Observation of large vibration-to-vibration energy transfer collisions (ΔE≳3500 cm−1) in quenching of highly excited NO2 by CO2 and N2O

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Abstract
Time‐resolved Fourier transform infrared emission spectra, recorded after 475 nm excitation of NO2 in a CO2 or 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 CO2 or 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 NO2 to bath gas and 1–2 Torr total pressure) a steady state concentration is reached in our experiments where 0.016±0.006 multiply excited CO2 molecules, or 0.03±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 vibration‐to‐vibration energy transfer are relaxed by extensive vibronic and vibrational couplings in highly excited NO2. In this model the energy‐dependent transition dipole of excited NO2 is derived from the time‐resolved IR emission spectra. The probability of Δv=1 energy transfer collisions for excited NO2 with CO2 or N2O can be accurately calculated. However, the number of multiply excited species produced (Δv≳1) is grossly underestimated. Analysis of the time‐resolved data shows that the probability for Δv≳1 V–V energy transfer is ca. two orders of magnitude larger than the probability predicted by the dipole coupling model, and that NO2 molecules with energies as low as 5000 cm−1 have a non‐negligible probability for exciting the overtone levels of CO2 and N2O. Finally, it was found that the dipole coupling model also underestimates the probability for the ΔE≳10 000 cm−1 supercollisions deduced in previous experiments (see Refs. ).

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