Prominent, and restricted, vibrational state mixing in the fluorescence excitation spectrum of benzophenone

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Abstract
We report the first successful observation of the fluorescence excitation spectrum of benzophenone in the collision-free environment of a supersonic jet. A comparison of this spectrum with an earlier prediction of Heller et al. [J. Chem. Phys. 73, 4720 (1980)], together with other experimental and theoretical data, reveals the existence of prominent vibrational state mixing of the totally (A) and nontotally (B) symmetric ring torsional modes in the S1 state, via a cubic anharmonicity term of the form λAB2. Fully 64 of the 66 vibrational degrees of freedom of this large molecule do not participate in this mixing process, suggesting that the phase space accessible to benzophenone following optical excitation to its S1 state is extremely restricted. The implications of these findings for intramolecular vibrational relaxation in the electronically excited states of this and other large molecules are discussed.