It is shown both experimentally and theoretically that a Rydberg circular atom keeps its maximum momentum only if it is submitted to a nonzero static electric field whose direction is constant or slowly varying. The minimum characteristic time of the field-allowed rotation is determined. The implications of this effect for a future high-resolution microwave spectroscopy experiment on circular Rydberg atoms is discussed.

• Received 19 September 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.3160