Abstract: The proton exchange of the benzoquinone radicals obtained from the laser photolysis of p-benzoquinone in ethylene glycol/water system was investigated by using time-resolved ESR(TRESR)spectrometer. The dependence of the TRESR spectra upon the pH value of the mixed solution was studied. In neutral solution, six superfine emission lines were observed and were attributed to radical BQH•, while the pH value of the solution changed to ～2.0, five emission superfine lines were observed and were assigned to cation BQH＋. For BQH＋, the decay rates differ in the superfine lines, lines involving greater fraction of the nuclear spin states decayed with slower rates than those involving smaller ones owing to the variation of the resonance position in the proton exchange.

Key words: Photolyzed p-benzoquinone radical, Chemically induced dynamic electron polarization (CIDEP), Time resolved electron spin resonsnce(TRESR), 　Proton exchange, Benzoquinone cation