Dynamics of Rydberg states in crossed E and B fields: Coherent elliptic states

The dynamics of the n = 25 shell of Li atoms in weak electric, E, and magne tic, B, fields was studied experimentally as a function of the rare of chan ge of the electric field strength, epsilon = \dE/dt\, the strength, B, of a constant magnetic field and the angle between the two fixed field directio ns, phi. Prior to the variation of E, it was held constant at E = E-0 with E-0 strong enough to dominate the Stark-Zeeman manifold for the shell. The uppermost state, which is a coherent elliptic state (CES), was populated se lectively by pulsed laser excitation. The field was subsequently varied wit hout rotation from +E-0 to -E-0 at the constant rate epsilon. It is shown e xperimentally, in accordance with theory, that the dynamics is described by a single dimensionless parameter zeta given by zeta = (sin phi/root n epsi lon)B in atomic units. When zeta greater than or equal to 5 the variation o f E is so slow that the electron has time to adjust and the population then remains in the uppermost state of the manifold (adiabatic transformation), for smaller zeta (similar or equal to 2) the population shifts to an inter val of states centred near the middle of the manifold and at zeta less than or equal to 0.6 the wavefunction is almost frozen and the lowermost part o f the manifold populated (diabatic transformation). The experimental findin gs for near-adiabatic evolution of the CES (zeta greater than or equal to 4 ) agree with theoretical results obtained in a non-relativistic, hydrogenic model but discrepancies are seen when non-hydrogenic states are populated for zeta < 4.