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 $1+<z<5+$).^{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 $z+1$ to the observed one $z$ in the stationary case) as function of the electron temperature $Te$ can be predicted by the Saha equation, once the ionization potential is known, with^{18}^{,}^{19}Display Formula
$nz+1nz=S(z,Te)\alpha r(z+1,Te)+\u2009\u2009ne\alpha 3b(z+1,Te),$(1)
where $S(z,Te)$ is the collisional ionization coefficient, $\alpha r(z;Te)$ is the radiative recombination coefficient, and $\alpha 3b(z;Te)$ is the three-body recombination coefficient. These coefficients are given by the following equations:^{18}Display Formula$S(z,Te)=9\xd710\u22126\xi z(Te\chi z)12\chi z32(4.88+Te\chi z)exp(\u2212\chi zTe),$(2)
Display Formula$\alpha r(z,Te)=5.2\xd710\u221214(\chi zTe)1/2z[0.429+0.5\u2009log(\chi zTe)+0.469(\chi zTe)1/2+],$(3)
Display Formula$\alpha 3b(z,Te)=2.97\xd710\u221227\xi zTe\chi z2(4.88+Te\chi z),$(4)
where $\chi z$ is the ionization potential and $\xi z$ is the number of open shell electrons corresponding to the state of charge $z$.