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Context. Level population inversion of hydrogen atoms in ionized gas may leadto stimulated emission of hydrogen recombination lines, and the levelpopulations can in turn be affected by powerful stimulated emissions. Aims. Inthis work the interaction of the radiation fields and the level populationinversion of hydrogen atoms is studied. The effect of the stimulated emissionson the line profiles is also investigated. Methods. Our previous nl-model forcalculating level populations of hydrogen atoms and hydrogen recombinationlines is improved. The effects of line and continuum radiation fields on thelevel populations are considered in the improved model. By using this methodthe properties of simulated hydrogen recombination lines and level populationsare used in analyses. Results. The simulations show that hydrogen radiorecombination lines are often emitted from the energy level with an invertedpopulation. The widths of Hn$\alpha$ lines can be significantly narrowed bystrong stimulated emissions to be even less than 10 km s$^{-1}$. Theamplification of hydrogen recombination lines is more affected by the lineoptical depth than by the total optical depth. The influence of stimulatedemission on the estimates of electron temperature and density of ionized gas isevaluated. We find that comparing multiple line-to-continuum ratios is areliable method for estimating the electron temperature, while theeffectiveness of the estimation of electron density is determined by therelative significance of stimulated emission.

Inverted level populations of hydrogen atoms in ionized gas