Photolysis of Cl 2 initiates the title reaction at a sharply defined collision energy of 0.24Ϯ0.03 eV. Nascent product rotational state distributions for HCl ͑vϭ0͒ are determined using resonance enhanced multiphoton ionization ͑REMPI͒, center-of-mass scattering distributions are measured by the core-extraction technique, and the average internal energy of the C 2 H 5 product is deduced from the dependence of the core-extracted signal on the photolysis polarization. The HCl product has little rotational excitation, but the scattering distribution is nearly isotropic. Although seemingly contradictory, both of these features can be accounted for by using the simple line-of-centers model presented to explain earlier results for the ClϩCH 4 reaction. In contrast to the ClϩCH 4 reaction, the data suggest that the ClϩC 2 H 6 reaction proceeds through a loosely constrained transition-state geometry. The reactions of atomic chlorine with ethane, C 2 H 6 , and perdeuteroethane, C 2 D 6 , yield virtually identical results. These findings, along with the low energy deposited by the reaction into the ethyl product ͑200Ϯ120 cm Ϫ1 ͒, demonstrate that the alkyl fragment acts largely as a spectator in this hydrogen abstraction reaction.

The title reaction has been studied by observing the ethyl radical product by means of resonance-enhanced multiphoton ionization (REMPI) that proceeds at 242 nm via highly dissociative Rydberg states. The method of core extraction was employed to measure the speed and spatial anisotropy distributions of the C2H5 product from which the differential cross section and the internal energy distribution of the products were deduced. The C2H5 product exhibits broad scattering that peaks sideways, and the internal modes of this product are not significantly excited. These results agree closely with those found from a previous study of the title reaction in which the HCl product was detected by REMPI under the same conditions. Additionally, we report REMPI detection of the propyl radical product in the analogous Cl + C3H8 reaction.

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.

No abstract