Slow electrons rotational excitation

The Rotational Excitation of Molecules by Slow Electrons, Kazuo Takayanagi and Yukikazu Itikawa... 

As excited atoms, molecules, or ions come to equilibrium with their surroundings at normal temperatures and pressures, the extra energy is dissipated to the surroundings. This dissipation causes the particles to slow as translational energy is lost, to rotate and vibrate more slowly as rovibrational energy is lost, and to emit light or x-rays as electronic energy is lost. 

We can specify the conditions for the above sequences of reactions to be possible in terms of thermodynamic quantities provided we can ascribe these to electronically excited states. This is normally possible for excited states in bulk condensed phases because these become thermalised , i.e. vibrationally and rotationally equilibrated with their enviromnent, extremely rapidly (usually within a few picoseconds), long before they undergo any chemical reaction. It is however not possible to assume thermalisation in space-quantised stractures such as quantum dots, in which relatively long-lived hot carriers are generated by photoexcitation. Indeed, the very slowness of thermalisation in space-quantised stractrrres makes it possible to envisage photoconversion devices in which hot carriers can deliver more work than wotrld be thermodynamically possible with thermalised carriers. Nozik discusses such possibihties in Chapter 3. 

When the energy of the electron drops below 5 eV (Case d) there is no more possibility for energy loss in electron excitation processes. The electron scatters in the medium losing energy in very slow energy transfer processes to the vibration/rotation levels of the molecules. 

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