DIFFERENTIAL CROSS-SECTION POLARIZATION MOMENTS - LOCATION OF THE D-ATOM TRANSFER IN THE TRANSITION-STATE REGION FOR THE REACTIONS CL-]DCL(V'=0,J'=1)+C2D5 AND CL+CD4-]DCL(V'=0,J'=1)+CD3(C2D6)

The photoloc technique can permit the measurement of not only the stat e-to-state differential cross section but also its complete product po larization dependence for all moments of orientation and alignment wit h k less than or equal to 2. We have realized this possibility for the reaction Cl + C2D6 --> DCl(upsilon' = 0,J' = 1) + C2D5 at a collision energy of 0.25 eV, for which we have measured the differential cross section, 1/sigma(d sigma(00)/d Omega(r)), and the four polarization-de pendent moments of the differential cross section, A(1)((1)stf), A(0)( (2)stf), A(1)((2)stf), and A(2)((2)stf), in the stationary target fram e (STF), which are defined by A(q)((k)stf) = (d sigma(kq)(stf)/d Omega (r))/(d sigma(00)/d Omega(r)). For the Cl + CD4 --> DCl(upsilon' = 0,J ' = 1) + CD3 reaction at a collision energy of 0.28 eV we have also de termined 1/sigma(d sigma(00)/d Omega(r)) and A(0)((2)stf). The laborat ory speed distributions of the DCl(upsilon' = 0,J' = 1) products are m easured using 2 + 1 resonance-enhanced multiphoton ionization (REMPI) and the core-extraction technique. The polarization-dependent differen tial cross sections are determined from the dependence of the core-ext racted profiles on the photolysis and probe polarizations. Recent stud ies have shown that the Cl + CD, and Cl + C2D6 both show scattering be havior described by the line-of-centers model and both yield rotationa lly cold DCl products with little energy in the alkyl fragments. Despi te these similarities, we measure DCl(upsilon' = 0,J' = 1) product pol arizations that differ greatly for these two reactions. For the Cl + C D4 reaction, we find that J(DCl) is maximally aligned perpendicular to an axis close to the product scattering direction, u(DCl). For the Cl + C2D6 reaction, we find that J(DCl) is half-maximally aligned perpen dicular to the line-of-centers direction. We interpret these results i n terms of the location of the D-atom transfer along the reaction coor dinate, positing that the D-atom transfer for the Cl + CD4 reaction oc curs late in the reactive process and the D-atom transfer for the Cl C2D6 reaction occurs earlier near the distance of closest approach. W e interpret the difference in the locations of the D-atom transfer to be the cause of the large differences in the Arrhenius pre-exponential factors of the Cl + CD4 and Cl + C2D6 reactions. (C) 1997 American In stitute of Physics.