HALF-CYCLE MICROWAVE IONIZATION OF RYDBERG ATOMS - BEYOND DIABATIC IONIZATION

Ionization of Rydberg atoms by short, half-cycle electrical pulses is investigated experimentally in the intermediate-pulse duration regime where the ionization threshold fields are determined both by the slew rate of the ionizing pulse and its duration. Electrical pulses with du rations ranging from 140 ps to 9 ns and amplitudes up to 200 V cm(-1) are applied to rubidium and lithium atoms prepared in high Rydberg sta tes by laser excitation (n ranges from 42 to 82). The amplitude of the 140 ps pulse required for ionization of the Rb and Li Rydberg populat ion is found to be a factor of two higher than the expected amplitude based on diabatic field ionization (F = 1/9n(4)) and recent theoretica l calculations by Reinhold et al (1993 J. Phys. B: At. Mol. Opt. Phys. 26 L659). Using the longer pulses we studied the transition from the diabatic/adabatic ionization towards ionization in the impulse-kick re gime.