We have excited wave packets in perturbed Rydberg states of barium, an atom with two optically active electrons, and have observed qualitatively different wave packets from those seen in one-electron atoms. In particular, we have found that electron-electron scattering quickly leads to excitation of a doubly excited state and subsequent passage of the population back and forth between different configurations or channels, the quantum analog of two coupled pendula. The experimental results can be successfully described by quantum defect theory. [S0031-9007 (97)03329-2] PACS numbers: 32.80.RmWave packets are formed in any system excited by a short enough pulse, and understanding their dynamics is essential [1]. Wave packets in single configurations, such as one-electron Rydberg wave packets or molecular vibrational wave packets correspond to a one dimensional classical oscillatory motion and are well understood [2][3][4]. Wave packets in perturbed systems, those exhibiting configuration interaction, correspond to the classical motion of two coupled oscillators. Although there have been related investigations [5], such wave packets have received little attention, in spite of the fact that they play a central role in several contexts such as zero kinetic energy spectroscopy and proposed methods for coherent control of chemical reactions [6,7]. The latter are based on the molecular analog of the energy transfer between coupled classical oscillators [8].Here we describe the experimental observation of radial wave packets of perturbed Ba Rydberg states, specifically the 6snd 1,3 D 2 states perturbed by the 5d7d 1 D 2 state near n 26. Although the configuration interaction only slightly perturbs the regularity of the Rydberg energy levels, it alters the oscillator strengths, with the result that the time dependence of the perturbed radial wave packet bears almost no relation to that of an unperturbed radial wave packet. This dramatic difference is implied by the excitation spectrum and can be understood qualitatively with a simple picture based on the atomic population's passing back and forth between the Rydberg states and the doubly excited perturbing level. This picture can be made quantitative using quantum defect theory (QDT), yielding good agreement with the experimental results. Furthermore, time resolved measurements of the doubly excited population show that the population does indeed flow back and forth between the Rydberg states and the perturber.The system we have studied is the three channel system shown in Fig. 1. The three channels are the 6snd 1 D 2 , 6snd 3 D 2 , and 5dnd 1 D 2 channels, although only the 5d7d state of the third channel is important. For convenience, we shall refer to these three channels as singlet, triplet, and perturber, respectively. In the absence of configuration interaction the energies of the two unperturbed Rydberg series and the 5d7d perturber are shown in Fig. 1(a). When the configuration interaction is included, the 5d7d state is inserted into the Rydberg series with the resu...
