabilities to analogues of 1P odd-parity states. Our calculations in the energy range 11-60 eV show a more complex picture of the excitation dynamics and detect large excitation probabilities for triplet and even-parity (2p --> 3p) states. A similar result is obtained for the excitation of the Ne atom. Although, to the authors's knowledge, these are the largest close-coupling electron-polyatomic-molecule scattering calculations to date, the results lead to questions concerning the convergence of the close-coupling approach in the case of Rydberg-state excitation. Conclusions are drawn in the context of earlier results for electron-impact excitation of the H, He, and Ne atoms.