  We present an experimental and theoretical study of the chaotic ionization of
quasi-one-dimensional potassium Rydberg wavepackets via a phase-space turnstile
mechanism. Turnstiles form a general transport mechanism for numerous chaotic
systems, and this study explicitly illuminates their relevance to atomic
ionization. We create time-dependent Rydberg wavepackets, subject them to
alternating applied electric-field "kicks", and measure the electron survival
probability. Ionization depends not only on the initial electron energy, but
also on the phase-space position of the electron with respect to the turnstile
--- that part of the electron packet inside the turnstile ionizes after the
applied ionization sequence, while that part outside the turnstile does not.
The survival data thus encode information on the geometry and location of the
turnstile, and are in good agreement with theoretical predictions.
