Abstract

When multiphoton ionization proceeds through an intermediate resonance in the Rydberg quasicontiuum a number of new features are introduced that are not found when the intermediate resonance is an isolated lower atomic state. First, a coherent superposition of the Rydberg states is excited that forms a radially localized wave packet. This wave packet then proceeds to move along a classical Kepler trajectory away from the nucleus. This introduces a new time scale into the problem. The rate of ionization depends critically on the relative size of the Kepler orbital period, the laser pulse duration, and the multiphoton Rabi period.When the wave packet moves away from the nucleus the rate of ionization is drastically reduced, and the population is trapped in a superposition of the Rydberg states. However, if the wave packet has time during the laser pulse to travel back to the nucleus, then the ionization rate is actually enhanced during the short period when the packet is near the nucleus, and the total ionization probability increases substantially. The total ionization signal is then increased by a discrete jump each time the wave packet returns. When the wave packet is produced by the bandwidth of the laser pulse due to its finite duration the orbital period is equal to the laser pulse length. In this case there will be no discrete jump. However, in the case of an intense pulse the finite bandwidth that produces the wave packet is instead produced by power broadening. In this case the orbital period can be substantially shorter than the laser pulse length. Discrete jumps can occur in the ionization signal. We report the observation of such discrete jumps.

© 1992 IQEC