FREQUENCY-RESOLVED AND TIME-RESOLVED STUDY OF THE DYNAMICS OF RUBIDIUM RYDBERG WAVE-PACKETS IN AN ELECTRIC-FIELD

The dynamics of an atomic wave packet in an electric field above the classical field ionization limit is studied in both the frequency and the time domain. We measure the photoionization yield of rubidium atoms in an electric field in the range 0.2-5 kV/cm as a function of the wavelength of the exciting laser. Fourier transforms of these spectra after multiplication with the spectrum of a short optical pulse allow us to make a direct comparison with time-resolved measurements performed by Broers et al. [Phys. Rev. A 49, 2498 (1994)]. The dynamics of electronic wave packets above the classical field ionization limit as a function of the excitation energy is studied in detail, showing angular-momentum recurrences of the wave packet to the core on a picosecond time scale, The observed lifetimes up to 50 ps depend on excitation energy and polarization of the laser Light. We also transform the frequency spectra to scaled-energy spetra, i.e., an alternative way of representing the dynamics of Rydberg wave packets. The frequency spectra allow us to compare the time-resolved spectra and the scaled-energy spectra directly. We find that near the saddle point (epsilon = -1.9) radial and angular recurrences correspond to actions of S = 0.77 and S = 2.4, respectively. Just above the classical field ionization limit long-lived states are observed with lifetimes up to microseconds. These long decay times are measured in an alternative way: detection of the ionization yield as a function of time after the laser excitation.