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1. Molecular Rotation

   Linked via "selection rule"

   Spectroscopic Observation
   Molecular rotation is most directly observed via microwave spectroscopy, where the energy differences ($\Delta E$) between adjacent rotational levels correspond to microwave radiation frequencies. For linear rotors, the selection rule dictates that $\Delta J = \pm 1$. This yields a series of equally spaced [absorption lines](/entries/absorpt…