Collisional Excitation and Deexcitation

Beside radiative processes, energy states of atoms are changed by collisions with other atoms and molecules, which are called collisional excitation and deexcitation. Since such transitions are proportional to the density of atoms or molecules, the number of transitions is written as follows: for excitation from n to n$_u$

$$C_{n\rightarrow n_u} N_n n_{\rm H},$$ (2.137)

and deexcitation from n$_u$ to n

$$C_{n_u\rightarrow n} N_{n_u} n_{\rm H},$$ (2.138)

where the coefficients $C$ are determined by laboratory experiments and quantum mechanics calculations.