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 Dopplerhyphen;free cw twohyphen;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 1.5times;107. Benzene is a prototype planar molecule taken to haveD6hsymmetry in the ground as well as in the first excited state. From our ultrahyphen;high resolution results it is found that benzene in the excitedS1state 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.
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