Infrared-Microwave Double Resonance of NH3 Using an N2O Laser

The infrared vibration-rotation transitions \ν₂[qQ-(8,7)] of ¹⁴ NH₃ and \ν₂[qQ-(4, 4)] of ¹³ NH₃ have been pumped by the P(13) and P(15) lines, respectively, of an N₂O-laser, and the resulting changes in the molecular populations of the rotational levles have been monitored by observing the microwave inversion lines. Large variations of the intensities of the monitored lines have been observed. In addition small variations of neighboring inversion lines, due to collision-induced transitions, have also been found. The dependence of the efficiency of pumping on the frequency difference between the laser line and the NH₂ absorption line (which was varied by using a Stark field), and on the pressure of the sample, was studied. The results can be explained by using Karplus and Schwinger’s theory of saturation and by taking into account a “hole burning” on the Doppler profile of the absorption line. The potential of this method for the studies of rotational energy transfer is discussed.