Coherent-state multipole moments in electron-hydrogen impact excitation

We present results of various model calculations for the density-matrix elements of hydrogen excited by electron impact. These density-matrix elements are discussed in terms of expectation values of physically understandable observable multipole moment operators and their time derivatives, thereby enabling us to isolate and understand central features of the collision dynamics hidden in the off-diagonal density-matrix elements. The strengths and weaknesses of these various scattering models, including plane-wave Born, distorted-wave Born, Glauber, classical trajectory, quasiclassical close-coupling, and close-coupling approximations are pointed out. The effects of angular momentum barriers, the impulsive nature of excitation, the mixing and splitting of degenerate energy eigenstates of the final state manifold, and the various ranges and times of the interactions occurring durng the collision are described. We observe that perturbative scattering models are inappropriate to describe certain of these multipole, even at very high energies. Experiments which determine these multipole-moment parameters are discussed.