Microwave Double-Photon Transitions in CD3CN and PF3

The microwave double-photon transitions $J=3\mathrm{}\twoheadleftarrow 2$ and $J=2\mathrm{}\twoheadleftarrow 0$ have been observed for the symmetric-top molecules C${\mathrm{D}}_3$CN and P${\mathrm{F}}_3$ using high-power microwave double resonance. The pumping of the $J=2\mathrm{}\twoheadleftarrow 0$ transition of C${\mathrm{D}}_3$CN was so efficient in the low-pressure region that a negative temperature was established between the $J=1\mathrm{}$ and the $J=0\mathrm{}$ levels, and induced emission $J=1\mathrm{}\to 0$ was observed. The pressure variation of the efficiency of the double-photon pumping agreed quantitiatively with theoretical values calculated using the Göppert-Mayer formula for the relevant dipole matrix element and Karplus and Schwinger's formula for saturation. The experiment provides interesting information on collision-induced transitions in C${\mathrm{D}}_3$CN.