INFRARED-INFRARED DOUBLE RESONANCE OF HOT BANDS IN METHYL FLUORIDE

Author Institution: Department of Chemistry, Michigan State UniversityInfrared-infrared four-level double resonance spectroscopy has been used in search for vibration-rotation transitions in the $3v_{3}-2v_{3}$ hot band of $^{12}CH_{3}F$. In these experiments, either the $^{Q}R(13,6)$ or the $^{Q}R(7,3)$ transition in the $2v_{3}-v_{3}$ hot band was pumped by a $CO_{2}$ laser coincidence while transitions in the $3v_{3}-2v_{3}$ band were recorded by scanning an infrared microwave sideband laser system. By using this technique six transitions in the $3v_{3}-2v_{3}$ hot band have been observed and assigned. Four-level double resonance experiments have also been performed in $^{12}CH_{3}F$ by pumping one of the transitions in the $2v_{3}-V_{3}$ hot band and probing transitions in the same band. The lineshapes of the observed double resonance effects provide information about the relative rates of collisionally induced rotational energy transfer and near resonant vibration-vibration energy transfer (V-V transfer) in the molecules. The double resonances in the $3v_{3}$-2$v_{3}$ band show only the effects of rotational energy transfer, while the double resonances in the $2v_{3}-v_{3}$ band show the effects of rotational and V-V transfer with opposite phase. The Doppler-free feature of three-level infrared-infrared double resonance lineshapes has been employed to obtain precise center frequencies for transitions used for laser pumping in methyl fluoride. For this purpose, three-level double resonance line shapes have been recorded with both copropagating and counterpropagating configurations for several pump and probe combinations. In this work, the center frequencies for the $^{Q}Q(12,1)$ and $^{Q}Q(12,2)$ fundamental transitions in the $v_{3}$ band in $^{12}CH_{3}F$, and for the $QR(4,3)$ transition in the corresponding band in $^{13}$$CH_{3}F$ have been determined to an accuracy of -0.1 MHz."