Generation of the simplest rotational wave packet in a diatomic molecule: Tracing a two-level superposition in the time domain

We introduce a time-domain approach to explore rotational dynamics caused by intramolecular coupling or the interaction with dissipative media. It pushes the time resolution toward the ultimate limit determined by the rotational period. Femtosecond pulses create a coherent superposition of two rotational states of carbon monoxide. The wave-packet motion is observed by subsequent Coulomb explosion, which results in a time-dependent asymmetry of spatial fragmentation patterns. The asymmetry oscillation prevails for at least 1 ns, covering more than 300 periods with no decoherence. Long time scans will allow weak perturbations of the order of $\ensuremath{\Delta}E/E={10}^{\ensuremath{-}4}$ to be discerned. Our conclusions are confirmed by a fully quantum-mechanical model.