Stroboscopic wave packets in sodium.

Abstract

Molecular Rydberg wave packets can exhibit entangled nuclear and electronic motions. Wave packets composed of stroboscopic states of Na₂ exhibit this entanglement in a regime where the spacing between electronic and rotational energy levels are commensurate. Applying the time-domain technique of Ramsey-separated oscillatory fields to Na₂ Rydberg states allows study of transitions between chaotic and regular electron motion, entanglement between nuclear and electronic motions, and dynamics of processes such as autoionization. Low-lying electronic states evolve on a short time scale compared to nuclear motions. For these the Born-Oppenheimer approximation is valid, and the angular momentum coupling follows Hund's case (a), in which the internuclear axis serves as an appropriate quantization axis for electronic angular momentum. Higher-lying electronic states will exhibit slower evolution, and have time scales more comparable to those of molecular rotation. The expected angular momentum coupling is then Hund's case (d), in which electron angular momentum decouples from the axis of the molecular ion core. However, for the high-lying stroboscopic states the angular momentum coupling reverts to case (a). A straightforward classical explanation comes from comparing the rotation period of the Na₂<super>+</super> ion core to the orbital period of the Rydberg electron. The Na₂<super> +</super> core undergoes an integer number of half-rotations in one electron Kepler period. The internuclear axis therefore has a consistent orientation each time the electron probes the core and once again provides a good quantization axis for the electron's orbital angular momentum. A supersonic expansion of sodium vapor provides rotationally and vibrationally cooled sodium dimers in their electronic ground state. A 10 ns dye laser pulse propagating perpendicular to the molecular beam excites a well-defined rovibrational level of the Na₂ <italic>A</italic> state. A subpicosecond pulse from a frequency-doubled Ti:sapphire laser launches a Rydberg wave packet several nanoseconds later, followed by an identical pulse at a variable delay. The molecules autoionize, and a microchannel plate detector collects the resulting ion signal. The variation of this signal with delay reveals the dynamics of wave packet evolution. Observed wave packets unexpectedly decayed on a picosecond time scale. Efforts to explain this dissipation are underway.