LINESHAPES OF INFRARED RADIOFREQUENCY DOUBLE RESONANCE SPECTRA IN $CH_{3}I^{1}$

$^{1}$ This research was supported by the U.S. National Science Foundation. Address of Fawzy and Schwendeman: Department of Chemistry, Michigan State University, East Lansing, MI 48824.Author Institution:The lineshapes of the $F, J, K = 23/2, 9, 8 \leftarrow 21/5, 9, 8$ ``pure'' nuclear quadrupole transition near 360.7 MHz in the ground vibrational state and the $25/2, 10, 9 \leftarrow 23/2, 10, 9$ transition near 382.9 MHz in the $v_{4} = 1$ state of $CH_{3}$ have been recorded by infrared radiofrequency double resonance (IR-RF DR) spectroscopy as a function of pressure and radiofrequency power. The infrared radiation was a $^{12}C^{14}O_{2}$ laser operating on the 10R(8) transition and the sample cell was outside the laser cavity. The spectra have been fit to a sum of two Lorentzian lineshapes to give the linewidths and center frequencies of each of the transitions. Plots of the linewidths vs. pressure at low RF power are linear within experimental error with slopes of $6.08 \pm 0.19$ MHz/Torr and $6.05 \pm 0.19$ MHz/Torr for the ground state and $v_{6} = 1$ state transitions, respectively. Further increase in pressure causes the upper state transition to disappear and changes the sign of the intensity of the lower state transition. The observed effects are not adequately represented by calculations based on simple three-level resonances. We have written a computer program to calculate the double resonance effect in a four-level system consisting of two coupled three-level systems and have obtained much better agreement between observed and calculated spectra. Lineshapes predicted by the new program will be compared to experimental spectra