Population diagram for the vibrational excitation of HC₃N. For the three rotational levels J = 10 (diamonds), J = 11 (circles), J = 12 (triangles) we compare the relative populations of the v = 0, v7 = 1, v7 = 2, and v6 = 1 vibrationally excited states. The vibrational temperature fit is shown as a dashed, dotted and dash-dotted line, for the J = 10–9, 11–10 , and 12–11 transition respectively. The three fits result in similar vibrational temperatures in the range 330−400 K.