16
M. Kono et al. / Chemical Physics Letters 418 (2006) 15–18
2
. Experimental method
1
0
0
.0
.5
.0
a
Because the experimental setup used in this work is
essentially the same as in our previous studies [12–14], the
following FP-LIF technique is described in brief. Ar-
dilute-gas mixtures of a small amount of HCl and an excess
of quencher were slowly introduced into a photochemical
reaction cell evacuated continuously by a rotary pump
through a liquid nitrogen trap. For the quencher gas mix-
e0
b
ture, Ar gas was bubbled through H O or D O liquid.
e-1
e-2
e-3
e-4
2
2
The concentrations of H O and D O in the gas mixture
2
2
were monitored at the gas stream before the cell by a
dew-point meter (General Eastern Optica+D-2). The total
pressure in the cell was measured by a capacitance manom-
eter (MKS, Baratron 622A). The HCl molecules in the cell
were photodissociated by a 193-nm pulsed laser. The quan-
tum yields in the photolysis of HCl at 193 nm are 0.41 and
0
50
100
150
Time / µs
2
2
0
.59 for Cl production in the P and P states, respec-
1/2 3/2
2
Fig. 1. Temporal profile, shown in: (a) linear and (b) logarithmic scales, of
the Cl( P1/2) concentration after the photodissociation of HCl at 193 nm.
The solid lines are exponential decay fits to the temporal profiles.
tively [15]. The Cl( P1/2) atoms were detected using VUV-
LIF excitation at 135.17 nm, corresponding to the
2
4
2
5 2
3
p 4s( P1/2)
3p ( P1/2) transition. The tunable VUV radi-
ation was generated by two-photon resonant four-wave dif-
ference frequency mixing in krypton gas [16]. The delay time
between the pump and probe laser pulses was controlled by
a pulse generator (Stanford Research, DG535). The VUV-
LIF signal was detected using a solar blind photo-multiplier
tube (EMR, 541J-08-17). The output of the photo-multi-
plier was pre-amplified and averaged over 10 laser pulses
using a gated integrator (Stanford Research, SR-250).
A 30% w/v aqueous H O solution was gently distilled
in which 7.8 mTorr of HCl was photolyzed at 193 nm in
the presence of 18.7 mTorr of H O and 1.23 Torr of Ar.
2
Single exponential decay curves were observed as the tem-
poral profiles under all experimental conditions. Since the
total removal rate constants in collisions with HCl and
ꢁ
12
ꢁ16
3
Ar are (7.8 ± 0.8) · 10
[13] and 65 · 10
[14], respectively, the contributions of HCl
and Ar to removal rates are insignificant. Both the contri-
butions of the photodissociation of H O, D O, or H
can be ignored. The
recommended reaction rate constant of the later process
cm mole-
ꢁ
1
ꢁ1
cules
s
2
2
undervacuum and stored in aglass bulb. The partial pressure
of H O in the reaction cell was calibrated by measuring
2
2
O
2 2
2
and the reaction of Cl( P1/2) + H O
2 2
2
2
photoabsorption at 193 nm. The photoabsorption cross sec-
ꢁ
19
2
ꢁ13
3
ꢁ1 ꢁ1
tion at 193 nm is reported as 5.89 · 10
cm [17]. The same
is only 4.1 · 10
Fig. 2 shows the plots of the decay rates versus the con-
centrations of H O and D O. The ordinate values have
cm molecules
s
[17].
cell (optical path length 325 mm) and 193-nm pulsed laser
were used as an absorption cell and a light source, respec-
tively. The laser beam was introduced into the cell through
an iris (4.5 mm in diameter) and a neutral-density filter
2
2
been corrected for the contributions of HCl. The straight
lines are the results of weighted least-squares fits analysis.
ꢁ
11
(
10% transmittance). The light intensity was measured at
The rate constants, (3.9 ± 0.2) · 10
and (4.7 ± 0.2) ·
ꢁ11
3
ꢁ1 ꢁ1
the exit window of the cell by a photodiode detector (Ham-
amatsu S1722-02). The signal output was acquired through
the same gated integrator and the data were averaged for
10
cm molecules
s , are given by the slopes of the
fit lines in Figs. 2a,b, respectively, where the analytical
uncertainties are taken as 1r. The upper limit of the estima-
tion uncertainty of the HCl contribution (5%), the system-
atic uncertainties (2%), the precision of the mass flow
controllers and the capacitance manometer (2%), and the
precision of the dew-point meter (1%) must be taken into
account. Having taken all into account, we determine
ꢀ
60 s. From the slope of the photoabsorbances measured
under 12 pressure conditions, the purity of the sample
vapour was determined. The other reagents, D O (Wako,
2
9
9.96%), Ar (Nihon Sanso, 99.999%) and HCl (Sumitomo
Seika, 99.8%) were obtained commercially and were used
in the experiments without further purification.
ꢁ
11
the values of (3.9 ± 0.3) · 10
and (4.7 ± 0.3) ·
as the rate constants of the col-
lisional quenching processes by H O, D O, respectively.
Fig. 3 shows the plots of the decay rates versus the con-
centration of H . The ordinate values have been cor-
rected for the contributions of HCl and H O. Although
the partial pressure of H O was about 37% of that for
in the photolysis cell, the contribution of H O to
the decay rate was very small, since the reaction rate con-
ꢁ11
3
ꢁ1 ꢁ1
10
cm molecules
s
3
. Results and discussion
2
2
2
The loss of Cl( P1/2) by all quenchers is regarded as fol-
O
2 2
lowing pseudo-first-order kinetics under the present exper-
2
2
imental conditions. To obtain temporal profiles of Cl( P1/2)
2
concentration, the LIF intensity was measured by changing
the delay time between photolysis laser and LIF-detection
laser pulses. A typical temporal profile is shown in Fig. 1,
H
O
2
2
2
2
stant of Cl( P1/2) with H
O
was ꢀ10 times larger than that
2
2
Kono, Mitsuhiko
