Double resonance measurements of far infrared lasing transitions in methanol

In this work, precise frequencies have been measured for two important far-infrared (FIR) laser transitions in methanol. These are optically pumped by the 9P(16) CO₂ laser line at an offset of +64 MHz from the line center, using infrared (IR)-millimetrewave (MMW) double resonance (DR) techniques. The two measured FIR lines are at 525,427.902 and 477,790.879 MHz, with an estimated precision of ± 50 kHz. Another line has been observed at 573,025.151 MHz. It corresponds to an absorption transition whose lower level is the upper pumped level. The measurements provide a valuable confirmation of the assignments of the IR as well as the FIR transitions. In addition, accurate frequencies for several transitions have been determined in the ground vibrational state in the range 109-127 GHz and provide assignments for additional microwave lines.