Population inversion mechanisms in

The condition set by equation (11.16) is, however, only a minimum requirement and even if this is satisfied, C.W. laser oscillation may not be possible for collision processes in the discliarge may prevent the attainment of a population inversion by creating lower-level atoms at an excessive rate, S O. Many gas laser systems therefore rely on a combination of both a favourable lifetime ratio and a selective population mechanism. To close this discussion we should point out that equations (11.15) and (11.16) illustrate an important application of the lifetime measurements discussed in Chapter 6, particularly for the case of neon and the other noble gases mentioned in section 6.3.5. 

The metastable atoms can be destroyed by collisions with atoms or electrons or by diffusion to the walls of the discharge tube. 

If a small amount of neon is added to the helium discharge the mean electron energy is not appreciably altered a helium-neon ratio of approximately 7 1 has been found to be optimum for discharge tubes of 6 mm bore. Collisions then occur between the metastable helium atoms and ground level neon atoms. As shown in Fig.11.3 there is a close coinci-dence between the 2 S. metastable level of helium and the 2s, 

A selection of the neon C.W. laser transitions excited in a helium-neon discharge 

Transition configurations Transition configurations Transition configurations  

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