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)
Tp. Rakitzis et al., 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 Journal of chemical physics, 107(22), 1997, pp. 9392-9405
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.