From left to right: Spectra for J = 2−1 (first) and 3−2 (second and third) transitions of HCN towards IRC +10216. The vibrational state of each transition is indicated in the top right corner of the respective spectrum. The vertical dashed blue lines denote the range of the emission in V_LSR, the vertical dotted line shows the systemic V_LSR of the star, and the horizontal dotted line is the baseline. The number below the vibrational quantum numbers in the panel is a code assigned to the date of observation of the particular transition. The corresponding dates of the codes are listed in the notes of Table 4.