The drastic perturbation of high Rydberg levels in a magnetic field is interpreted as a prototype effect of wave propagation on a ridge with formation of standing waves and with diffraction from radial into angular excitation modes. Analogs of this phenomenon include two-electron excitations into continuum or high Rydberg levels and may well be quite numerous since ridges occur generally on molecular potential surfaces. The Wannier-Peterkop-Rau theory of two-electron detachment at threshold is shown to provide the essential mathematical model for the description of these phenomena in terms of a base set of two eigenmodes that mediate the diffraction process. The apparent limitation of the original theory to an infinitesimal range of coordinates straddling the ridge is lifted. The problem remains of utilizing the eigenmodes as mediators of the coupling among the complete sets of adiabatic modes that seem to afford an adequate description of phenomena away from the ridges, that is, in potential valleys.

• Received 20 May 1980

DOI:https://doi.org/10.1103/PhysRevA.22.2660