JOURNAL OF CHEMICAL PHYSICS
VOLUME 108, NUMBER 21
1 JUNE 1998
Collisions of O„ 1 D … with HF, F2 , XeF2 , NF3 , and CF4 : Deactivation
and reaction
V. I. Sorokin, N. P. Gritsan, and A. I. Chichinin
Institute of Chemical Kinetics and Combustion, 630090, Novosibirsk, Russia
~Received 9 December 1997; accepted 19 February 1998!
The reactions of O( 1 D! atoms with fluorides have been investigated by time-resolved laser magnetic
resonance ~LMR!. O( 1 D! atoms were produced by the dissociation of ozone with an excimer laser
~KrF, 248 nm!. By monitoring Cl atoms ~when HCl or Cl2 is added! or FO radicals, the rate
constants for total removal of O~1D) (310 211 cm3/s! by HF~5.161.0!, F2~0.8160.20!,
XeF2~1663.0!, NF3~1.260.25!, and CF4~,0.016! have been determined at 298 K. Reaction rate
constants (310 211 cm3/s! have been obtained for HF~1.560.3!, F2~0.8160.2!, XeF2~1463.0!, and
NF3~1.060.3!. The deactivation of O( 1 D! by HX ~X5F, Cl, Br! is discussed. Ab initio calculations
have been performed with the aim of qualitative comparison of deactivation of O( 1 D! by F2 and Cl2.
© 1998 American Institute of Physics. @S0021-9606~98!01820-0#
I. INTRODUCTION
II. EXPERIMENT
The reactions of excited oxygen atoms O( 1 D)
~[O~2 1 D 2 )! with fluorides are of interest both for furthering
our understanding of the rates and mechanisms of small molecule reactions and also as potential laboratory sources of FO
radical. In this paper we report the results of a study of the
following processes:
O~ 1 D ! 1HF→F1OH,
→O~ 3 P ! 1HF,
O~ 1 D ! 1F2→FO1F,
→O~ 3 P ! 1F2,
O~ 1 D ! 1XeF2→FO1F1Xe,
→O~ 3 P ! 1XeF2,
O~ 1 D ! 1NF3→FO1NF2,
The experimental arrangement used in the present work
has been described previously.6–9 Briefly, it involves timeresolved LMR detection of Cl atoms, FO, and NF2 radicals;
details are presented in Table I. These species were produced
photolytically by an excimer KrF-laser ~ELI-94, 248 nm, 50
mJ/pulse, 3 Hz! or in chemical reactions initiated by the
photolysis. Gas mixtures were pumped through a photolysis
cell ~2.9 cm i.d.! at a rate of ;3 m/s. The cell was inserted
into the cavity of a CO2 -laser and was subjected to oscillating and constant magnetic fields. The unfocused excimer laser beam was aligned to the direction of the IR beam at an
angle of about 3°. The outlet CO2 -laser radiation went to a
GeHg photoresistor, cooled by solid N2 ~53 K!. The signal of
the photoresistor was detected by a lock-in amplifier, digitized, and transferred to a computer.
The main experimental problems were connected with
decomposition and premature reactions of F2 , XeF2 and HF.
A greaseless flow system was used; gas flow lines exterior to
the cell were constructed of either stainless steel or copper
with clamped Teflon O–ring joints; the cell was constructed
of Teflon entirely. All the gas handling and storage components were passivated by F2 at ;1 atm during several
months before experiments. The main gas flow line made
from stainless steel ~3 mm i.d.! leaded to the cell, reactants
were added at a series of addition ports. The F2 /HF addition
port was the nearest to the cell; there were no offshoots from
the line downstream of this port. Thus, the time of flight
from this port to the cell was minimized, it was no more than
10 ms. Despite of these precautions, we have observed two
anomalies. The first one was the slow disappearance of HF in
the gas flow lines during several-hours storage. The second
anomaly was a rather long time ~seconds! for passage of HF
from the addition port to the cell. Note that the first problem
is mentioned in the paper of Wine et al.14 To avoid this
problem, we purified HF by trap-to-trap distillation and pre-
~1r!
~1q!
~2r!
~2q!
~3r!
~3q!
~4r!
→O~ 3 P ! 1NF3,
~4q!
O~ 1 D ! 1CF4→O~ 3 P ! 1CF4,
~5!
where both overall rate constants and the channel specific
rate constants are determined. To the best of our knowledge
all the systems studied in this work, except for O( 1 D!1HF
and O( 1 D!1CF4 , have not been investigated previously.
In review of Atkinson et al.1 on chemical kinetics data
for atmospheric chemistry the k 1 rate constant for removal of
O( 1 D! by HF is assumed to be comparable to most other
O~1D! rate constants; hence a value of 1.0310210 cm3 /s is
recommended. According to the private communication of
Wine et al.,2 k 1 51.4310210 cm3 /s, k 1q ,0.04k 1 .
Quenching of O( 1 D! by CF4 was studied by Fletcher
and Husain,3 by Force and Wiesenfeld,4 and by Shi and
Barker.5 Deactivation rate constant measured in the two latter studies is two orders of magnitude smaller than that reported in the first study.
0021-9606/98/108(21)/8995/9/$15.00
8995
© 1998 American Institute of Physics
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8996
J. Chem. Phys., Vol. 108, No. 21, 1 June 1998
Sorokin, Gritsan, and Chichinin
TABLE I. LMR registration of Cl, FO, NF2 in this study; E'B polarization is used.
Transition
P 1/2← P 3/2
Cl
2
2
FO
NF2
v ,J:1,1.5←0,1.5
n 1 : 1←0
CO2 -laser
n ~cm21 ! a
13
882.287
12
1033.488
1041.279
CO2 , 11P~36!
CO2 , 9P~34!
CO2 , 9P~26!
12
s ~cm2 !c
B ~kG!b
d
0.919,* 0.976
3.476, 3.570*
0.947, 1.002*
0.54660.033f
1.07~219!
3.43~219!
2.29~218!
g
Ref.
e
10,11
10,11
7,12
13
a
Transition frequency.
Magnetic flux density for the LMR signal maxima.
c
Calculated absorption cross section for the LMR signal maximum marked by
d
The LMR signal is slightly stronger at the magnetic field marked by the .
*
e
a(b)5a310b .
f
Measured in the present study.
g
Signal to noise ratio for NF2 is '9.4 times lower than that for FO radicals.
b
pared HF/He mixture just before use. The second anomaly
have disappeared after rather long work with large flows of
HF.
The cell incorporated two NaCl windows for CO2 -laser
radiation and one quartz window for UV radiation. The
spaces between the windows and reaction zone of the cell
(;30 cm! were continuously flushed with He. In order to
increase the accuracy of measurements, the same gas flow
line and flow-meter system were used for F2 and HF gases.
XeF2 /~He or N2 ) gas mixtures were prepared by slow passage of buffer gases over crystalline XeF2 at room temperature. Downstream these mixtures were passed through a
cooled trap at 5–18 °C before entering the low pressure reactor. The XeF2 pressure in the reactor was determined by
the magnitude of the gas flow and the temperature of the
trap.15 The second technique used to measure the pressure of
XeF2 was UV photometry. The XeF2 concentration in the
flowing stream after the cooled trap but before entering the
low pressure reactor was monitored by the absorption at
248.5 nm ~Ref. 16! in a 10 cm absorption cell by means of a
spectrophotometer.
The pressure of HF in gas flow lines and intermediate
storage vessel was no more then 0.1 atm. At this pressure the
deviation of association factor from unity is ,0.012.17
Hence, the association was negligibly small.
Cl2 , HCl, and COCl2 were prepared by standard
techniques18 and contained ,2% of impurities. All other
gases were commercial grades stated by the manufacturer to
have the following purities: NF3 , 97%, SF6 , 99.2%, HF,
99%, F2 , 98%, CF4 , 99.6%, N2 , 99.99%, He, 99.99%. The
purity of Cl2 , COCl2 , and F2 was checked by UV photometry, the purity of HCl, HF, CF4 , SF6 , and NF3 was controlled by mass spectrometry. We measured the vapor pressure of the XeF2 sample in the temperature range of 0 to
25 °C, the result was in good agreement with the literature.15
O3 was prepared just prior to experiments by a 10 kV–50 Hz
ac-discharge in a vessel with O2 cooled by liquid N2 . Before
use, it was degassed at 77 K to remove O2 .
and O( 1 D!1M reaction rate constant, respectively. For a
O( 1 D)1R i system, r i and k i denote the reaction rate constant and overall quenching rate constant, respectively;
r i [k ir , k i [k R i 5k ir 1k iq , i51,...,4. k(M1 , M2 , . . . )
[k M1@ M1# 1k M2@ M2# 1 . . . denotes the pseudo-first-order
rate constant for deactivation of O( 1 D! by M1 , M2 , . . . molecules. The probability of A atom production from M molecule via photolysis at 248 nm can be calculated as
F F A/M s M , where s M is the absorption cross section of
M, F A/M is the quantum yield of A atoms, and F is the
number of UV photons in one laser pulse divided by cross
section of laser beam. Table II lists the s M and F A/M values
of the gases used in our experiments.
Several experimental approaches have been used in the
present study. Preliminary results obtained by all the approaches are collected in Table III. The relative and absolute
values from Table III were combined to compute a set of
mean values. A final complete summary of the rates determined by this investigation is given in Table IV. The literature data on O( 1 D! deactivation processes discussed in the
present paper are also presented in Table IV.
A. O„ 1 D …1NF3 , XeF2 : Risetime measurements for Cl
atoms
The method of observing the time-resolved decay
of O( 1 D! atoms has been described previously.8
Briefly, the experiments were performed by irradiating
R i /O3 /HCl/SF6 /He gas mixture; all conditions except R i concentration were held constant; the LMR signal of Cl atoms
was monitored. The photolysis of O3 by irradiation with l
5248 nm yields dominantly O( 1 D! atoms, see Table II. Removal of these atoms is first order, with a decay time given
TABLE II. Absorption cross sections and quantum yields of the A atom
from the photolysis of the M molecule at 248 nm.
III. RESULTS
The following designations are introduced in this paper.
HF, F2 , XeF2 , NF3 , and CF4 are denoted as R i ,
i51, . . . , 5, respectively. k M and r M denote the overall deactivation of O( 1 D! atoms by M molecules rate constants
*.
a
M
A
F A/M
s M ~cm2 !
Ref.
O3
F2
XeF2
Cl2
COCl2
O( 1 D!
F
F
Cl
Cl
0.960.1
2
2
2
2
1.043~217!a
1.17~220!
1.44~219!
1.154~221!
9.024~220!
1
19
16
1
1
a(b)5a310b .
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J. Chem. Phys., Vol. 108, No. 21, 1 June 1998
Sorokin, Gritsan, and Chichinin
8997
TABLE III. Preliminary results of the present study, quoted errors are 2s.
Measured parameter
211
By monitoring
a
3
k i , 10
cm /s
r i , 10211 cm3 /s
k i , 10211 cm3 /s
k i /k 4
r i /r 4
(r i /k i )/(r 4 /k 4 )
r i /k i
r i , 10211 cm3 /s
k CF4 , 10213 cm3 /s
Cl
FOb
FOc
FOd
FOe
FOf
Clg
Clh
FOi
NF3
XeF2
1.160.2
1763.5
1461.4
1566
11.563
1461
1.060.15
1.360.4
1
1
1
F2
HF
CF4
0.860.3
0.6760.12
0.7060.10
1.0560.10
1.1260.15
0.8360.30
4.060.16
3.560.5
1.2860.10
0.2760.03
0.3060.02
,1.6
a
Measurements of risetime of the Cl signal; see Sec. III A, Fig. 1.
Experiments with the photolysis of O3 and XeF2 ; see Sec. III C, Eq. ~14!, Fig. 3.
c
Determined from the B i values; see Sec. III B, Eq. ~13!, Fig. 2.
d
Determined from the B i values; see Sec. III B, Eq. ~12!, Fig. 2.
e
Determined from the A i /B i values; see Sec. III B, Eq. ~10!, Fig. 2.
f
Determined from the A i values; see Sec. III B, Eq. ~11!, Fig. 2.
g
Measurements in excess of HF, or F2 , or Cl2 ; see Sec. III D, Eq. ~20!, Fig. 4.
h
See Sec. III D, Eq. ~18!, Fig. 4.
i
See Sec. III B.
b
TABLE IV. Rate constants for reactive and nonreactive quenching of O( 1 D! by M molecules at room temperature.
M
Products
HF
a
a
F1OH
F1OH
O1HF
O1HF
H1OF
HCl
a
Cl1OH
O1HCl
H1OCl
HBr
a
Br1OH
O1HBr
H1OBr
F2
a
FO1F
XeF2
a
FO1Xe1F
NF3
CF4
Cl2
SF6
O3
N2
He
a
FO1NF2
O1CF4
O1CF4
O1CF4
O1CF4
a
ClO1Cl
O1SF6
FO1 SF5
a
O1N2
O1He
2DH 298
~kcal/mol!
Rate constant
(cm3/s!
210
1.4310
~5.161.0!310211
1.4310210
~1.560.3!310211
,5.6310212
~3.660.7!310211
~1.560.1!310210
~1.060.2!310210
~1.3560.8!310211
~3.661!310212
~1.4860.16!310210
~1.1460.36!310210
~3.061.2!310211
,8310212
~8.162.0!310212
~8.162.0!310212
~1.660.3!310210
~1.460.3!310210
~1.1560.2!310211
~1.160.2!310211
~3.060.4!310211
~1.860.1!310213
,6.0310213
,1.6310213
~2.560.5!310210
~1.960.32!310210
~1.860.26!310214
e
210
~2.460.24!310
~2.660.5!310211
,3310216
11.2
11.2
b
b
238.6
44.5
b
6.4
60.1
b
14.2
59.9
5.2d
10.9
b
b
b
b
51.5
b
r M /k M
Refs.
1
1
.0.96
0.3060.02
,0.04
0.7060.02
0
1
0.6560.1
0.0960.05
0.2460.05
1
0.8060.12
0.2060.07
,0.045
1
.0.97
1
.0.88
1
.0.90
1
1
1
1
1
0.7660.12
1
c
227
e
b
1
1
1
b
this work
c
this work
c
this work
8,14,29,30
8,14
14
14
14
14
14
14
this work
this work
this work
this work
this work
this work
3
4
5
this work
3,8,20
8,20,31
29,32
1
1
33
a
Overall deactivation of O( 1 D!.
2DH 2985DE @ O( 1 D)-O( 3 P) # 545.4 kcal/mol.
c
Private communication of Wine et al., 1984, published in Ref. 2.
d
D(XeF–F)1D(Xe–F)568.1 kcal/mol ~Ref. 34!.
e
Since the O( 1 D!1SF6 reaction is 24 kcal/mol endodermic ~Ref. 35!, r SF6 is negligible.
b
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8998
J. Chem. Phys., Vol. 108, No. 21, 1 June 1998
Sorokin, Gritsan, and Chichinin
FIG. 1. Plots of pseudo-first order rate constants for the removal of O( 1 D!
vs concentration of deactivating gases, the slopes of the lines yield the
quenching rate constants. The formation of Cl atoms is monitored after the
photolysis of NF3 ~or XeF2 )/O3 /HCl/SF6 /He gas mixture at 248 nm. Right
figure: the temperature of the trap on XeF2 /He line was T515 °C (d) and
8 °C (n). @O3 #53.531014, @HCl#53.531014, @SF6 #52.531015, @He#52.5
31017 cm23 .
by 1/t 5k(R i ,O3 ,HCl,SF6 ,He). We assume that the kinetics
of Cl atoms is determined dominantly by the reaction
O~ D ! 1HCl→Cl1OH.
1
~6!
Hence, the Cl first order appearance rate should be equal to
the O( 1 D! first order disappearance rate. Note that the reactions O( 3 P)1HCl ~3310214 cm3 /s! ~Ref. 20! and F1HCl
~1.6310211 cm3 /s! ~Ref. 21! are slow on the time scale of
the present experiments. The experimental curves of LMR
signal of Cl atoms were fitted by the expression S Cl(1
2exp(2t/t)). Plots of 1/t vs @NF3 # and @XeF2 # are presented
in Fig. 1. The slopes of the least squares lines yield the rate
constants k 3 and k 4 . Note that this determination is the only
absolute measurement of rate constants in the present paper.
In the subsequent text, only relative rate data are presented.
To deactivate spin-orbitally excited atoms Cl( 2 P 1/2) produced in reaction ~6!,8,22 in all experiments a considerable
amount of SF6 was present, see Sec. IV A.
FIG. 2. Amplitudes of FO radicals signal, S FO , vs @R i # in experiments with
the photolysis of R i /O3 /SF6 /He mixtures; plots for R i 5HF, F2 , XeF2 and
NF3 are shown. Solid curves are simulated by Eq. ~9!. The initial slope of
each curve is proportional to the r R i rate constant, limiting value of the
curve is proportional to the r R i /k R i ratio. @O3 #50.731014, @SF6 #52.5
31015, @He#52.531017 cm23 .
The maximum FO concentration after the complete decay of O( 1 D! is given by the expression
@ FO# 5
B. O„ D … 1 fluorides: Detection of FO
The method of data analysis has been described
previously.8 The experiments were performed by irradiating
R i /O3 /SF6 /He gas mixture; all conditions except R i concentration were held constant; the LMR signal of FO ~and NF2
when R i 5NF3 ) radicals was monitored.
O( 1 D! atoms formed by the photolysis of O3 can produce FO radicals, in reactions ~2r!,~3r!,~4r!, or F atoms, in
reactions ~1r!,~2r!,~3r!. In the latter case FO radicals arise in
the subsequent reaction of F with O3 :
F1O3→FO1O2,
~7!
k 115~1.3360.33!310211
cm3 /s,23
~6.260.3!310212
3 24
cm /s. The LMR signal of FO radicals reached a maximum
at several microseconds and decayed slowly over several
milliseconds. The maximum amplitude of the signal, hereafter denoted S FO , was measured.
~8!
where g i takes into account the rate of the reaction of F with
O3 . That is, g 1 5 g 4 50, and the values for g 2 and g 3 depend
on @O3 #. Under our experimental conditions g 2 5 g 3 '0,
when @O3 #,1014 cm23 , because in this case the time of reaction ~7! is longer than the kinetics decay time; g 2 5 g 3 '1,
when @O3 #.631014 cm23 . In the intermediate case,
@O3 #5~1–6!31014 cm23 , the analysis of the form of kinetics
is required. Usually we avoided this case. The contribution to
FO signal from the photolysis of F2 or XeF2 is given by the
last term in Eq. ~8!. Under our experimental conditions it
was negligible, that is, g i s R i @ R i # ! s O3@ O3# . All secondary
radical–radical reactions are also negligible over the time
scale of the present experiments. Expression ~8! reduces to
S FO5
1
~ 11 g i ! r i @ O~ 1 D !#@ R i #
1F g i s R i F F/R i ,
k~ R i ,O3 ,SF6 ,He!
~ 11 g i ! A i @ R i #
,
B i1 @ R i#
~9!
where A i and B i are parameters.
As it follows from Eqs. ~8! and ~9!, the following relations exist:
r i /r j 5 ~ A i /B i ! / ~ A j /B j ! ,
~10!
~ r i /k i ! / ~ r j /k j ! 5A i /A j ,
~11!
k i /k j 5B j /B i ,
~12!
k i 5k~ O3 ,SF6 ,He! /B i .
~13!
Typical plots of S FO vs @R i # are presented in Fig. 2. A least
squares fit of the plots yields A i and B i parameters. Using
these parameters and Eqs. ~10!–~13!, we determine r i /r j ,
(r i /k i )/(r j /k j ), and k i values. The results are presented in
Table III.
Inspection of Table III suggests that the values of k 3 and
k 4 derived by using Eq. ~13! are in good agreement with
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J. Chem. Phys., Vol. 108, No. 21, 1 June 1998
Sorokin, Gritsan, and Chichinin
8999
FIG. 4. Amplitudes of Cl atoms signal, S Cl , vs @X#; X5Cl2 ~A!, HF ~B!, F2
~C!, and COCl2 ~D!. For X5Cl2 and HF, the maximum signal level at high
@X# is proportional to the r X /k X ratio. For X5COCl2 and F2 , the slopes of
the linear plots obtained at high @X# are proportional to the absorption cross
sections s X . ~A!,~B!,~C!: @O3 #51.031014; ~B!,~C!: @Cl2 #50.531014 cm23 .
The buffer gas is a SF6 /He mixture ~1/100!, P He510 Torr.
FIG. 3. Amplitudes of FO radicals signal, S FO , vs @XeF2 # in experiments
with the photolysis of XeF2 /O3 /SF6 /~N2 or He! mixtures. The temperature of
the trap on XeF2 /He line was T519 °C (s) and 13 °C ~all other symbols!.
Upper curve: Solid curve is simulated by Eq. ~9!; @O3 #50.731014 cm23 .
Lower curve: @N2 #50.831014 cm23 ; @O3 #54.831017 (d), @O3 #50.8
31014 cm23 (n). The k O3 /r XeF2 ratio can be restored from the ratio of
initial slopes of these curves.
those determined by monitoring the formation of Cl atoms as
described in previous section. We emphasize two limitations
of the present method: first, only ratios between r i reaction
rate constants are obtainable; second, the uncertainty in the
values determined using Eq. ~13! is relatively large, probably
due to impurities in gas mixtures. It should be mentioned that
the results obtained by monitoring NF2 and FO radicals were
in close agreement.
Experiments with the photolysis of the O3 /NF3 /SF6 /He
gas mixture have shown that the addition of CF4 , up to
3.531016 cm23 , had no effect upon the amplitude of the FO
radicals signal. Hence only upper limit for k CF4 was estimated.
C. Photolysis of XeF2 : Determination of r 3
The experiments were performed by irradiating
O3 /XeF2 /SF6 /B gas mixtures, B denotes the buffer gas, He or
N2 . The amplitude of the LMR signal of FO radicals was
measured at different pressures of XeF2 , all other concentrations were held constant. The dominant source of FO radicals was different in these two sets of experiments: for the
studies with B5He, it was the photolysis of O3 with the
subsequent reaction of O( 1 D! atoms with XeF2 ; when B5N
2 , it was the photolysis of XeF2 followed by reaction ~7!, the
reaction of O( 1 D! with XeF2 was unimportant because of
rapid quenching of O( 1 D! by N2 . Typical S FO vs @XeF2 #
plots obtained in experiments with different buffer gases, He
and N2 , are illustrated in Fig. 3. For the studies with B5N2 ,
the plot is linear, S FO5C @ XeF2# . When B5He, the plot is
given by Eq. ~9!. As it follows from Eqs. ~8! and ~9!, the
ratio of initial slopes of such plots is given by
k O3 F F/XeF2s XeF2
C
5
.
A 3 /B 3 r 3 F O~ 1 D ! /O s O
3
3
~14!
In experiments in which N2 was the carrier gas, @O3 # was
varied by a factor of 2. To within experimental error, no
variation in S FO was detected, see Fig. 3. Hence, the reaction
of O( 1 D! with XeF2 in such experiments was really negligible.
D. O„ 1 D …1fluorides: Detection of Cl
The analysis of data described in the previous sections is
not straightforward. Hence, re-examination of our results on
O( 1 D!1HF and O( 1 D!1F2 systems was carried out in order
to assess the precision of the experimental methods outlined
above. The present approach is close to that described in Sec.
III B. The main difference is the presence of constant a
amount of Cl2 in gas mixture, this allows to monitor Cl atoms instead of FO radicals.
The determinations described in the current section include monitoring of Cl atoms in experiments with the photolysis of COCl2 and O3 . In the latter case, production of Cl
atoms is due both to the reaction of O( 1 D! with Cl2 and to
the reaction
F1Cl2→FCl1Cl,
210
~15!
k 155~1.660.5!310
cm /s. The temporal profile of
LMR signal of Cl atoms is similar to that of FO radicals. It
displays a fast (,100 m s! rise followed by a slow (;5 ms!
decay. Measurements of the maximum amplitude of the
LMR signal, S Cl , were carried out during the course of the
following experiments:
~a!
3
25
The O3 /Cl2 /SF6 /He gas mixture was photolyzed; @Cl2 #
was varied, all other concentrations were kept constant;
see Fig. 4~A!. This kind of measurements is described
in our previous study.8 At large @Cl2 # the amplitude of
Cl atoms signal is constant,
S~Cla ! 5 a ~ r Cl2 /k Cl2!@ O~ 1 D !# ,
~16!
where the coefficient a ([S Cl / @ Cl# ) converts the Cl
atom concentration to the amplitude of the LMR signal.
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9000
~b!
~c!
J. Chem. Phys., Vol. 108, No. 21, 1 June 1998
Sorokin, Gritsan, and Chichinin
Addition of an excess of HF to the photolysis mixture
described in ~a! resulted in the constant S Cl , see
Fig. 4~B!. In these experiments deactivation of O( 1 D!
mainly by HF is assumed, hence the amplitude of Cl
atom signal is given by
S~Clb ! 5 a ~ r 1 /k 1 !@ O~ 1 D !# .
~17!
Note that the reaction of OH with Cl2 is slow
(8310 214 cm3/s!26 and therefore it can be neglected.
Replacement of HF by F2 makes the analysis somewhat more elaborate, because the production of Cl atoms is due both to the reactions of O( 1 D! with Cl2 and
F2 and to the photolysis of F2 . The variation in S Cl with
@F2 # is given by
r 2 @ F2# 1r Cl2@ Cl2#
SCl5 a @ O~ 1 D !#
k~ F2 ,O3 ,Cl2 ,SF6 ,He!
S
D
1FF F/F2s F2@ F2# .
~18!
The variation obtained experimentally is shown in Fig.
4~C!. At large @F2 # the plot dependence is linear, with
the intercept given by
S ~Clc ! 5 a ~ r 2 /k 2 !@ O~ 1 D !# .
~19!
Using Eqs. ~16!,~17!, and ~19!, we derive
r 1 /k 1 :r 2 /k 2 :r Cl2 /k Cl25S ~Cla ! :S ~Clb ! :S ~Clc ! .
~20!
Since all the values in the right-hand side of this relationship
are determined from experiment and the ratio r Cl2 /k Cl2 is
known from the literature ~it is equal 0.7560.1, see Table
IV!, one can obtain the ratios r 1 /k 1 and r 2 /k 2 . Using
Eq. ~20!, we have obtained r 2 /k 2 51.1260.15. Note that the
ratio can not exceed unity; hence it is equal unity rather
accurately.
In order to check our kinetic scheme of O( 1 D!1F2 system for self-consistency, additional experiments were performed by irradiating COCl2 in a He/SF6 carrier. The variation of S Cl with @COCl2 # was studied and found to be linear
with zero intercept, see Fig. 4~D!. The slope of the plot was
compared with the slope of the linear plot obtained in experiments ~c! at large @F2 #. The ratio of these slopes was found to
be ~0.1560.02!, in agreement with the calculated value,
F F/F2s F2 /F Cl/COCl2s COCl25 0.13.
In addition, the value for r 2 was determined from
Fig. 4~C! using least-squares fitting by Eq. ~18! to be
r 2 5~8.363.0!310212 cm3 /s, in good agreement with the
previous determinations. The greater uncertainty in the value
derived reflects the complexity of the data analysis method.
Hence, no effort was made to obtain r 1 in the same manner.
IV. DISCUSSION
Ab initio molecular orbital calculations were performed
with the GAUSSIAN-92 program.27 The methods include SCF,
Mo” ller–Plesset perturbation theory ~MP2 through MP4! and
CI method ~QCISD level!. The 6–31G** basis was employed.
A. Justification of assumptions
To deactivate spin-orbitally excited atoms Cl( 2 P 1/2) produced in reactions ~6! and ~15!, or vibrationally excited
FO( v .0) radicals, considerable amount of SF6 was present
in all experiments. The SF6 molecule is an effective
quencher of Cl( 2 P 1/2) atoms ~1.5310210 cm3 /s! ~Ref. 28!
and FO( v 51! radicals ~1.2310211 cm3 /s! ~Ref. 7! and has a
slight effect upon O( 1 D! atoms, see Table IV. Thus, the
population of the excited states Cl( 2 P 1/2) and FO( v .0), due
to chemical reactions or stimulated by CO2 -laser radiation6
was not of significance in our experiments.
The fraction of FO radicals at the vibrational–
rotational–spin–orbit–Zeeman ( v 50, J51.5, V53/2,
M I 520.5, M J 521.5,20.5,0.5) ~Ref. 7! sublevels employed for monitoring the radicals should to be the same in
all experiments. We assume that the relaxation between the
sublevels within the vibrational FO state occurs, except for
M I sublevels, in each gas-kinetic collision of FO with buffer
gas atoms; the M I -relaxation proceeds ;16 times slower.7
As the current experiments were all carried out at P He;10
Torr, the relaxation time is expected to be about several microseconds. This time is negligible in comparison with the
decay time of LMR signal of FO radicals ~milliseconds! and
hence these relaxation processes can not introduce an error in
the present measurements. Support for this conclusion is also
provided by the remeasurement of certain rate constants in
the experiments with monitoring of Cl and NF2 rather than
FO.
Note that all reactions of the excited molecular oxygen,
O2 ( 1 D), produced photolytically from O3 with reactants of
the present study are exothermic and hence they are of no
importance.
Preliminary and final results of the present study are
summarized in Tables III and IV, respectively. Errors quoted
in Table III are 2 s and represent reproducibility of the data
only. Errors quoted in Table IV represent the absolute accuracy. The absolute accuracy of r F2 and k F2 rate constants is
estimated to be 625%, the accuracy of other rate constants is
estimated to be 620%. The absolute accuracy of r i /k i ratios
is assumed to be rather high, see Table IV. This assumption
is based on our belief that the r F2 /k F2 ratio is very close to
unity. Usually authors believe that their major source of error
is the determination of reactant concentrations; in all our
measurements of r i /k i ratios this source was of no importance. This advantage is especially significant for the experiments with HF.
B. O„ 1 D …1HF
To date there exists the sole experimental study on the
reaction of O( 1 D! with HF carried out by Wine et al.; see
Ref. 14 and private communication in Ref. 2. The rough
estimate of k 1 reported in Ref. 14 is k 1 56310211 –
2.4310210 cm3 /s, in agreement with our measurements.
Probably, the k 1 value published in Ref. 2 is the average of
this estimate. According to Ref. 2, reactive deactivation
dominates, r 1 /k 1 .0.96, in strong disagreement with our re-
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J. Chem. Phys., Vol. 108, No. 21, 1 June 1998
FIG. 5. Adiabatic electronic state correlations for reactions of O( 1 D! with
HF and HCl. The energies of HOF and HOCl are the values for the equilibrium geometries ~shown at the bottom! of the ground electronic state.
Dotted lines correspond to triplet state correlations. The probabilities of the
channels for the reactions O( 1 D!1HX ~X5F, Cl! are shown.
sult, r 1 /k 1 50.30. The results of Wine et al. are published
without details as a private communication, hence the discussion of this discrepancy is hindered.
The present experiments demonstrate that the deactivation of O( 1 D! by HF is dominated by nonreactive quenching
and is '3 times slower than the deactivation of O( 1 D! by
HCl and HBr. These features are discussed below.
Let us compare deactivation of O( 1 D! by HF and HCl.
The correlations and energetically accessible levels relevant
to these systems are shown in Fig. 5. The energies of excited
states of HOCl and HOF for the equilibrium geometry of the
ground electronic state are taken from the SCF–CI calculations of Peyerimhoff and Buenker36 and of Nambu et al.,37
respectively. Note that there are no triplet state energies of
HOF in the latter study. Hence the energy difference between
the ground state 1 1 A 8 and the first triplet state 1 3 A 9 for
HOF has been calculated in the present study to be 3.960.1
eV at MP3, MP4 levels of theory. The energy of the second
triplet
state
is
estimated
from
the
relation
DE(2 1 A 8 – 1 3 A 8 )5DE(1 1 A 9 21 3 A 9 ), which is valid for
the HOCl molecule.36
It is well established that the reaction of O( 1 D! with HCl
proceeds mainly via formation of an intermediate
HOCl* complex, which dissociates mainly into Cl1OH.38–40
It seems likely that the reaction of O( 1 D! with HF proceeds
via an intermediate complex also.
It is worth noting that the energies of HOX*→X1OH
decay processes are almost equal for X5F and X5Cl, but
the energy of HOF* intermediate is considerably lower than
* 50.56 and 0.82
that of HOCl*. That is, D 0 (OH–X)/E HOX
* it the energy of
for X5Cl and X5F, respectively; E HOX
HOX*. Hence one can expect HOF* to have a longer lifetime than HOCl*. Note that there is no barrier in the
HOCl*→OH1Cl decomposition pathway,40 hence it is
likely that the barrier is absent for the decay of HOF* also.
There is a competition between the HOX* decomposition
channels, leading to X1OH and O( 3 P!1HX products. In the
latter case, a nonadiabatic curve crossing mechanism is usually assumed. If, as we suggest, the HOF* complex has a
long lifetime, the nonadiabatic transition channel may be
Sorokin, Gritsan, and Chichinin
9001
preferred over the F1OH channel. Note that this comparison
may be incorrect because, according to Kruus et al.,41 physical quenching of O( 1 D) by HCl may proceed via the excited
HOCl(1 1 A9! potential energy surface.
It is interesting to note that the channel
O( 1 D!1HF→F1OH( v 51) is only 0.043 eV exothermic.
Thus, as suggested by statistical consideration, it is unlikely
compared to F1OH( v 50) channel. Hence one can expect
vibrational excitation of OH produced in O( 1 D!1HF reac* !1 (E V is vibrational energy of
tion to be small, E V /E HOF
OH!, in contrast with O( 1 D)1HCl→Cl1OH reaction, in
* 50.58.41,39
which E V /E HOCl
Now let us consider the relation 3k HF'k HCl'k HBr obtained experimentally, see Table IV. The following qualitative notes can be proposed as a basis for interpretation of this
relation:
~1! The rate constant for a reaction determined by potential
can
be
expressed
as42
U(R)52C 6 /R 6
1/3
k 8 53 p ( v C 6 /2m ) where m is the reduced mass of the
colliding pair and v is the relative velocity. The C 6 coefficient can be estimated from polarizabilities ( a M) and
ionization potentials. Using a M50.61, 2.66, and 3.61
8 52.67, 3.75, 3.75310210 cm3 /s, for
Å 3 ,43 one obtains k M
M5HF, HCl and HBr, respectively. Thus, the relatively
small k HF value is partially due to the small polarizability of HF.
~2! If, as has been suggested,41 the deactivation of O( 1 D) by
HCl proceeds partly via excited state HOCl(1 1 A 9 ), the
relation k HF,k HCl is partly due to the inaccessibility of
the HOF(1 1 A 9 ) excited surface for O( 1 D!1HF reactants.
~3! The decomposition of HOF* back to reactants
O( 1 D!1HF is more probable, than for HOCl*, because
of the longer lifetime and because of the rather small
energy difference between O( 1 D!1HF and F1OH decomposition channels. This note also lends credibility to
the relation k HF,k HCl .
C. O„ 1 D …1F2
The present experiments demonstrate that the rates for
deactivation of O( 1 D! by F2 and Cl2 differ significantly,
k F2' 301 k Cl2 . In order to understand this fact, we have performed ab initio calculations of certain features of the potential energy surfaces for these reactions.
The calculations have shown that there are two stable
isomers, more stable Cl–O–Cl and less stable O–Cl–Cl, see
Table V. The energy difference between these isomers was
found to be 1.47 eV and 1.15 eV at the MP4 and QCISD
levels of theory, respectively. Taking an average, 1.3160.16
eV, we obtain the nonclassical isomer O–Cl–Cl to be
2.3560.16 eV more stable than the O( 1 D!1Cl2 reactants.
Existence of the O–Cl–Cl isomer is also confirmed by Rochkind and Pimentel50 who analyzed infrared spectra that result
from photolysis of matrix-isolated Cl2 O at T520 K.
The SCF potential energy curves connecting the O( 1 D!
atom and Cl2 molecule with the O–Cl–Cl isomer were calculated whereby the internuclear angle was held constant.
According to these calculations, the reaction of O( 1 D! with
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9002
J. Chem. Phys., Vol. 108, No. 21, 1 June 1998
Sorokin, Gritsan, and Chichinin
TABLE V. Molecular parameters of certain reactants and intermediate complexes; units are Å, degrees and eV.
O–Cl–Cl a
Cs
r~Cl–Cl!51.53
r~Cl–Cl!52.37
/O–Cl–Cl5116.7
D 0 (O–Cl2 )50.3960.16
Cl–O–Cl
ONF3b
NF3c
OXeF2d
XeF2e
C 2v
r~O–Cl!51.70
C 3v
r~N–F!51.43
r~O–N!51.15
/F–N–F5100.5
D 0 (O–NF3)54.25
C 3v
r~N–F!51.37
D `h
r~Xe–F!51.95
r~O–Xe!51.76
/F–Xe–F5180
unknown
C 2v
r~Xe–F!51.98
/Cl–O–Cl5110.76
D 0 (O–Cl2 )51.70
/F–N–F5102.4
/F–Xe–F5180
a
Calculated in the present study.
References 44 and 45.
c
Reference 46.
d
References 47 and 48.
e
Reference 49.
b
Cl2 produce the O–Cl–Cl isomer without activation energy
in a wide range of O–Cl–Cl angles. The subsequent
O–Cl–Cl→Cl–O–Cl interconversion seems probable. Thus,
the deactivation of O( 1 D! by Cl2 is expected to be fast, in
agreement with the experimental evidence.
Analogous calculations were made for the O( 1 D!1F2
system. Completely repulsive curves for the O( 1 D!1F2 interaction were obtained in a wide range of O–F–F angles.
Moreover, a stable O–F–F isomer was not found at all. Because of this, the low rate for deactivation of O( 1 D! by F2
seems reasonable, probably due to the favourable electronegativity difference between O and Cl, as well as the greater
polarizability of Cl.
D. O„ 1 D …1NF3 and XeF2
It is worth remembering that the reaction of O( 1 D! with
XeF2 may produce the XeF radical @ D e ~Xe–F!51073 cm21 ,
n 015210 cm21 ].51,52 We have calculated the thermally averaged probability for the XeF( v 50)1He→XeF( v 51)1He
vibrational excitation using conventional classical53 and
quantum-mechanical54 estimates. These estimates show that
the probability is very high, '0.4 per collision. Hence under
our experimental conditions XeF radicals survive for at most
a few microseconds, this time is negligible on the time scale
for F atoms removal.
It is interesting to speculate upon the mechanism of deactivation of O( 1 D! by XeF2 and NF3 . We assume that the
deactivation proceeds via formation of intermediate collisional complexes. Note that the structures of NF3 and XeF2
are almost the same in free molecules and in the collisional
complexes, ONF3 and OXeF2 , see Table V. Hence the
‘‘sticking’’ of O( 1 D! to NF3 or XeF2 seems to be easy because it does not require significant rearrangements of atoms.
This speculation explains, among other things, the
k XeF2@k NF3 ratio obtained experimentally. For O( 1 D!1NF3 ,
the only relative orientation of the reactants leads to reaction,
along C 3 v axis of NF3 molecule; for O( 1 D!1XeF2 , all orientations for which the /F–Xe–O590° are appropriate.
E. O„ 1 D …1CF4
Our estimate for the rate of deactivation of O( 1 D! by CF
213
cm3 /s, compares favorably to the esti4 , k CF4,1.6310
mate of Shi and Barker5 and to the determination of Force
and Wiesenfeld, k CF45(1.860.1)310213 cm3 /s,4 while dis-
agreeing two orders of magnitude with that of Fletcher and
Husain;3 see Table IV. Note that purity of CF4 used by Force
and Wiesenfeld was only 99.7%. According to Matheson
Gas Products, the main impurities in CF4 were N2 , O2 , CO2 ,
CO and H2 O. Based on the data, the upper limit for k CF4 can
be readily determined, k CF4,1.0310213 cm3 /s. This upper
limit is found on the assumption that N2 , the less efficient
quencher for O( 1 D!, is the dominant impurity.
ACKNOWLEDGMENTS
We would like to thank Professor Ch. Hadad, Ohio State
University, for a careful reading of this paper and for
his helpful comments. This work was supported by the Russian Foundation for Basic Research through Grant
No. 97-03-33649a.
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