Knowledge Bank

Three-level and four-level infrared-infrared double resonance experiments were performed on the $v10$ band of cyclopropane. The purpose of these experiments was to extend our recent studies of velocity dependence of collisional energy transfer to a molecule that not only does not have a permanent dipole moment, but which also has rather non-polar bonds. Both the observed three-level and the four-level double resonance spectra consisted of broad lines with widths comparable to the expected Doppler widths; no velocity-conserving spikes in the three-level spectra or transferred spikes in the four-level spectra were observed. The absence of spikes in the three-level spectra is especially puzzling, because we know of no proven mechanism for such an observation under high resolution for our experimental conditions. The absence of spikes in the four-level spectra is less surprising given the appearance of the three-level lineshapes. Possible processes that could broaden the three-level spectra will be discussed.