Abstract
High-resolution fluorescence-excitation spectra of the S1S0 transitions of 1- and 2-methylnaphthalene have been obtained in the collision-free environment of a molecular beam. Each spectrum is composed of two sets of spectral lines that can be fit to different rotational Hamiltonians. The two sets of lines belong to the ground torsional levels A and E, respectively. The A lines can be fit to a rigid-rotor rotational Hamiltonian; the E lines, however, are perturbed by a coupling between the torsional motion of the methyl group and the overall rotation of the molecule. The first-order and second-order perturbation coefficients can be obtained from the fit of both A and E lines, thus allowing us to determine the torsional barriers of both electronic states. The values obtained, which depend significantly on the position of methyl substitution as well as on both the vibrational and electronic state of the molecule, will be interpreted within the framework of molecular orbital theory.
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