The plasma atomic processes that lead to radiation emission in the VUV range include spontaneous emission from both high and low states of charges (on the order of ).16 Photoabsorption processes are also observed in this wavelength region with the use, for example, of the dual pulsed laser technique.17 Emission in this wavelength region occurs for planar targets within a few mm from the target.17 In this case, the plasma can be optically considered thin. Assuming a collisional-radiative equilibrium, the ion fractions (i.e., the ionic chain, which relates the fraction of the next ion stage to the observed one in the stationary case) as function of the electron temperature can be predicted by the Saha equation, once the ionization potential is known, with18,19Display Formula
(1)where is the collisional ionization coefficient, is the radiative recombination coefficient, and is the three-body recombination coefficient. These coefficients are given by the following equations:18Display Formula
(4)where is the ionization potential and is the number of open shell electrons corresponding to the state of charge .