ORIENTATION MODULATION IN FOUR-LEVEL INFRARED-INFRARED DOUBLE RESONANCE IN METHYL FLUORIDE AND AMMONIA
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Date
1994
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Publisher
Ohio State University
Abstract
Four-level IR-IR double resonance spectra of $^{l}3CH_{3}F$ and $^{1}5NH_{3}$ were taken under conditions of population modulation with circularly polarized pump and probe radiation using the method of Shin and $Schwendeman.^{1}$ For population modulation, a mechanically-chopped pump beam was fixed at either right or left circular polarization while the probe team was kept at fixed circular polarization. Four-level double resonance spectra were also taken under conditions of orientation modulation in which the polarization of the pump beam was switched between right and left circular polarization by an electro optic crystal modulator while the circular polarization of the probe beam was fixed. Thus, the difference in intensity between spectra taken under population modulation with right and left circularly polarized pump beam was observed directly with orientation modulation. The difference is direct evidence for partial conservation of the spatial orientation of the angular momentum during collision-induced rotational transitions. It can be shown that the intensity of the spectrum recorded under orientation modulation is proportional to the n=1 spherical tensor combination of the populations (orientation) of the degenerate m states of the lower level of the probe transition. Orientation modulation spectra are observable only when the quantum number k for the component of the angular momentum referred to the molecule-fixed symmetry axis is the same for the upper level of the pump and lower level of the probe transitions. The data were fit to theoretical lineshapes that were calculated by assuming that the tensor combinations created in the upper level of the pump transition were transferred intact to the lower level of the probe transition by collision. Analysis of the lineshapes shows that a loss of orientation occurs during collision-induced transitions. Also, only transitions that result from collisions that occur with large impact parameters and therefore cause little change in the velocity of the molecules show a tendency for conservation of the orientation. A description of the apparatus and samples of the observed spectra will be included in the discussion.
Description
1. Uhyon Shin and R.H. Schwendeman, J. Chem. Phys 96, 8699 - 8712 (1992).
Author Institution: Department of Chemistry, Michigan State University
Author Institution: Department of Chemistry, Michigan State University