Level populations vs. density

In addition to enhancing the total recombination rate, high densities enhance the importance of collisional processes relative to radiative processes in bound-bound transitions. Level populations approach their LTE values, which may greatly exceed the recombination values for levels with dipole allowed decays. This is illustrated in figure 10, which shows the ratio of level populations to LTE populations (departure coefficients) as functions of density for H I at log($\xi$)=-5 and T=10$^4$K (panel a) and for O VIII at log($\xi$)=-5 and T=10$^6$K (panel b). Departure coefficients of all bound spectroscopic levels decrease proportional to density, approaching assymptotic values at densities greater than 10$^{17}$ cm$^{-3}$. The superlevels exhibit slower dependence on density, reflecting the fact that they are likely to be in LTE with the continuum at lower density than the spectroscopic levels.