Knowledge Bank

The method of Shin and $Schwendeman1$ has been used to record infrared-infrared four-level double-resonance spectra of $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CH3F$ under conditions of population modulation with the planes of polarization of the pump and probe lasers either parallel or perpendicular to one another. The technique has also been used to record spectra under conditions of alignment modulation in which the polarization of the pump laser is switched between vertical and horizontal planes while the plane of polarization of the probe beam remains vertical. The spectra taken under conditions of population modulation show different intensity for the two different relative orientations of the pump and probe beams, which requires some conservation of the spatial orientation of the angular momentum during collisionally-induced transitions. The intensity of the spectrum taken under conditions of alignment modulation can be shown to be proportional to the $n=2$ spherical tensor combination of the populations of the degenerate m states (alignment) of the lower level of the probe transition. Analysis of the lineshapes of the double resonance shows that the collisions that preserve the alignment are those that occur with very large impact parameter and therefore cause very little change in the velocity of absorber molecule while apparently preserving the orientation of the angular momentum. The alignment modulation spectrum, which is observable only when the component of the rotational angular momentum along the molecular symmetry axis (K) is unchanged, is very strong when the total rotational angular momentum (J) of the pumped and probed levels differ by only one unit, but is still observable when J changes by 5 units. The experimental apparatus will be described and the results of lineshape analyses of the various spectra will be shown.