ROTATION-TRANSLATION TRANSFER

It has been suggested84,85 that owing to anharmonicity, truly resonant collisions will only occur between two molecules in the same vibrational level, so that the fastest process will be 

Rotational quanta are much smaller than vibrational quanta, and rotational energy is therefore much more easily degraded to translational energy. For most molecules the collision number for rotation-translation transfer is less than 10, corresponding to relaxation times smaller than 10 9 sec at one atmosphere pressure. 

It is thus more difficult to investigate experimentally by acoustic methods, which will require very high fjp values at which classical dispersion and absorption must also be taken into account87. Theoretical treatment is also much less satisfactory molecules are usually distributed in a variety of rotational energy levels, so that a number of different transition probabilities have to be considered, and observed relaxation times usually represent averages over sets of transitions involving a range of /-states. 

This was attributed to the stronger coupling required for the double quantum vibrational transition. 

The rotational relaxation of polyatomic spherical top molecules can be treated approximately on the classical rough sphere model. This has been done for homo-molecular collisions by Wang Chang and Uhlenbeck101. They find a simple expression resembling that obtained by Brout for diatomic molecules 

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The extremely narrowband emission of a laser allows the specific excitation of molecular states. The non-Boltzmann distribution produced by the excitation process is quickly destroyed by radiation processes and collisional deactivation. The relative contribution of these different deactivation channels depends on the nature of the level excited as shown in Fig. 3. In the microwave region where rotational levels are excited, the radiative life time is very long compared to the very efficient rotational relaxation processes (R—R rotation—rotation transfer and R—T rotation—translation transfer). Therefore, the absorbed radiation energy is transformed within a few gas kinetic collisions into translational energy. The situation is similar for... 

Wang Z S, Darling G R and Holloway S 2000 Translation-to-rotational energy transfer in seattering of H2 moleeules from Cu(111) surfaees Surf. Sc/. 458 63... 

The effect of rotation on transfer to a translating sphere has been studied for both screw motion (El, F6, T2) and top spin (N3, T2) with Re > 1500. The effect of rotation on the transfer rate is less than 10% for vJU < 0.5. The ratio of the Sherwood number in screw motion to that in pure translation at the same Ur is correlated within 10% by... 

The photofragmentation that occurs as a consequence of absorption of a photon is frequently viewed as a "half-collision" process (16)- The photon absorption prepares the molecule in assorted rovibrational states of an excited electronic pes and is followed by the half-collision event in which translational, vibrational, and rotational energy transfer may occur. It is the prediction of the corresponding product energy distributions and their correlation to features of the excited pes that is a major goal of theoretical efforts. In this section we summarize some of the quantum dynamical approaches that have been developed for polyatomic photodissociation. For ease of presentation we limit consideration to triatomic molecules and, further, follow in part the presentation of Heather and Light (17). 

First, collisional relaxation of rotation is fast. One to ten collisions are sufficient for rotation-translation energy transfer. At 1 torr at least one collision per microsecond will occur for both 02 and HC1. In contrast, rotational relaxation by radiation, when allowed, is very slow, of the order of 102 seconds. The absence of... 

Whether rotation-vibration transfer occurs, and how important it is, are questions of considerable dispute. The experimental observation by Millikan106,107, that vibrational deactivation of CO in collision with p-H2 is more than twice as efficient as in collision with o-H2, seems to provide some evidence that rotational energy participates in vibrational relaxation. The only significant difference between o- and p-H2 in the context of this experiment would appear to be the difference in rotational energy states, as illustrated by the fact that at 288 °K (the temperature of the experiment) the rotational specific heat of o-H2 is 2.22, while that of p-H2 is 1.80 cal.mole-1.deg-1. Cottrell et a/.108-110 have measured the vibrational relaxation times of a number of hydrides and the corresponding deuterides. On the basis of SSH theory for vibration-translation transfer the relaxation times of the deuterides should be systematically shorter than those of the hydrides. The... 

Miklavc, A. and Smith, I.W.M. (1988) Vibrational relaxation of C2H2 and C2D2 by vibration-rotation, translation (V-R,T) energy transfer. J. Chem. Soc., Faraday Trans. 2 84,227-238. 

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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