Inverted level populations of hydrogen atoms in ionized gas

TL;DR

This study investigates how stimulated emissions influence hydrogen recombination lines and level populations in ionized gas, revealing that stimulated emission can significantly narrow line widths and affect plasma diagnostics.

Contribution

An improved model incorporating radiation fields effects on hydrogen level populations and line profiles, enhancing understanding of stimulated emission impacts in ionized gas.

Findings

Hydrogen recombination lines often originate from inverted population levels.

Strong stimulated emissions can narrow line widths to less than 10 km/s.

Line optical depth influences line amplification more than total optical depth.

Abstract

Context. Level population inversion of hydrogen atoms in ionized gas may lead to stimulated emission of hydrogen recombination lines, and the level populations can in turn be affected by powerful stimulated emissions. Aims. In this work the interaction of the radiation fields and the level population inversion of hydrogen atoms is studied. The effect of the stimulated emissions on the line profiles is also investigated. Methods. Our previous nl-model for calculating level populations of hydrogen atoms and hydrogen recombination lines is improved. The effects of line and continuum radiation fields on the level populations are considered in the improved model. By using this method the properties of simulated hydrogen recombination lines and level populations are used in analyses. Results. The simulations show that hydrogen radio recombination lines are often emitted from the energy level with an inverted population. The widths of Hn$\alpha$ lines can be significantly narrowed by strong stimulated emissions to be even less than 10 km s$^{-1}$. The amplification of hydrogen recombination lines is more affected by the line optical depth than by the total optical depth. The influence of stimulated emission on the estimates of electron temperature and density of ionized gas is evaluated. We find that comparing multiple line-to-continuum ratios is a reliable method for estimating the electron temperature, while the effectiveness of the estimation of electron density is determined by the relative significance of stimulated emission.