Study on Excited State Interaction by Time-Resolved Spectroscopy

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The visible absorption bands of NO_2 in the 588-591-nm range have been proved to contan a vibronic transition to the dark C^2A_2 state by rotational analysis using the optical-optical double-resonance (OODR) techniques. The intermediate state is verified to be a mixed state of dark C^2A_2 and light A^2B_2 coupled by spin-orbit inter- action with the selection rules DELTAN=*1 and DELTAKa=0. Although the C^2A_2 state is dark in the v_1 transition from X^2A_1, it becomes a light state in the v_2 transition to 2^2B_2.Transient absorption and time-resolved thermal lensing techniques have been used to study the photophysical behavior of C_<60> in benzene solution at room tempeature. The riplet state was the only transient species detectable with the 10 ns time- resolution. The triplet-triplet absorption spectrum with apparent band peaks at 330 and 745 nm covers the whole spectral range of 290-850 nm. The riplet lifetime in air-free benzene is 49*1 mus and essentially all the photon energy absorbed is released as heat, predominantly through the triplet state.A time-resolved thermal lensing (TRTL) technique was applied to the determina- tion of the dissociation quantum yield of benzophenone in the highly-excited triplet state yielding two phenyl radicals and a carbon monoxide. The TRTL signals were simulated using a kinetic model inculuding the S_1*S_0 and T_n*T_1 absorptions of benzophenone and subsequent dissociation of the Tn molecule. The reaction quantum yield and the heat of reaction in benzene solution were determined to be 0.36 and 126 kcal mol^<-1>, respectively, based on the experiment and simulation.