Radiationless transitions in excited electronic states of the benzene cation in the gas phase

Emission from electronic excited states of the benzene-h₆ (Bz(h₆)) and benzene-d₆ (Bz(d₆)) gas-phase cations was searched for by using the PIFCO technique in which mass-selected photoion-fluorescence photon coincidences are counted. The maximum fluorescence efficiency was found to be 4 × 10^-5 for benzene ions produced with He I 584-Å excitation. From the derived maximum quantum yields and an estimate of the fluorescent decay rate, the nonradiative relaxation rate k_nr of electronic excited states of the benzene cation was determined to be greater than 5 × 10¹² s^-1 for the C̃²A_2u state and greater than 8 × 10¹⁰ s^-1 for the B̃²E_2g state. The absence of an observed deuterium effect on the fluorescence efficiency is consistent with these high k_nr rates. A discussion is given of possible relaxation pathways of the excited states; for the C̃ and B̃ states it is shown that relaxation could take place by C̃ /\/\/\> B̃ /\/\/\> X̃ and C̃ /\/\/\> X̃ interelectronic couplings and by isomerization to Dewar benzene and/or benzvalene cations.