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 the extraneous ionization background due to other causes is suppressed, resulting in higher accuracy and sensitivity than otherwise achievable. The method is applied to a series in the calcium spectrum which exhibits an interesting anomaly around n = 63 in previous experiments by laser spectroscopy using thermionic diode detection. In the present experiments, collisions ale highly improbable, and it is demonstrated that electric field ionization then occurs without any detectable anomaly around n = 63. The implications of this for the interpretation of the effect are discussed.