Isomeric dimethylbenzyl radicals were identified by analysis of the vibronic emission spectrum recorded from the corona excited supersonic expansion through a pinhole-type glass nozzle of precursor 1,2,4-trimethylbenzene seeded in a large amount of carrier gas helium. The visible spectrum showed several vibronic bands reflecting the characteristics of the band shape of each isomer generated. An analysis based on monosubstituted methylbenzyl radicals confirmed that all three of the dimethylbenzyl isomers were formed in the corona discharge of precursor as a result of the removal of a hydrogen atom from the methyl group at the different substitution position. For the three isomeric products, the D1 → D0 electronic transition and the vibrational mode frequencies in the ground electronic state were determined by comparing with those from both an ab initio calculation and the known data of the precursor.
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