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 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 weil below the
Doppler profile. The resolution so achieved is 1.5 X 10'. Benzene
is a prototype planar molecule taken to have D •• symmetry in the
ground as weil as in the first excited state. From our ultra-high
resolution results it is found that benzene in the excited SI state
i8 a symmetrical rotor to a high degree. A negative inertial defect
is found for the excited state. The origin of this inertial defect
is discused.