Temperature Dependence of Nearly Resonant Vibration → Vibration Energy Transfer in CO2 Mixtures

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Abstract
Laser-excited vibrational fluorescence measurements have given rates for nearly resonant vibrational energy exchange between the asymmetric stretch of CO2 and the stretching vibrations of 13CO2, N2O, CO, and 15N2 as a function of temperature and of 13CO and D2 at room temperature. For molecules with vibrational transition dipole moments the energy transfer cross sections, σ vv, decrease rapidly with increasing vibrational energy discrepancy, ΔE. Their magnitudes at resonance are accurately fit by first order calculations using the transition—dipole interaction, but that theory underestimates σ vv for Δ E ≥ 66 cm−1, especially at low temperatures. At resonance, σ vv is inversely proportional to temperature. As ΔE increases, σ vv decreases less rapidly with temperature (13CO2, Δ E = 66 cm−1), becomes temperature-insensitive (N2O, Δ E = 125 cm−1), and finally increases with temperature (12CO, Δ E = 206 cm−1). It appears that rotation—translation coupling terms in the intermolecular potential operate simultaneously with the transition dipole interaction to transfer more of ΔE into rotation than is allowed by the first-order selection rules. When the leading vibrational transition multipole term is dipole quadrupole (CO2–N2), σ vv is much less sensitive to ΔE. First-order calculations match CO2 – 14N2 (Δ E = 19 cm−1) well, but show some breakdown for CO2–15N2 (Δ E = 99 cm−1). The vibrational deactivation rates have been measured in mixtures of CO2 with N2O, 15N2, CO, and 18O2 as a function of temperature. The diatomics are similar in behavior to the rare gases. The rate for deactivation in N2O–CO2 collisions is much faster than the rates for pure CO2 or N2O and is nearly temperature independent. Nearly resonant processes are likely to be responsible. The rate of relaxation in pure N2O is also obtained.