We show that molecules and nuclei with well-expressed rotational degrees of freedom can be excited by means of a multistep process, initiated by a short and sufficiently strong electromagnetic pulse, into a coherent superposition of rotational states. Properties of a molecule or a nucleus in such a nonstationary state change (quasi-) periodically in time, due to the fact that the energies of the rotational states satisfy (approximately) the rule ωJ(J+1). Accordingly, their interaction with an electromagnetic field or with other particles changes (quasi-) periodically in time. A gas of molecules in a coherent state exhibits a (quasi-) periodic change of the refractive index, an effect which has been observed for the case of ${\mathrm{CS}}_2$ gas at 325 K. The nuclear case does not lead to direct experimental detection, because of its short coherence period.

• Received 11 August 1986

DOI:https://doi.org/10.1103/PhysRevA.35.4132