One-photon ionization from the 6s27d ${}_{}{}^1{}_{}{}^{}$${\mathit{D}}_2$ state in barium is measured with short (0.25–2.7 ps), high-intensity laser pulses. Fermi’s golden rule predicts that only the fluence (time-integrated intensity) determines the yield. We observed a decrease in the yield for fixed-fluence pulses shorter than the Kepler orbit time of the Rydberg electron (2.2 ps). This is explained semiclassically: The wave function of a Rydberg electron performs a Kepler-like orbit. Only the wave function near the core can be ionized. Not all of the wave function nears the core during a short pulse, and therefore the wave function far away from the core is stable against ionization. A quantum-mechanical calculation based on Raman transitions over the continuum agrees well with experimental observations and the semiclassical explanation.

• Received 25 May 1994

DOI:https://doi.org/10.1103/PhysRevA.50.4133