VELOCITY DEPENDENCE OF COLLISIONALLY TRANSFERRED SPIKES OBSERVED BY INFRARED. INFRARED FOUR-LEVEL DOUBLE RESONANCE$^{1}$

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1990

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Ohio State University

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The center frequency of a collisionally-induced transferred spike observed by infrared-infrared four-level double resonance depends on the difference between the frequency of the pump laser and the center frequency of the pumped transition. This difference determines the component of the velocity of the pumped molecules in the direction of propagation of the pump laser. The r.m.s. speed of the pumped molecules increases with this difference. In order to change the r.m.s. speed of the pumped molecules, we have locked the frequency of the pump laser to the frequency of the saturation dip in a laser-Stark absorption in a separate sample cell outside the laser cavity. By varying the Stark field it has been possible to stabilize the laser at any frequency within $\pm 15$ MHz from the usual saturation dip in the $CO_{2}$ fluorescence. This technique has been used to stabilize the frequency of the 9P(32) transition of a $^{12}C^{1} {^{6}}O_{2}$ laser by means of the J.k.m.$- 5,1,5 - 4,1,4$ transition in $^{13}CH_{3}F$. The laser stabilization scheme just described has been used in double resonance experiments to pump the $^{Q}R(4,3)$ transition in 13CH3F with offset frequencies between 8 and 38 MHz. The variation in the collisionally-transferred spikes in several transitions in the $2\nu_{3} - \nu_{3}$ band of the same species were probed by means of an infrared microwave sideband laser spectrometer. The lineshapes of the transferred spikes have been analyzed by means of an equation derived from the $Keilson-Storer^{2}$ collision kernel. The r.m.s. change in velocity for the overall collisionally-induced transition from the upper level of the pump transition to the lower level of the probe transition has been deduced for each lineshape by this analysis. The change in velocity has been found to be - 15\% larger for 38-MHz offsets than for 8-MHz offsets. Some differences in the lineshapes with increasing offsets have also been noted. Possible interpretations of these results will be discussed

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$^{1}$ Supported in part by the National Science Foundation $^{2}$ J. Keilson and J. E. Storer, Q. Appl. Math. 10, 243-253 (1952).
Author Institution: Department of Chemistry, Michigan State University

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