Infrared-microwave double-resonance spectroscopy of CH3OH by use of sidebands of CO2 laser lines

An infrared-microwave double-resonance technique using microwave sidebands of CO2 laser lines as an infrared sour ce has been applied for observation of rotational lines of the methanol molecule. Frequencies of more than 50 r otational lines in the excited CO stretching vibrational state (v(co) = 1) have been measured with good precision and have been compared with those re ported in infrared studies. Many of them agree within several megahertz, al though some lines show differences of >10 MHz. The pressure dependence of t he double-resonance signals for two low-J microwave transitions belonging t o the ground and the v(co) = 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 an alysis explains the observed pressure dependence quantitatively and allows us to understand the physical processes involved in the double resonance. ( C) 1999 Optical Society of America [S0740-3224(99)01109-1].