We report on an experimental study of the frequency dependence of the microwave ‘‘ionization’’ probability for laser-excited Rb(np${)}^2$${\mathit{P}}_{3/2}$(‖${\mathit{M}}_{\mathit{J}}$‖=1/2) Rydberg atoms. Most data are for n=84. The microwave field was produced by a low-noise frequency synthesizer, and the laser-excited atoms subsequently driven by microwaves were kept in an environment cooled to 77 K in order to minimize the perturbing effects of the thermal, blackbody-radiation field. The driving frequency covered 4–14-MHz intervals around three frequencies between 8.87 and 16.14 GHz. Data are shown for 1- and 5-μs interaction times, which correspond to the driving frequency being resolved by the atoms to about 1 and 0.2 MHz, respectively. The turnon and turnoff parts of the microwave pulse shape were slow, lasting about 6 ns (circa 70 field oscillations). For the narrow frequency intervals used, the microwave field amplitude was calibrated in situ with use of two-photon Rabi-nutation measurements, which also directly demonstrated the spatial uniformity of the driving field amplitude and its coherence over at least a 1-μs time scale. Within the frequency intervals studied experimentally, we have observed no sharp, frequency-dependent structure outside of that expected from experimental counting statistics. We give physical arguments based on experimental data and previous experimental and theoretical work on microwave ‘‘ionization’’ that show that the present experiments have been in the range of the scaled frequency above 1. Thus, these experimental data examine in a continuous fashion the frequency dependence of microwave ‘‘ionization’’ in the high-scaled-frequency regime. Finally, we discuss our results in the light of published theoretical discussions of what might be expected to be the frequency dependence of the microwave ‘‘ionization’’ probability for various quantal models for the one-dimensional hydrogen atom in this high-frequency regime.

• Received 28 October 1993

DOI:https://doi.org/10.1103/PhysRevA.49.3831