Nonexponential Decay of Autoionizing Shock Wave Packets

Femtosecond laser pulses have been used to promote calcium atoms in stationary $4snd$ Rydberg levels to superpositions of $4pnd$ autoionizing states. The rapid excitation of the $4s$ core electron generates a ``shock wave packet'' that propagates through the previously stationary Rydberg wave function. The nonexponential decay of the autoionizing wave packet is probed using a second isolated-core excitation from the $4pnd$ to the $4dnd$ autoionizing series. Our measurements are in good agreement with the predictions of time-dependent multichannel quantum defect theory.