Hydrogen atom reactions with molecular halogens: The rate constants for H + F2 and H + Cl2 at 298 K

Article abstract of DOI:10.1021/jp020293d

The rate constants for H atom reactions with Cl₂ and F₂ have been measured by monitoring the loss rate of hydrogen atoms in the presence of excess [Cl₂] and [F₂] via time-resolved, laser-induced Lyman-α fluoresc

Full text of DOI:10.1021/jp020293d

J. Phys. Chem. A 2002, 106, 8417-8421
8417
Hydrogen Atom Reactions with Molecular Halogens: The Rate Constants for H + F2 and
H + Cl2 at 298 K†
Jiande Han and Michael C. Heaven*
Department of Chemistry, Emory UniVersity, Atlanta, Georgia 30322
Gerald C. Manke II
Air Force Research Laboratory, Directed Energy Directorate, Kirtland AFB, New Mexico 87117
ReceiVed: January 30, 2002; In Final Form: April 1, 2002
The rate constants for H atom reactions with Cl
hydrogen atoms in the presence of excess [Cl ] and [F
The rate constants for H + F and H + Cl
were found to be 2.4 ( 0.4 (2σ) × 10 and 2.52 ( 0.18 (2σ)
10 cm s , respectively. The result for H + F is consistent with the recommendation of Baulch et al.
J. Phys. Chem. Ref. Data 1981, 10 (Suppl. 1)], and our k(H + Cl ) value is consistent with the majority of
previous measurements.
2
and F
2
have been measured by monitoring the loss rate of
2
2
] via time-resolved, laser-induced Lyman-R fluorescence.
-
12
2
2
-
11
3
-1
×
2
[
2
-11
3 -1
I. Introduction
measured k3(298 K) ) 0.96 ( 0.04 × 10 cm s . This result
is a factor of 2 smaller than the generally accepted value13
Our intention in the present study was to reduce the
uncertainty of the room-temperature H + X2 rate constants by
applying a novel experimental approach. With the exception of
the work reported in ref 17, previous investigations of these
reactions have used molecular hydrogen as the H atom source.
In some cases, a discharge was applied to H2 directly, whereas
in others, a discharge based F atom source and F + H2 prereactor
were used to generate a known flow of H atoms. As H2 reacts
rapidly with atomic fluorine or chlorine to regenerate H atoms,
the presence of undissociated H2 in these systems complicated
the analysis and tied the accuracy of the measurements to
assumptions about the rate constants for the secondary reactions.
Here we report direct measurements of the rate constants for
the reaction of H atoms with molecular fluorine and chlorine.
Low concentrations of H atoms were produced by photolysis
of H2S, thereby avoiding complications stemming from the
presence of H2. The H atoms were generated in the presence of
a large excess of F2 or Cl2 and monitored via laser induced
fluorescence of the Lyman-R transition. Typically, the initial
pool of H atoms was monitored for 100-200 microseconds or
less, so that complications related to secondary reactions and
wall losses could be avoided.
In the HF chemical laser, vibrationally excited HF is
generated primarily by the F atom reaction with molecular
hydrogen
F + H f HF(V ) 0-3) + H
(1)
2
or via the secondary reaction
H + F f HF(V ) 0-9) + F
(2)
2
The rate constant1,2 and nascent HF(v) distribution for reaction
3,4
-
10
1
(
are well established, k1(T) ) 1.1 ( 0.1 × 10
exp(-(450
3
-1
50)/T) cm s and 0.0:0.15:0.55:0.30 for P0:P1:P2:P3. The
detailed dynamics of reaction 1 have been exhaustively studied,
both theoretically5,6 and experimentally
vestigators.
7-12
by numerous in-
Surprisingly, the same cannot be said for reaction 2. The 1981
13
kinetics review by Baulch et al. lists only eight previous
measurements of the rate constant (compared to 22 for reaction
-
10
3
1
s
) and recommends k2(T) ) 1.46 × 10
and k2(298 K) ) 2.5 × 10 cm s . A 1983 review by
Cohen & Westberg evaluated the same ensemble of measure-
exp(-1210/T) cm
-1
-12
3
-1
14
-
15 1.4
3
ments and selected k2(T) ) 4.8 × 10
and k2(298 K) ) 1.5 × 10 cm s . The five room-
temperature rate constants reported for reaction 2 range from
T
exp(-667/T) cm
-
1
-12
3
-1
s
II. Experimental Methods
-
12
-12
3
-1
1
.0 × 10
to 4.2 × 10
cm s .
To observe the reaction of H atoms with F2 or Cl2, the H2S
was present as a minor constituent of F2/He or Cl2/Ar mix-
tures. Partial pressures of the halogen/rare gas mixtures were
The reaction of H with Cl2
H + Cl f HCl(V ) 0-4) + Cl
(3)
2
0.5-6.0 Torr of 5% Cl2 in Ar (Matheson) or 0.5-1.75 Torr of
10% F2 in He (Matheson). The flow of H2S was adjusted to
is an important source of HCl(V) in the HCl chemical laser
provide a partial pressure of approximately 5 mTorr. The main
carrier gas was Ar, and the total reactor pressure was typically
system and has been used as a reference for competitive H atom
1
5,16
17
reaction studies.
Dobis and Benson recently applied their
1
00 Torr. The flow rates of H2S and Ar were controlled by
very low-pressure reactor (VLPR) technique to this reaction and
needle valves, whereas a mass flow controller was used for
F2/He or Cl2/Ar addition.
Hydrogen sulfide was photolyzed at 193 nm by a Lumonics
TE-860-4 excimer laser operating at 10 Hz. The laser emission
†
Part of the special issue “Donald Setser Festschrift”.
*
To whom correspondence should be addressed. E-mail: heaven@
euch4e.chem.emory.edu.
1
0.1021/jp020293d CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/09/2002

8418 J. Phys. Chem. A, Vol. 106, No. 36, 2002
Han et al.
(
∼60 mJ/pulse) was focused by a 50 cm focal length lens,
-2
providing a power density (150 MW cm ) that was sufficient
to dissociate all of the H2S in the focal region. Photodissociation
at 193 nm produces translationally hot H and SH fragments.
18,19
The high pressure of Ar buffer gas was used to thermalize the
H atoms and limit their diffusion out of the detection region.
Both F2 and Cl2 have very weak absorption cross-sections at
193 nm and were not photodissociated to any significant degree.
LIF detection of H atoms was accomplished by two-photon
excitation of the 2s-1s transition of atomic hydrogen. Collisions
rapidly quench the 2s state to 2p, which then emits Lyman-R
radiation as it relaxes back to the ground state.20 A tunable dye
laser (Lambda-Physik EMG-203/FL3002 system) was used to
generate 486 nm light. This was frequency doubled using a BBO
crystal. A short (20 cm) focal length lens was used to focus the
UV in the center of the photolysis cell. The photolysis and LIF
lasers were arranged in an overlapping, counter-propagating
configuration. The delays between the photolysis and probe laser
pulses were controlled with a precision pulse delay generator
(
(
SRS model DG535). A solar blind photomultiplier tube
Hammamatsu R6835), and narrow band interference filter (121
nm, 10 nm fwhm) combination was used to detect the vacuum
ultraviolet fluorescence while discriminating against scattered
UV light from the photolysis and probe lasers. The probe laser
was scanned across the Lyman-R line to characterize the line
-
1
shape. The Gaussian profile exhibited a line width of 1.2 cm ,
which demonstrated that the translational energy distribution
had been relaxed to the ambient temperature by collisions with
the buffer gas.
Some difficulty was encountered in finding appropriate
conditions for the H + F2 reaction. It was found that H2S and
F2 could not be mixed too far upstream of the photolysis region
-
16
3
-1 21
because of a slow prereaction (k e 6.4 × 10
cm s ).
Figure 1. Representative H + F data are shown. The slow decay in
2
This problem was manifested by a total loss of the H atom LIF
signal at high [F2]. To limit the consumption of [H2S], the
reaction time was minimized by mixing the reagents just prior
to their injection into the photolysis reactor. A second and more
troublesome complication for the collection of reliable H + F2
data was then discovered. For moderate to high [F2] and long
delays between the photolysis and probe laser pulses, the LIF
signal deviated significantly from single exponential decay.
Vibrationally excited HF is produced by H + F2. The bond
strength of HF is 136.3 kcal mol , and a single 243 nm photon
can photolyze HF(V g 2). The deviation from single exponential
decay was associated with the regeneration of H atoms from
probe laser photolysis of vibrationally excited HF. This problem
was minimized by considering only the first 50-100 µs of the
decay curve and keeping [F2] relatively low.
2
the absence of F in the upper panel is attributed to diffusion and H
atom loss via reactions with photoproducts. The lower panel demon-
strates the quality of the fit to a single-exponential decay. The best
-
1
least-squares fit gives b ) 8927 s
.
TABLE 1: Conditions and Results for H + F
Measurements
2
a
[
F
cm
2
]
H atom decay rate
[F
2
]
H atom decay rate
15
-3
5
-1
1015 cm
-3
5 -1
1
0
10 s
10 s
1
1
.063
.578
0.044
0.070
0.038
0.067
0.088
0.074
0.089
0.092
0.084
3.928
3.928
3.928
4.540
4.991
5.120
5.152
5.571
0.114
0.117
0.119
0.141
0.164
0.189
0.150
0.145
-1
1.578
2.318
2
2
2
3
3
.415
.512
.512
.252
.928
Measurements for the H+Cl2 system were straightforward.
We did not see any evidence of a prereaction between H2S and
Cl2, and the H atom decay curves were single exponentials for
all of the conditions investigated.
a
These are the raw observed decay rates, uncorrected for the decay
rate (Γ) in the absence of F . The initial concentration of H S for these
2
2
14
-3
measurements was 1.6 × 10 cm . Photolysis caused complete
14
-3
dissociation, giving [H]
0
) [SH]
0
) 1.6 × 10 cm
.
III. Results and Discussion
where
A. H + F2. A pair of representative H atom decay curves
are shown in the upper panel of Figure 1. For our conditions,
b ) k [F ] + Γ
(5)
-
1
2
2
the slow decay in the absence of added F2 (Γ∼2700 s ) was
dominated by diffusion out of the small volume sampled by
and Γ is the loss rate when [F2] is not present. The baseline
intensity, y0, was determined from the pre-photolysis signal
level. For the faster decay shown in Figure 1, an exponential
fit gave b ) 8927 s . The decay rate vs [F2] data are
summarized in Table 1 and plotted in Figure 2. The slope of
15
-3
the probe laser. The addition of [F2] ) 2.5 × 10 cm clearly
increased the H atom loss rate. Within the 0-100 µs range, the
decay curves were single exponential and the rate constant was
extracted by fitting the data to the expression
-
1
-
12
Figure 2 gives the rate constant, k2 ) 2.4 ( 0.4 (2σ) × 10
-bt
y ) y + Ae
(4)
3
-1
0
cm s .

Hydrogen Atom Reactions with Molecular Halogens
J. Phys. Chem. A, Vol. 106, No. 36, 2002 8419
TABLE 3: Conditions and Results for H + Cl
Measurementsa
2
a
a
[
Cl H atom decay rate
2
]
[Cl
2
]
H atom decay rate
5
-3
5
-1
1015 cm
-3
5 -1
1
01 cm
10 s
10 s
0
1
1
1
2
2
2
2
2
2
3
3
3
4
.74
.66
.66
.98
.11
.11
.13
.29
.83
.83
.22
.27
.75
.06
0.32
0.40
0.50
0.56
0.60
0.64
0.59
0.79
0.71
0.88
0.90
0.89
0.64
1.28
1.32
1.15
0.87
4.81
5.02
5.23
5.47
5.55
5.59
6.44
6.50
7.12
7.28
7.53
7.63
8.60
8.63
9.05
10.55
1.27
1.11
1.56
1.53
1.67
1.50
1.83
1.77
2.06
1.74
1.56
1.98
2.47
2.22
2.34
2.79
Figure 2. H atom decay rate vs [F
2
]. The rate constant for reaction 2
-
12
3
-1
4.31
4
4
is given by the slope, k
2
) 2.4 ( 0.4 (2σ) × 10 cm s
.
.35
.73
TABLE 2: Comparison of Measured Rate Constants for H
+
F
2
at 298 K
a
These are the raw observed decay rates, uncorrected for the decay
rate (Γ) in the absence of Cl . The initial concentration of H S for these
measurements was 1.6 × 10 cm . Photolysis caused complete
k
2
(298 K)
2
2
ref
Homann, et al.2
(× 10 cm s-1)
-
12
3
14
-3
14
-3
dissociation, giving [H]
0
) [SH]
0
) 1.6 × 10 cm
.
experiments
Reviews
3
1.6 ( 0.4
3.0 ( 0.4
TABLE 4: Comparison of Measured Rate Constants for H
Cl at 298 K
2
2
2,26
Albright, et al.
+
1
5
a
Sung, et al.
Levy & Copeland
1.0
2
4
b
1.0 ( 0.15
4.2 ( 0.3
2.4 ( 0.4
3
k (298 K)
2
5
(10 cm s-1)
-11
3
Rabideau, et al.
this work
ref
27
a,b
Klein & Wolfsberg
0.46 ( 0.19
Davidow, Lee, & Armstrong29
Jardine, Ballash, & Armstrong
0.49 ( 0.03
b,c
1
4
c
28
b,d
Cohen & Westberg
1.5
2.5
0.43 ( 0.04
1
3
c
Ambidge, Bradley, & Whytock30
0.70 ( 0.14
0.96 ( 0.04
2.97 ( 0.89
3.5 ( 1.2
Baulch, et al.
Dobis & Benson17
a
Authors reported k
2
/k
3
) 0.053, absolute value calculated using k
3
22,26
Dodonov, et al.
Stedman, Steffenson, and Niki
Wagner, Welzbacher and Zellner
-
11
b
)
)
1.9 × 10 from ref 13. Authors reported k
2
/k(H + O
2
+ M)[M]
39
3
3
4.6 ( 0.7, absolute value calculated using k(H + O
2
+ M)[M] from
1.91 ( 0.25
2.19 ( 0.32
1.6 ( 0.1
c
32
ref 13. Recommendation based on literature review.
Bemand & Clyne
Michael & Lee4
0
4
1
Jaffe and Clyne
2.13 ( 0.78
1.7 ( 0.26
Photolysis of H2S also yields SH radicals, but it is unlikely
that the present results are significantly influenced by the
reactions of this fragment. As F2 is the most abundant reactant
available, it is most pertinent to consider the reaction of SH
with F2. This reaction does not appear to have been studied
previously, but the possible products are HSF + F and S + HF
Kita and Stedman42
Seeley, Jayne, and Molina
this work
43
e
1.8 ( 0.5
2.52 ( 0.18
a
Authors reported k
(298 K) calculated from k
Authors reported k /k
6
/k
3
) 0.143 ( 0.033 exp(-(1540 ( 130)/RT).
b
13
-14
3
-1
k
3
6
(298 K) ) 4.25 × 10
cm s .
c
d
6
3
) 0.0088 ( 0.0005 at 298 K. Authors reported
e
+
F. As far as the present measurements are concerned, the
k /k ) 0.01 ( 0.001 at 298 K. Authors do not give a numerical result,
6
3
only deleterious effect these reactions can have is to reduce the
concentration of F2. An upper bound for the error that this would
introduce can be estimated by examining the measurement with
but the value in the table is estimated from Figure 16 of ref 43.
et al.22,26 (k2(298 K) ) 3.0 ( 0.4 × 10
-12
cm s ). Although
3
-1
15
-3
the lowest F2 concentration ([F2] ) 1.6 × 10 cm ). Assuming
Dodonov et al.’s method has been criticized for its poor
-12
3
-1
23
a rate constant of 2.4 × 10 cm s for reaction 2, this process
definition of the reaction time and the unknown role of mixing,
the results appear to be accurate not only for the H + F2 reaction
but also for H + Cl2, (see Table 4 and the discussion below).
13
-3
would reduce the F2 concentration by 5 × 10 cm within
the first 100 µs. If the reaction of SH with F2 goes to completion
1
4
-3
within this time, a further reduction of F2 by 1.6 × 10 cm
B. H + Cl . We tested the reliability of our method by
2
will occur. Modeling of this limiting situation shows that the
rate constant derived from the H atom decay under the
assumption of first-order removal would be underestimated by
applying it to the well-studied H + Cl reaction. As the rate
2
constant for H atom removal by Cl is large, this system should
2
be less sensitive to variations in Γ, the baseline decay rate.
Representative temporal profiles of the [H] dependent VUV
12%. This error decreases to 5% for measurements made with
the highest pressure of F2.
fluorescence signal with and without added Cl are shown in
2
Table 2 summarizes the results from the five previous reports
the upper panel of Figure 3. The addition of [Cl2] (∼50 mTorr)
of k2 measured at room temperature. Our result compares well
dramatically increased the H atom decay rate, and the entire
time history was consistent with a single-exponential decay.
Clearly, no detectable amounts of reaction products underwent
photolysis to yield H atoms.
13
with the recommended value of Baulch et al. and the
22
experimental results of Dodonov et al. The agreement with
Homann et al.23 is marginal (just inside the combined error bars),
1
5
and we are in poor agreement with Sung et al., Levy and
The lower panel of Figure 3 shows an example of a single-
exponential fit to the H atom decay data. Figure 4 and Table 3
summarize the conditions and results for reaction 3. The slope
Copeland,24 and Rabideau et al. Interestingly, Baulch et al.
25
13
chose a k2 value that lies halfway between the results of Homann
et al.23 (k2(298 K) ) 1.6 ( 0.4 × 10 cm s ) and Dodonov
-12
3
-1
of the plot in Figure 4 gives k3 ) 2.52 ( 0.18 (2σ) × 10
-11

8420 J. Phys. Chem. A, Vol. 106, No. 36, 2002
Han et al.
between Dobis and Benson’s17 value and ours is outside of the
combined uncertainty.
Table 4 shows that the results from previous measurements
of k3 can be roughly separated into two groups. A majority of
-
11
3 -1
the previous studies report k3 ) 2 ( 1 × 10 cm s , whereas
-11
3 -1
a smaller number give k3 e 1 × 10 cm s . A more detailed
examination of this second group of experiments shows that
nearly all were performed under conditions where secondary
reactions were important, and the primary data must be care-
fully processed to extract k3. For example, the experiments of
2
7
28,29
Klein and Wolfsberg and Armstrong and co-workers
provided estimates for the ratio of the rate constants for reactions
and 6
3
H + HCl f Cl + H2
by measuring the yield of H2 following long-term irradiation
(6)
(
i.e., many minutes of reaction time) of a variety of gaseous
2
7,29
mixtures containing HCl. Their results
1
range from k4/k3 )
-
2
-3
13
.07 ( 0.15 × 10 to 8.8 ( 0.5 × 10 . If the Baulch et al.
-
14
recommendation for k6 is adopted (k6(298 K) ) 4.25 × 10
3
-1
-12
cm s ), then k3 ranges from 4.1 ( 0.6 × 10
to 4.8 ( 0.3
-
12
3
-1
×
10
cm s . Because of the long reaction times and the
chemical complexity of the system, a complicated mechanism
must be deconvoluted to extract the ratio k6/k3. In addition, the
role of wall reactions could not be determined, and considerable
error can occur if the model is not complete.
The present result is also in disagreement with the k3 value
3
0,31
reported by Whytock and co-workers
(k3(298) ) 0.7 ( 0.22
-
11
3
-1
×
10
cm s ). The accuracy of this result has been
1
7,32,33
questioned on several occasions.
The measurements of
did not correspond to pseudo-first-
30,31
Whytock and co-workers
Figure 3. Representative H + Cl
2
data are shown. The slow decay in
the absence of Cl in the upper panel is attributed mainly to diffusion
order conditions, and they were working in an extremely difficult
H atom regime for detection by electron spin resonance.
2
out of the observation zone with a minor amount of H atom loss via
reactions with photoproducts. The lower panel demonstrates the quality
of the fit to a single-exponential decay. The corresponding decay rate
17
Dobis and Benson used a phosphoric acid coated discharge
10
-3
tube to create a small concentration of H atoms (∼10 cm )
-
1
to which Cl2 was added. Mass spectrometry was used to monitor
is 5000 s
.
[
Cl2], [Cl], and [HCl] as a function of [Cl2]0 under steady-state
1
7
flow conditions. Dobis and Benson attribute the difference
between their result and the larger k3 values listed in Table 4 to
unaccounted-for wall reactions that remove H atoms. The
difference between our result, which is unaffected by wall
effects, and Dobis and Benson’s is not clear. We note that
Dobis and Benson’s
1
7
3
4,35
application of their VLPR apparatus
to the Cl + HBr reaction also resulted in a rate constant that
was a factor of 2 smaller than the majority of previous
3
6-38
results.
IV. Summary
The H + F2 and H + Cl2 reactions were examined under
pseudo-first-order conditions. Pulsed photolysis of low concen-
trations of H2S was used to generate H atoms. The subsequent
removal of H atoms was monitored by two-photon laser induced
fluorescence. For H + Cl2, simple exponential decay curves
-
11
were observed, and a rate constant of 2.52 ( 0.18 (2σ) × 10
3
-1
cm s was obtained. This value was in good agreement with
several earlier determinations.
Figure 4. H atom decay rate vs [Cl
2
]. The rate constant for reaction
-
11
3
-1
3
is given by the slope, k
3
) 2.52 ( 0.18 (2σ) × 10 cm s
.
Study of the reaction H + F2 was complicated by the
production of vibrationally excited HF, which could be photo-
dissociated by the probe laser. Measurements were made under
conditions that minimized the influence of this secondary
process. The initial H atom decay rate as a function of [F2]
3
-1
cm s , and the intercept is consistent with the measured decay
rate in the absence of Cl2.
Our result is in good agreement (i.e., within the combined
error bars) with the recommended value of Baulch et al. and
other previous measurements, see Table 4. The difference
1
3
-12
3 -1
defined a rate constant of 2.4 ( 0.4 (2σ) × 10 cm s . This
result is in good agreement with the value obtained by Baulch

Hydrogen Atom Reactions with Molecular Halogens
J. Phys. Chem. A, Vol. 106, No. 36, 2002 8421
et al.13 from a critical review of previous determinations. We
recommend continued use of the Baulch et al. Arrhenius
parameters in computational models of HF chemical lasers.
(19) Berg, P. A.; Sloan, J. J. J. Chem. Phys. 1994, 100, 1075.
(20) Hansch, T. W.; Lee, S. A.; Wallenstein, R.; Wieman, C. Phys. ReV.
Lett. 1975, 34, 307.
13
(21) Turnipseed, A. A.; Birks, J. W. J. Phys. Chem. 1991, 95, 6569.
(22) Albright, R. G.; Dodonov, A. F.; Lavrovskaya, G. K.; Morosov, I.
Acknowledgment. We gratefully acknowledge support of
this work by AFOSR under Grant F49620-01-1-0070.
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(
(
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(
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(
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