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LARIN et al.
.
tored. The amount of ozone entering the reactor in 1 s
was determined using the formula
IO radicals. The zone L, whose upper boundary was
at a distance of 1 cm from the nozzle cut and whose
lower boundary coincided with the inlet through which
2.3D10
3.25 × 1016σLcellPcell
-------------------------------------------------
Q(O3) = Q(O2 + He)
NO was fed into the reactor, is shown in Fig. 1. In the
.
(1)
zone L, all of the active iodine exists as IO . The length
[cm3 Torr s–1],
of the zone L could be varied between 0 and 20 cm by
.
moving the nozzle. The concentration of the IO radi-
.
where Q(O2 + He) is the total flow rate in the ozonizer
line (cm3 Torr s–1), D10 is absorbance in decimal loga-
rithms (spectrophotometer reading), σ253.7 = 1.15 ×
10−17 cm2 is the absorption cross section of ozone at λ =
253.7 nm, Lcell is the optical path length of the cell, and
Pcell is the gas pressure in the cell (Torr).
cals at the entrance end of the zone L ([IO ]0) was
equal to the concentration of iodine atoms introduced
through the nozzle. The change in the IO concentra-
tion across the zone L was determined by the heteroge-
neous loss reaction
.
The absolute sensitivity of this technique to iodine
atoms was calibrated before each series of experiments.
The absolute sensitivity of the technique in different
series of experiments varied between 3 × 107 and
108 cm–3 at a signal-to-noise ratio of 1 and a pulse accu-
mulation time of ~100 s.
k6
IO + wall
products
(VI)
.
and by the reaction between the IO radical and the RH
molecule (reaction (III)).
.
Let the concentration of IO radicals at the exit end
.
of the zone L be designated [IO ]fin. At the exit end of
the zone L, NO (~1014 cm–3) was fed into the vessel
.
Study of Reactions of the IO Radical
.
through a side inlet. The IO radicals that had not
reacted in the zone L entered into a chain reaction,
with Hydrogen-Containing Compounds
without Formation of Iodine Atoms (Procedure 1)
whose first step was
The reaction rate constant was determined as fol-
lows: iodine atoms (~1 × 1010 atom/cm3) were intro-
duced through the mobile nozzle into a dioxygen flow
containing ~1 × 1015 molecule/cm3 of ozone. Iodine
k7
IO + NO
I + NO2
(VII)
and whose second step was reaction (IV).
The characteristic time of the establishment of the
atoms could be introduced into the flow at different dis-
.
steady-state concentrations of iodine atoms and IO
.
.
tances from the detection zone. The IO radical
radicals ([IO ]st, [I]st), which obeyed the equation
resulted from the fast reaction
.
k4
k7[IO]st[NO] = k4[I]st[O3],
(4)
.
I + O3
IO + O2,
(IV)
was τ ~ 1/(k7[NO] + k4[O3]) ~ 3 × 10–4 s. Since the IO
concentration at the exit end of the zone L, [IO ]fin, is
whose rate constant at the experimental temperature
was k4 = 1.2 × 10–12 cm3/s [10]. Due to this reaction and
iodine loss on the walls,
.
.
.
[IO ]fin = [IO ]st + [I]st,
(5)
k5
I + wall
products,
(V)
it can readily be seen that the steady-state concentration
of iodine atoms in the detection zone is related to
the concentration of iodine atoms decreased rapidly
according to the expression
.
[IO ]fin by the equation
ln([I]/[I]0) = –(k4[O3] + k5)t.
(2)
k7[NO]
----------------------------------------
= α[IO]fin. (6)
The characteristic time of 98% conversion was
[I]st = [IO]
fink4[O3] + k7[NO]
4
ln0.02
τ98 = –--------------------------- <
k4[O3] + k5
---------------
k4[O3]
.
(3)
Thus, the iodine atom concentration detected by the
resonance fluorescence method, [I]st, was proportional
to the IO concentration at the exit end of the zone L.
The proportionality coefficient was determined only
by the NO and é3 concentrations, which were constant
in each entry, and was independent of the nozzle posi-
tion or the RH concentration added to the flow. From
entry to entry, α varied between 0.67 and 0.88. After
NO was introduced into the flow, the following reaction
could occur along with reactions (VII) and (IV):
.
Expression (2) is independent of the IO loss on the
.
walls if there are no reactions converting IO back
into iodine. Thus, it can be seen from Eq. (3) that 98%
of the iodine atoms were surely converted into IO in
a time equal to the quadruple characteristic time of
.
reaction (IV).
The typical conversion length in our experiments
was ~0.25 cm. Thus, at a distance of 1 cm from the noz-
zle cut, 98% of the iodine atoms were converted into
k8
I + NO + O2
INO + O2.
(VIII)
KINETICS AND CATALYSIS Vol. 48 No. 5 2007