Energy Transfer in Near-Resonant Molecular Collisions due to Long-Range Forces with Application to Transfer of Vibrational Energy from ν3 Mode of CO2 to N2

Abstract

The cross section for the near-resonant transfer of vibrational energy from CO2(001) to N2(0), CO2(001) + N2(0)→CO2(000) + N2(1) + ΔE, is calculated for the isotopes N214 (ΔE = 18 cm−1) and N215 (ΔE = 97 cm−1). The impact parameter (semi-classical) approximation is used, and it is assumed that the vibrational-energy transfer is caused by the interaction of the instantaneous CO2 dipole moment with the N2 quadrupole moment. When proper account is taken of the rotational motions of the molecules it is found that in collisions of CO2 with 14N2 only the low rotational levels of the CO2 and N2 molecules contribute to Reaction (1). In collisions of CO2 with 15N2, only those rotational levels contribute which undergo transitions cancelling most of the relatively large (97 cm−1) vibrational-resonance defect. For 14N2 below about 1000°K, where the cross section displays a negative temperature dependence, the results are in excellent qualitative and quantitative agreement with available experimental data, with no adjustable parameters in the theory. Above about 1000°K the experimental cross-section data display a positive temperature dependence indicating that some other mechanism becomes important there. For 15N2 the calculations are in good agreement with an experimental measurement at 300°K. Data at other temperatures are not presently available. The theoretical results indicate that the cross section for the vibrational-energy transfer should have maximum around 200°K. Below this temperature the higher rotational levels which dominate the energy-transfer process are unfavorably weighted in the Boltzmann distribution.