We present a comparative study of optical zero-quantum two-dimensional coherent spectroscopy (2DCS) in alkali-metal vapors of potassium (K) and rubidium (Rb). Both systems exhibit pronounced sideband structures in their respective zero-quantum spectra. In K vapor, these sidebands are consistent with electronic fine-structure coherence between the spin-orbit split sublevels of a single atom. In contrast, Rb vapor exhibits anomalous sidebands that cannot be explained within a single-atom picture. Numerical simulations based on a correlated two-atom model attribute these anomalous features to nonradiative coherences involving collective doubly excited states. By monitoring the temporal evolution of these sidebands, we find that the dependence of the decay time on vapor density is consistent with an interaction-induced dephasing picture associated with interatomic dipole-dipole interactions. These findings illustrate the crucial role of collective effects in contributing to the zero-quantum spectra and establish zero-quantum 2DCS as a sensitive probe for interatomic correlations.

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• Atomic & molecular processes in external fields
• Collective effects in atomic physics
• Nonlinear optics
• Atomic gases
• Two-dimensional coherent spectroscopy