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In a collision between two hydrogen atoms in their ground states, the electron spins may exchange Such exchanges affect the relative populations of the hyperfine levels and are therefore important in determining the rate of 21-cm transitions in an assembly of atoms undergoing collision A cross-section is computed for electron exchange over a range of gas kinetic temperatures, and the competition between collisions and radiation in establishing the hyperfine populations is analyzed The results are applied to typical situations encountered in 21-cm radio astronomy
Purcell et al. (Thu,) studied this question.