Rotational-Translational RT Energy Transfer

Both the general formulations of such a theory, and the numerous explicit results for different systems are available [98, 343, 353, 463, 484]. Here it should only be noted that the rotational quantum number j usually changes by several units. Moreover, the mean square of the transferred energy ((AE)rot) is proportional to the square of the Fourier component at a frequency cOrot of the anisotropic part of the interaction potential. It is the latter that expresses the manifestation of the adiabatic principle the lower the rate of change in perturbation acting on the system (here the rotor), the smaller the probability of transition between the states of the system. 

Recent calculations in terms of classical mechanics show that the mean number of collisions Zfot = l/ rot o needed for the onset of the equilibrium rotational distribution at 1000 K is about 10 for molecules of the O2 or Ng type and 200 to 300 K for H2. This difference is due to the large cojot value for H2 as the average thermal angular velocity of H2 is much higher than that of O2 and N2. 

As regards the dependence of transition probabilities Pjj/ on rotational quantum numbers, it can approximately be represented by the exponential function of AEjj/ [373, 376]. Only in the case of large rotational quanta (e.g. hydrogen halides) are transitions with Aj = il dominant. Moreover, Pjj+i decreases with increasing 

Most measurements of rotational relaxation have been carried out for N2, usually either at or near-room temperature. The Zj-ot values obtained are 3.3 to 7.2. The mean value 4.8 d 0.8 is in good agreement with that obtained theoretically Zrot (300 K) 4.0. 

Measurements for O2 have also been conducted either at or close to room temperature. The resulting Z ot values range from 2.5 to 4.5, on the average 3.6 di 0.6 and are also in good agreement with the theoretical value Zrot(300 K) = 3.5. In both cases, Z ot increases with, temperature [3]. 

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Rotational-rotational (RR) and rotational-translational (RT) energy transfer processes are usually non-adiabatic and fast, because rotational quanta and, therefore, the Massey parameter are small. As a result, the collision of a rotator with an atom or another rotator can be considered a classical collision accompanied by essential energy transfer. The Parker formula for calculation of number of collisions, Zrot, required for RT relaxation was proposed by Parker (1959) and Bray and Jonkman (1970) ... 

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