Observation of large vibrationhyphen;tohyphen;vibration energy transfer collisions (Dgr;Egsim;3500 cmminus;1) in quenching of highly excited NO2by CO2and N2O

摘要

Timehyphen;resolved Fourier transform infrared emission spectra, recorded after 475 nm excitation of NO2in a CO2or N2O bath, show IR emission from collisionally populated vibrational levels of the bath gas. The frequency of the observed bands proves that the emission arises from either the (1,00,1), (0,2l,1), and/or (0,00,2) levels of CO2or N2O. From the pressure dependence of the emission intensity it was determined that these levels are populated by single collisions with excited NO2. Under typical conditions (1:10 ratio of NO2to bath gas and 1ndash;2 Torr total pressure) a steady state concentration is reached in our experiments where 0.016plusmn;0.006 multiply excited CO2molecules, or 0.03plusmn;0.01 multiply excited N2O molecules were generated per laser excited NO2. A transition dipole coupling model is applied to explain these results, where the resonance conditions for vibrationhyphen;tohyphen;vibration energy transfer are relaxed by extensive vibronic and vibrational couplings in highly excited NO2. In this model the energyhyphen;dependent transition dipole of excited NO2is derived from the timehyphen;resolved IR emission spectra. The probability of Dgr;v=1 energy transfer collisions for excited NO2with CO2or N2O can be accurately calculated. However, the number of multiply excited species produced (Dgr;vgsim;1) is grossly underestimated. Analysis of the timehyphen;resolved data shows that the probability for Dgr;vgsim;1Vndash;Venergy transfer is ca. two orders of magnitude larger than the probability predicted by the dipole coupling model, and that NO2molecules with energies as low as 5000 cmminus;1have a nonhyphen;negligible probability for exciting the overtone levels of CO2and N2O. Finally, it was found that the dipole coupling model also underestimates the probability for the Dgr;Egsim;10thinsp;000 cmminus;1supercollisions deduced in previous experiments (see Refs. ).