Electron momentum-transfer, collision frequency,

A necessary condition for the two-term expansion of the distribution function of equation (2) to be valid is that the electron collision frequency for momentum transfer must be larger than the total electron collision frequency for excitation for all values of electron energy. Under these conditions electron-heavy particle momentum-transfer collisions are of major importance in reducing the asymmetry in the distribution function. In many cases as pointed out by Phelps in ref. 34, this condition is not met in the analysis of N2, CO, and C02 transport data primarily because of large vibrational excitation cross sections. The effect on the accuracy of the determination of distribution functions as a result is a factor still remaining to be assessed. 

To explain the photoelectric effect, light of frequency, v, is considered a stream of particle-like entities (photons), each of which travels with speed c and carries an energy given by equation (8.3). Emission of a photoelectron is imagined to occur as the result of a collision between a photon of the incident light and an electron in the target. As a result of the collision, the energy and momentum of the photon are transferred to the electron. 

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