Direct measurement of fe

Troe and his co-workers [27] have recently measured directly the lifetimes of excited molecules undergoing unimolecular decomposition, under essentially collision-free conditions. In these experiments, cyclo-heptatriene, 7-methylcycloheptatriene, 7,7-dimethylcycloheptatriene and 

7-ethylcycloheptatriene were each excited electronically with a short pulse of laser radiation. This is followed by a rapid internal conversion to generate highly vibrationally excited, electronic ground state molecules which absorb in the ultraviolet, at longer wavelengths than the unexcited parent. Their decay (isomerisation to alkylbenzenes) was monitored directly with a continuous background source. 

In fact, the lifetime of excited CHT itself was too short to measure, but k(i) values of 2.5x 10 , 1.6 x 10 and 2x 10 s were recorded, respectively, for Me-CHT, Et-CHT, and Me2-CHT. These variations of lifetime with structure are as predicted by RRKM theory. The individual values could be matched to within a factor of 2. 

Direct measurement of the lifetimes of excited molecules represents 

Robinson and K.A. Holbrook, Unimolecular Reactions, Wiley, New York, 1972. 

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