Differential cross section polarization moments: Location of the D-atom transfer in the transition-state region for the reactions Cl+C2D6→DCl(v′=0,J′=1)+C2D5 and Cl+CD4→DCl(v′=0,J′=1)+CD3

Abstract

The photoloc technique can permit the measurement of not only the state-to-state differential cross section but also its complete product polarization dependence for all moments of orientation and alignment with k⩽2. We have realized this possibility for the reaction Cl+C2D6→DCl(v′=0,J′=1)+C2D5 at a collision energy of 0.25 eV, for which we have measured the differential cross section, 1/σ(dσ00/dΩr), and the four polarization-dependent moments of the differential cross section, A1(1)stf, A0(2)stf, A1(2)stf, and A2(2)stf, in the stationary target frame (STF), which are defined by Aq(k)stf=(dσkqstf/dΩr)/(dσ00/dΩr). For the Cl+CD4→DCl(v′=0,J′=1)+CD3 reaction at a collision energy of 0.28 eV we have also determined 1/σ(dσ00/dΩr) and A0(2)stf. The laboratory speed distributions of the DCl(v′=0,J′=1) products are measured using 2+1 resonance-enhanced multiphoton ionization (REMPI) and the core-extraction technique. The polarization-dependent differential cross sections are determined from the dependence of the core-extracted profiles on the photolysis and probe polarizations. Recent studies have shown that the Cl+CD4 and Cl+C2D6 both show scattering behavior described by the line-of-centers model and both yield rotationally cold DCl products with little energy in the alkyl fragments. Despite these similarities, we measure DCl(v′=0,J′=1) product polarizations that differ greatly for these two reactions. For the Cl+CD4 reaction, we find that JDCl is maximally aligned perpendicular to an axis close to the product scattering direction, uDCl. For the Cl+C2D6 reaction, we find that JDCl is half-maximally aligned perpendicular to the line-of-centers direction. We interpret these results in terms of the location of the D-atom transfer along the reaction coordinate, positing that the D-atom transfer for the Cl+CD4 reaction occurs late in the reactive process and the D-atom transfer for the Cl+C2D6 reaction occurs earlier near the distance of closest approach. We 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.