Abstract
Infrared‐microwave (IR‐MW) double resonance was demonstrated on the ν2 sQ (3,3) line of NH3 using a tunable diode laser. The laser beam was focused into a waveguide cell. The double‐resonance effect was observed as a variation of infrared absorption when the J=K=3 inversion line of NH3 was saturated in the ground vibrational state. At low pressure the signal reversed its sign, indicating saturation of the infrared transition by the laser. Application of the technique to microwave spectroscopy of vibrationally excited molecules is discussed.