Ionization of high Rydberg states of calcium by electric fields

Measurements of electric field ionization of high Rydberg states populated by two-photon excitation are reported. We use a novel technique, in which t he extraneous ionization background due to other causes is suppressed, resu lting in higher accuracy and sensitivity than otherwise achievable. The met hod is applied to a series in the calcium spectrum which exhibits an intere sting anomaly around n = 63 in previous experiments by laser spectroscopy u sing thermionic diode detection. In the present experiments, collisions ale highly improbable, and it is demonstrated that electric field ionization t hen occurs without any detectable anomaly around n = 63. The implications o f this for the interpretation of the effect are discussed.