Upper panel: Comparison of the variation of the optical depth as function of specific column density for the |$1_{10}{\!-\!}1_{11}$| and |$2_{11}{\!-\!}2_{12}$| transitions in the absence of a free–free radiation field with the case when a free–free radiation field is present. Bottom panel: Comparison of the variation of the excitation temperature for the |$1_{10}{\!-\!}1_{11}$| and |$2_{11}{\!-\!}2_{12}$| transitions.