POLARIZATION MODULATION EFFECTS IN INFRARED-INFRARED 4-LEVEL DOUBLE-RESONANCE IN (CH3F)-C-13 AND (NH3)-N-15

The elements of the Jones matrices for an optically pumped sample have been derived and used to predict four-level double resonance absorpti on coefficients that are functions of the velocity component of the mo lecules in the direction of the pump beam for different polarizations of the probe beam. When a saturating pump and weak probe are used in f our-level double resonance experiments under population modulation con ditions, these absorption coefficients are found to depend only on the first three statistical tensor ranks: n = 0 (population), 1 (orientat ion), and 2 (alignment). It is also found that for polarization modula tion experiments with plane-polarized radiation, the absorption coeffi cient depends only on the alignment of the rn-state populations. Simil arly, for polarization modulation with circularly polarized radiation, the absorption coefficient depends only on the orientation. The theor y was used to interpret double-resonance polarization modulation exper iments in (CH3F)-C-13 and (NH3)-N-15 in order to examine the effects o f collisions on the initial anisotropy of the projection of J on a spa ce-fixed Z axis. The four-level double-resonance lineshapes were fit b y least squares to absorption coefficients predicted by the theory. Th e collisional effects were modeled by a sum of Keilson-Storer collisio n kernels. The results of the fits were much improved when the value o f the effective rate constant for the transfer of the n = 0 tensor fro m the upper level of the pump to the lower level of the probe was larg er than the values of the effective rate constants for the transfer of the ii = 1 and 2 populations. The best ratio of the rate constant for it > 0 to that for n = 0 is about 2/3 for( 13)CH(3)F and 1/3 for (NH3 )-N-15. Additional analysis of the lineshapes showed the importance of long-range dipole-dipole interactions, elastic realignment and reorie ntation, and V-V mechanisms for collision-induced rotational energy tr ansfer in( 13)CH(3)F and (NH3)-N-15. (C) 1998 Academic Press.