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

New experimental results for the collisional energy transfer of highly vibr ationally excited toluene and pyrazine employing the method of "kinetically controlled selective ionization (KCSI)" are presented. By means of a maste r equation approach we determine complete and detailed collisional transiti on probabilities P(E',E) for energies up to 50 000 cm(-1). The same monoexp onential representation P(E',E) proportional to exp[ - ((E - E')/alpha (1)( E))(Y)] (for E' less than or equal to E) with a parametric exponent Y in th e argument and linearly energy dependent alpha (1)(E) = C-0 + C1E successfu lly 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 an d pyrazine results for the whole range of bath gases studied. The parameter s Y, C-0 and C-1 of P(E',E) show a smooth increase with the size of the col lider. An approximately linear energy dependence of the first moment of ene rgy transfer [DeltaE] is observed for all bath gases. Literature data from infrared fluorescence (IRF) experiments in general show significantly small er - [DeltaE] 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 collid ers observed in the KCSI experiments are easily rationalized on the basis o f accompanying trajectory calculations on the deactivation of highly vibrat ionally 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.