Probabilities for adiabatic or near-adiabatic state transformation within a highly excited shell of Li(n = 25) were studied experimentally and theoretically for a time dependent electric field, E⃗(t), and a constant magnetic field, B⃗. The fields were sufficiently weak and the time dependence slow enough such that only states belonging to the chosen shell were involved. The studies show that the dynamics are governed by the approximate hydrogenic character of the system in most cases, but for some specific time dependences it is influenced strongly by core interactions as expressed through the quantum defects, δl. The s-state is effectively decoupled from the rest of the n = 25 manifold due to a very large quantum defect. However the quantum defects of the p, d and f states are shown to play a decisive role in the dynamics. The core interactions lead to avoided crossings, non-adiabatic state transformations, and possibly even phase-interference effects. When a resonance condition pertaining to the hydrogenic character of the system is fulfilled, a linear Stark state is transformed completely into a circular Stark state oriented along E⃗f.

A new type of resonance is discovered in Rydberg atoms placed in a constant magnetic field -->B and a transient electric field that rotates at the constant frequency -->omega in a plane perpendicular to -->B. The dynamics is explained in terms of two pseudoparticles with spin 1 / 2 in two generalized magnetic fields. The resonance frequency is predicted and found at -->omega = (e/2m)-->B, where -e/m is the electron's charge-to-(reduced)mass ratio. We discuss the applicability of the resonance to accurate magnetic field measurements and the prospects for determining e/m with improved precision.

The evolution of Rydberg states of Li in time-dependent fields was studied with special emphasis on the influence of the Li(1s2)+ core. The strengths and rates-of-change of the fields were such that the dynamics involves states of only a single shell (intrashell dynamics). For sufficiently slow variations, the Rydberg states are transformed adiabatically but for faster variations, intrashell transitions do occur. We present experimental approximate probabilities for adiabatic transformation of the Rydberg states and compare with theoretical predictions with and without core effects included. The core can perturb the dynamics significantly. Circumstances under which this is likely to be the case are discussed.