Knowledge Bank

The resonance frequency of the transition in the $\nu 3$ fundamental band of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH3F$ is approximately 26 MHz below the frequency of the 9P(22) laser transition in $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C118O2$. We have used this laser coincidence to pump molecules into the $\nu 3-1$ vibrational state of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}CH3F$ while observing double resonance effects in transitions in the $\nu 3$ fundamental band and in the $2\nu 3\leftarrow \nu 3$ hot band by means of an infrared microwave sideband laser spectrometer. Both three-level and four-level double resonance effects were observed and were separated from single resonance absorption by means of a double-modulation scheme. The experiments were performed at room temperature with sample pressures of 10-40 mTorr in a cell with one-meter pith. The double resonance effects observed in the hot-band transitions strongly parallel the effects recently reported for $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CH3F.2$ Sharp transferred spikes are seen in transitions originating in states with $K-9$ and with . Evidence for broader spikes are seen for transitions with $K-3$ and $K-6$, confirming the $\Delta k-\pm 3n$ selection rules for collisionally-induced rotational transitions. The lineshapes of the transferred spikes have been analyzed by means of a theoretical equation derived by assuming a $Keilson-Storer3$ collision kernel. The width of a spike, which is related to the r.m.s. change in velocity upon collision, is greater for larger values of the absolute value of difference between the J value of the lower level of the probe transition and the J value (12) of the level pumped. This result can be explained by assuming that the number of collisions required to transfer molecules from the pumped to the probed level increases with this difference. The widths of the transferred spikes observed for several fundamental transitions are not so easily explained because both levels involved in the transition are pumped. Nevertheless, a rationalization will be described