TIME-RESOLVED DYNAMICS OF ELECTRONIC WAVE-PACKETS ABOVE THE CLASSICAL FIELD-IONIZATION THRESHOLD

In a picosecond time-resolved study, Rb atoms in an electric field are excited above the classical field-ionization threshold by a laser. Part of the electron wave function is observed to return to the core before escaping over the saddle point. At energies just above the saddle point all Stark states in the wave packet contribute to the returning part. Close to the zero-field-ionization limit, only bluest Stark components are involved. Also, a wave packet is observed performing simultaneous quantum beats in the radial coordinate and angular momentum. Simple calculations reproduce the orbit times.