The evolution of wave packets is investigated in crossed magnetic and electric fields. We report a direct observation of their propagation along classical trajectories in the time domain. Rydberg states of rubidium are excited with a short laser pulse. We monitor the evolution of the wave packet using a delayed, ionizing probe pulse. When the wave packet approaches the nucleus, the ionization signal is enhanced. The first return to the nucleus is observed and is in agreement with semiclassical predictions. Further evolution of the wave packet is also observed.

• Received 25 January 1993

DOI:https://doi.org/10.1103/PhysRevLett.70.2884