Infrared–microwave double-resonance spectroscopy of CH_3OH by use of sidebands of CO_2 laser lines

An infrared–microwave double-resonance technique using microwave sidebands of CO2 laser lines as an infrared source has been applied for observation of rotational lines of the methanol molecule. Frequencies of more than 50 rotational lines in the excited C—O stretching vibrational state (vco=1) have been measured with good precision and have been compared with those reported in infrared studies. Many of them agree within several megahertz, although some lines show differences of >10 MHz. The pressure dependence of the double-resonance signals for two low-J microwave transitions belonging to the ground and the vco=1 states, respectively, have been observed for sample pressures as high as 0.4 Torr. For the former transition the signal has been observed to change its sign at higher pressures. Rate equation analysis explains the observed pressure dependence quantitatively and allows us to understand the physical processes involved in the double resonance.