The decay of triplet pyrazine in supersonic jets

The decay rates of optically excited triplet states of pyrazine in supersonic expansion were measured by using three different methods. The excess energy dependence of the radiationless rate constants in the energy range between the T₁ and the S₁ electronic origins of the isolated molecule was explored. Decay rates between 7×10 ₂-2.5×10⁴ s^-1 were found in the 1500 cm^-1 range of excess vibrational energy from the origin of the T ₁ state. The decay rates are free of mode specificity and rotational effects. The pure radiative lifetime in the measured range is rovibronic independent. The results support a model which suggests that certain vibrational modes, those which undergo large frequency changes in the excited state, control the strong vibrational energy dependence of the T₁ → S₀ intersystem crossing of pyrazine.