PX282 - C1 - the physics of line strength

the two main factors controlling line strength are:

• excitation: thermal energy excites electrons to higher energy states
• ionization: electrons gain enough energy to escape the nuclear well

eg: for H, the ionization energy is $-13.6eV$, ie: if an electron absorbs more than that, it escapes

the number of electron in any state are given by the boltzmann equation