EXCITATION OF CO2 BY ENERGY-TRANSFER FROM HIGHLY VIBRATIONALLY EXCITED BENZENE-DERIVATIVES

The time-resolved infrared fluorescence technique has been used to study V-V and V-T/R energy transfer to carbon dioxide from highly excited benzene, benzene-d6, toluene, and toluene-d8. The highly vibrationally excited aromatics in the electronic ground state are obtained by radiationless transitions after pumping with a KrF laser at 248 nm to the S1 excited electronic level. The V-V energy transfer from the excited parent to the asymmetric stretch mode of CO2 was measured by observing the characteristic emission of CO2* near 4.3 mu-m. From these measurements, the probability per collision of formation of CO2* was determined as a function of the internal energy in the excited aromatic. In all cases investigated, this probability is less-than-or-equal-to 0.1% at the initial excitation energy of 40 000 cm-1 and it is approximately directly proportional to the vibrational energy of the excited aromatic. The total concentration of CO2* produced as a result of the many collisions needed to totally deactivate the excited aromatic amounted to > 5% of the initial concentration of the excited aromatic and the quantitative values obtained are in excellent agreement with other work. A simple dipole-dipole interaction model is shown to explain the observed magnitude of V-V energy transfer and it is used to predict the amount of energy transferred to the bending mode of CO2. A key feature of this model is that the states of the highly vibrationally excited polyatomic are assumed to be broadened by rapid intramolecular vibrational redistribution of energy. In addition to the V-V energy-transfer measurements, the average energy lost per collision by the excited aromatic was determined as a function of the vibrational energy of the aromatic, and the rate constants were determined for CO2* deactivation by the nondeuterated species. For the deuterated species, the results implicated a contribution from resonant V-V transfer between the C-D stretch modes and the asymmetric stretch mode of CO2. The overall results for the CO2 collider gas indicate that V-V energy transfer contributes a relatively small portion of the total energy transfer, and that portion can be described with the dipole-dipole interactions model.