Detailed balance of absorption and emission of resonant radiation in dense media

A detailed balance of absorption and emission of resonant radiation is considered. We will show the existence of a two-temperature distribution for the spectral density of resonant states meeting the condition of detailed balance with temperatures describing translational degrees of freedom and the distribution in atomic states. Our analysis reveals an asymmetry in absorption coefficients for resonant radiation, with absorption coefficients being different on the far blue and red wings of the spectral line.