Ground state vibrational randomization in alkylbenzenes

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Abstract
Measurements of intramolecular vibrational randomization (IVR) previously carried out on the S1 excited state of a series of jet‐cooled alkylbenzenes have been extended to the ground electronic state. The ground state ring modes 6b, 1, and 12 were conveniently activatd by pulsed laser excitation of the 000 vibronic band of the S1←S0 ultraviolet absorption system, followed by fluorescence. The onset of IVR was then probed by resonant two‐photon ionization (R2PI) of these vibrationally activated ground state alkylbenzenes. When the alkyl chain exceeded three carbon atoms in length, the hot band 1201 (Ev∼1010 cm−1) was found to be absent from the R2PI‐probed spectrum, while the 6b01 hot band (Ev∼625 cm−1) remained easily observable through n‐pentylbenzene. This overall pattern of IVR is essentially the same as that observed previously in the S1 excited state. In both electronic states, the only effective barrier to vibrational energy decay from the ring to the chain appears to be an inadequate density of states. Above the critical state density, IVR appears to be complete, and this critical state density appears to be about the same in both the ground and excited electronic states.