Atomic pseudospin resonance.

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.