Lines from the J = 6 → 5 rotational transition of CH₃CN observed at a pressure of 10⁻⁵ mbar (top panel) and 10⁻⁴ mbar (bottom panel) with integration times of 90 and 100 s, respectively. The observing mode is ON/OFF. Intensity is expressed in K and frequency in MHz. The intensities are proportional to pressure as collisional broadening is marginal at these pressures. The hyperfine structure of the K = 3 component is clearly seen in the bottom panel. The K = 2 component exhibits an emerging left shoulder due to the hyperfine splitting of the line.