Population diagram for vibrationally excited HNC and HC₃N. For HNC, the data points relative to the J = 3, v = 0, and v₂ = 1 states are shown as open diamonds. For HC₃N, the J = 30, v = 0 state is drawn as a diamond, while the v₆ = 1, v₇ = 1, and v₇ = 2 values, derived from all the observed transitions, are shown as stars, circles, and a triangle. The solid lines show the vibrational temperature fit, while error bars represent 1σ uncertainties. For a discussion see Sect. 4.2.2.