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

摘要

We report the first successful observation of the fluorescence excitation spectrum of benzophenone in the collisionhyphen;free environment of a supersonic jet. A comparison of this spectrum with an earlier prediction of Helleretal. lsqb;J. Chem. Phys.73, 4720 (1980)rsqb;, 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 theS1state, via a cubic anharmonicity term of the form lgr;AB2. 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 itsS1state 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.