Collisional energy transfer of highly vibrationally excited toluene and pyrazine: Transition probabilities and relaxation pathways from KCSI experiments and trajectory calculations

Abstract

New experimental results for the collisional energy transfer of highly vibrationally excited toluene and pyrazine employing the method of “kinetically controlled selective ionization (KCSI)” are presented. By means of a master equation approach we determine complete and detailed collisional transition probabilities P(E′,E) for energies up to 50000 cm⁻¹. The same monoexponential representation P(E′,E)∝exp[ − ((E − E′)/α₁(E))^Y] (for E′⩽E) with a parametric exponent Y in the argument and linearly energy dependent α₁(E) = C₀ + C₁E successfully used in our earlier investigation [T. Lenzer, K. Luther, K. Reihs and A. C. Symonds, J. Chem. Phys., 2000, 112, 4090] can reproduce the toluene and pyrazine results for the whole range of bath gases studied. The parameters Y, C₀ and C₁ of P(E′,E) show a smooth increase with the size of the collider. An approximately linear energy dependence of the first moment of energy transfer 〈ΔE〉 is observed for all bath gases. Literature data from infrared fluorescence (IRF) experiments in general show significantly smaller − 〈ΔE〉 values outside the uncertainty limits of the KCSI results. It is shown that this can mainly be traced back to the critical dependence of the IRF data on small uncertainties in the calibration curve. Some of the trends with respect to the energy transfer efficiencies of different colliders observed in the KCSI experiments are easily rationalized on the basis of accompanying trajectory calculations on the deactivation of highly vibrationally excited pyrazine by n-propane and CO₂. The negligible influence of the V–V relaxation channel in the pyrazine + CO₂ system observed in earlier IR diode laser studies is confirmed.