K. Seki et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 200–203
203
We consider the mechanism for Cl detachment process using
our experimental results, that is, acetylene formation is one of
the main photochemical process (˚(C H ) = 0.47) and no Cl is
In conclusion we have measured the absolute quantum yields
of acetylene and chloroacetylene, which is 0.47 and 0.35, respec-
tively, at 193 nm. Consideration of the reaction mechanism gives
the quantum yields of Cl (˚ = 0.94) and HCl (˚ = 0.35). These results
imply that two Cl atoms with different kinetic energies are succes-
sively dissociated from the photolysis of one trans-DCE molecule
at 193 nm. We plan to measure the quantum yields for cis-DCE and
2
2
2
observed. It is impossible to explain these results, if only one Cl
atom is produced from one trans-DCE after photolysis and the
counterpart:C H Cl does not dissociate into another Cl atom. Then
2
2
we suggest the following scheme,
1
,1-DCE at 193 nm photolysis.
#
trans-C H Cl + hꢀ (193 nm) → C H Cl + Cl :
2
2
2
2
2
Eavl = 59 kcal mol−1,
(7a)
(7b)
Acknowledgement
#
C H Cl → C H + Cl,
Part of this work was supported by the Sumitomo Foundation.
2
2
2
2
References
trans-C H Cl + hꢀ (193 nm) → C H + Cl + Cl :
2
2
2
2
2
[
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[
[
.
(8)
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(
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[
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#
(
C H Cl ) produced by reaction (7a) is energetically possible to
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2
2
6
dissociate to C H + Cl at 193 nm photolysis [23]. Since total trans-
2
2
−
1
lational energy after (7a) reaction is 26 kcal mol for high energy
Cl atom [8], internal energy of C H Cl is 23.4 kcal mol (calcla-
[
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#
−1
2
2
−
1
tion from 59.4 to 26) and larger than the ꢁH (22.9 kcal mol ) of
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5
(
1
7b) reaction [24,25]. Sato also evaluated the reaction rate constant:
[
1
3
−1
× 10
s using RRK calculation and suggested (7b) reaction has
no barrier. The scheme for (7a) and (7b) sequential reactions can
produce the acetylene and two kinds Cl atoms. On the other hand it
is difficult to explain the presence of two kinds of Cl atoms produced
from reaction (8).
[
1
[
[
Next we try to explain other studies using the sequential
scheme. Suzuki et al. [12] reported the translational energy dis-
tributions of Cl atom produced from the photolysis of trans-DCE
at 193, 210 and 235 nm using a two-dimensional ion imaging
technique. The yields of the two different Cl energy channels in
trans-DCE vary with the wavelength of the exciting light. At 193 nm
the yield of the lower energy component (Plow) is comparable to
that of higher energy (Phigh). On the other hand at 235 nm Plow
is about 4 times smaller than Phigh. They explained these results
by an avoided crossing between the (,*) and repulsive states in
the C C twist coordinate. Now we can show a simple explanation
for the Cl yields based on the sequential scheme. The wavelength
dependence indicates that the second Cl detachment depends on
the excess energy. That is, at 235 nm excitation the excess energy
5
(
[
[
[
[
[
[
2
[
[
of C H Cl as a counterpart of the first Cl detachment is not enough
2
2
[27] R. Atokinson, S.M. Aschmann, Int. J. Chem. Kinet. 19 (1987) 1097–1105.
[28] C.-J. Weng, T.-I. Ho, T.-M. Su, J. Phys. Chem. 91 (1987) 5235–5238.
to dissociate the second Cl atom completely.