Electronic spectra of polyatomic molecules with resolved individual rotational transitions
The density of rotational transitions for a polyatomic molecule is so large that in general many such\ntransitions are hidden under the Doppler profile, this being a fundamental limit of conventional high\nresolution electronic spectroscopy. We present here the first Doppler-free cw two-photon spectrum of a\npolyatomic molecule. In the case of benzene, 400 lines are observed of which 300 are due to single rotational\ntransitions, their spacing being weil below the Doppler profile. The resolution so achieved is 1.5 X 10'.\nBenzene is a prototype planar molecule taken to have D \u2022\u2022 symmetry in the ground as weil as in the first\nexcited state. From our ultra-high resolution results it is found that benzene in the excited SI state i8 a\nsymmetrical rotor to a high degree. A negative inertial defect is found for the excited state. The origin of this\ninertial defect is discused.