Time evolution studies of triplet toluene by two-color photoionization

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Abstract
Two‐color photoionization has been applied to measurement of the time evolution of laser‐excited 1B2(ππ*) vibronic levels of toluene. The observed photoionization signals exhibit a biexponential dependence upon the time delay between the pump and ionizing lasers. This photon intensity is shown to be proportional to the population of the excited singlet (S1) and triplet (T†) electronic states. The data is fit to a simple kinetic model which yields the rates of S1 fluorescent decay, S1→T†, and T†→S‡0 intersystem crossings. The measured quantum yield for intersystem crossing was found to agree with previous work. The triplet lifetimes measured are considerably shorter than values reported for collisionally relaxed triplets states due to the steep excess vibrational energy dependence of the nonradiative rate. This excess energy dependence of the triplet decay rate has been measured over a limited range of energy and the observed rates are found not to extrapolate smoothly to the decay rate measured for the vibrationless T1 level. This behavior is rationalized as a dilution of excitation in good accepting modes for the T→S‡0 transition due to increasing dominance of lower frequency, poor accepting modes in the triplet density of states.