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Electronic spectra of polyatomic molecules with resolved individual rotational transitions: Benzene

The density of rotational transitions for a polyatomic molecule is so large that in general many such transitions are hidden under the Doppler profile, this being a fundamental limit of conventional high resolution electronic spectroscopy. We present here the first Doppler‐free cw two‐photon spectrum of a polyatomic molecule. In the case of benzene, 400 lines are observed of which 300 are due to single rotational transitions, their spacing being well below the Doppler profile. The resolution so achieved is 1.5×107. Benzene is a prototype planar molecule taken to have D6h symmetry in the ground as well as in the first excited state. From our ultra‐high resolution results it is found that benzene in the excited S1 state is a symmetric rotor to a high degree. A negative inertial defect is found for the excited state. The origin of this inertial defect is discussed.