Knowledge Bank

A recent report from this laboratory describes the observation of collisionally-induced transferred spikes in the lineshapes of transitions in the $2\nu 3\leftarrow \nu 3$ hot band of $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CH3F.2$ The spikes, which were produced by pumping the transition in the $\nu 3$ fundamental band by means of a $9P(32)CO2$ laser, were recorded by means of an infrared microwave sideband laser spectrometer. Sample pressures were 10-50 mTorr. The transferred spikes were observed in many transitions originating in $K-3$ levels of the $\nu 3-1$ state. We have now succeeded in time-resolving the probe signals both upon switching on and upon switching off the pump laser. The pump laser output is turned on or off by the combination of an electro-optic crystal and a Fresnel rhomb. By switching an electric field across the crystal between appropriate positive and negative values, the plane of polarization of the radiation can be changed by $90\circ$ in $\sim 0.5\mu s$. A polarizer blocks the beam during one-half the cycle. Although the electronics allow switching at a rate up to 10 kHz under TTL control, most of the experiments to date have been performed at 100 Hz to allow establishment of near steady-state conditions before each change in the pump power applied to the sample. The steady-state lineshape of the four-level double-resonance transition in the hot band is a superimposition of a narrow transferred spike and a Gaussian component that has the expected Doppler width. We have interpreted the spike to be the result of collisionally-induced rotational transitions bringing a pumped molecule to the lower level of the probe transition, whereas the Gaussian part is the result of a swapping of vibrational enery between a pumped molecule and a molecule in the ground vibrational state. The time-resolved spectra show that the effect of double-resonance pumping of the transferred spike is a much faster process than pumping the Gaussian part. The rate of appearance or disappearance of the transferred spike also decreases as J(probe). J(pump) increases. Other characteristics of the time-resolved spectra will be described