DISSOCIATION OF HIGHLY EXCITED TRIPLET BENZOPHENONE INTO PHENYL RADICALS AND CARBON-MONOXIDE - DETERMINATION OF THE REACTION QUANTUM YIELD AND THE HEAT OF REACTION BY TIME-RESOLVED THERMAL LENSING TECHNIQUE

The photodissociation path and quantum yield of benzophenone in the hi ghly excited triplet (T(n)) state were determined in benzene solution at room temperature using two-photon absorption (TPA) and time-resolve d thermal lensing (TRTL) techniques. The TRTL signals measured were co mposed of fast and slow components: the fast one rose with a time cons tant less than the instrumental response time (almost-equal-to 100 ns) , while the slower rose with a time constant of about 3 mus. The fract ions of the fast and slow TRTL signals increased and decreased, respec tively, with the laser power. The TRTL signals were quantitatively ana lyzed on the basis of the probe-light tracing in the nonuniform refrac tive index medium. The efficiency of thermal lensing (TRTL signal inte nsity/heat available to thermal lens) depends on the spatial profile o f thermal distribution, Therefore, the TRTL signal due to TPA is more intense than that due to single photon absorption (SPA): TPA was about four times more sensitive in the TRTL signal than SPA under the exper imental conditions employed. The TRTL signals were simulated using a k inetic model including the S1<--S0 and T(n)<--T1 absorptions of benzop henone and the subsequent dissociation of the T(n) molecule. The react ion quantum yield (PHI(r)) and the heat of reaction (DELTAH) in benzen e solution were determined to be 0.39 and 117 kcal mol-1, respectively , based on the experiment and simulation. The dissociation path open f or benzophenone in the T(n) state was thus confirmed to be Ph2CO-->2Ph +CO.