Large molecules energy transfer

If, at different points of any gas in a closed volume for a short time, a different temperature is created and the gas is then left to itself, the temperature at all gas points is equalized owing to the process of heat transfer. From a macroscopic point of view, the phenomenon of heat transfer in gases consists of temperature transport from hotter to colder places. Within the framework of the molecular kinetic theory, the process of heat transfer consists of the fact that molecules from a heated site of the gas, where they have large kinetic energy, transfer the energy to cooler areas via collisions a flow of heat is thus created. 

In the experimental and theoretical study of energy transfer processes which involve some of the above mechanisms, one should distingiush processes in atoms and small molecules and in large polyatomic molecules. For small molecules a frill theoretical quantum treatment is possible and even computer program packages are available [, and ], with full state to state characterization. A good example are rotational energy transfer theory and experiments on Fie + CO [M] ... 

Experimental access to the probabilities P(E ,E) for energy transfer in large molecules usually involves teclmiques providing just the first moment of this distribution, i.e. the average energy (AE) transferred in a collision. Such methods include UV absorption, infrared fluorescence and related spectroscopic teclmiques [11. 28. 71. 72, 73 and 74]. More advanced teclmiques, such as kinetically controlled selective ionization (KCSI [74]) have also provided infonnation on higher moments of P(E ,E), such as ((AE) ). 

The standard mechanisms of collisional energy transfer for both small and large molecules have been treated extensively and a variety of scaling laws have been proposed to simplify the complicated body of data [58, 59, 75]. To conclude, one of the most efficient special mechanisms for energy transfer is the quasi-reactive process involving chemically bound intennediates, as in the example of the reaction ... 

Oref I and Tardy D 0 1990 Energy transfer in highly excited large polyatomic molecules Chem. Rev. 90 1407 5... 

Barker J R 1984 Direct measurement of energy transfer in rotating large molecules in the electronic ground state J. Chem. Phys. 88 11... 

Here t. is the intrinsic lifetime of tire excitation residing on molecule (i.e. tire fluorescence lifetime one would observe for tire isolated molecule), is tire pairwise energy transfer rate and F. is tire rate of excitation of tire molecule by the external source (tire photon flux multiplied by tire absorjDtion cross section). The master equation system (C3.4.4) allows one to calculate tire complete dynamics of energy migration between all molecules in an ensemble, but tire computation can become quite complicated if tire number of molecules is large. Moreover, it is commonly tire case that tire ensemble contains molecules of two, tliree or more spectral types, and experimentally it is practically impossible to distinguish tire contributions of individual molecules from each spectral pool. 

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