Fundamentals of Absorption and Emission

Atomic absorption and emission require the prevalence of free atoms which, in most instances, is achieved in a plasma or a plasma-like state. When a plasma is contained in a closed system and is in thermal equilibrium, the population of the excited levels may be described for one species by Boltzmann s law  

When the plasma is in the steady-state, the number of particles that leave an energy level per time unit is equal to that of those that arrive at this level. A number of phenomena are responsible for the transition of species between energy levels in a plasma  

and B are the Einstein transition probabihties for spontaneous emission, stimulated emission, and absorption, respectively and a, a, and p are the cross sections of the respective processes (which are also a function of the velocity distribution of the particles involved), is the electron density is the radiation density at a given frequency v. 

When the system is in thermodynamic equihbrium, the rate of formation and disappearance of charged particles and neutrals is equal, and at a given temperature T we can state  

The number of charged and uncharged species remains constant through excitation and de-excitation by collisions with neutrals, ions and electrons. Absorption and emission, as they occur in a real radiation source, certainly also have to be considered, but they normally contribute only very little to the energy balance which is in the so-called local thermal equihbrium (LTE)  

---