Photodissociation dynamics of n-butylbenzene molecular ion has been investigated on a nanosecond time scale. The rate constants for production of C 7 H 8 •+ and C 7 H 7 + , their branching ratios, and the kinetic energy release distributions have been determined by the photodissociation method using mass-analyzed ion kinetic energy spectrometry. The branching ratios have been found to be in excellent agreement with the previously established results. All the experimental data could be explained with statistical theories such as RiceRamsperger-Kassel-Marcus (RRKM) and phase space theories. RRKM fittings for these reactions have been improved. The present result supports the previous suggestion that the dissociation to C 7 H 8 •+ occurs via a stepwise McLafferty rearrangement.

Production of C 7 H 7 + in the unimolecular and photoinduced dissociations of ethylbenzene; methylcycloheptatriene; and o-, m-, and p-xylene ions has been investigated using mass-analyzed ion kinetic energy spectrometry. Both the benzylium and tropylium ions were produced near the reaction threshold, whereas only the benzylium ion was observed at higher internal energy achieved by photoabsorption. Experimental data suggest that interconversion between ethylbenzene and methylcycloheptatriene ions occurs more rapidly than their dissociations near the reaction threshold, whereas xylene ions undergo rate-determining isomerization to the methylcycloheptatriene structure, and dissociation occurs from the ethylbenzene/methylcycloheptatriene ion mixture. The processes occurring at higher internal energy are quite similar, except that the tropylium channel cannot compete against the benzylium channel because either the ethylbenzene ion-to-methylcycloheptatriene ion conversion or the dissociation of the methylcycloheptatriene ion to the tropylium ion is slower than the direct dissociation of the ethylbenzene ion. The above mechanistic picture is in disagreement with the prediction of a dynamic model reported previously. A series of investigations performed for the production of C 7 H 7 + from alkylbenzene ions shows an interesting trend of increasing benzylium production with the increase in the total number of carbon atoms in the alkyl substituents.

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