By means of a technique of corona excited supersonic expansion coupled with a pinhole-type glass nozzle, vibronically excited but jet-cooled 3,4-dichlorobenzyl radical was formed from precursor 3,4-dichlorotoluene in corona discharge. From an analysis of the visible vibronic emission spectrum observed, we determined the electronic energy of the D-1 -> D-0 transition and vibrational mode frequencies in the D-0 state of the 3,4-dichlorobenzyl radical by comparing with the known vibrational data of precursor and an ab initio calculation. Also, the substituent effect on the electronic transition energy was explained using Huckel's molecular orbital theory for the first time. (c) 2013 Elsevier B.V. All rights reserved.