Geometrical structures of the isolated benzene and naphthalene molecules have been accurately determined by using ultrahigh-resolution laser spectroscopy and ab initio calculation in a complementary manner. The benzene molecule has been identified to be planar and hexagonal (D6h) and the structure has been determined with accuracies of 2 × 10−14 m (0.2 mÅ; 1 Å = 1 × 10−10 m) for the C–C bond length and 1.0 × 10−13 m (1.0 mÅ) for the C–H bond length. The naphthalene molecule has been identified to be symmetric with respect to three coordinate axes (D2h) and the structure has been determined with comparable accuracies. We discuss the effect of vibrational averaging that is a consequence of zero-point motions on the uncertainty in determining the bond lengths.

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