Einstein's Coefficients

Figure 2.11:

In this section, we describe the absorption coefficient $\kappa$ and the emissivity $\epsilon$ for the line emission and absorption. Here, consider a hypothetical atom which has only three levels named n$_d$, n, n$_u$. and assume the energy of each level is, respectively, $E_{n_d}$, $E_n$, and $E_{n_u}$. When the atom changes its state from n$_u$ to n, an photon with energy of $h\nu_{n_u\rightarrow n}=E_{n_u}-E_{n}$ is emitted, if this transition is radiative. Besides the radiative process, atom changes its state exchanging its energy with other atoms, molecules and electrons.