KrF excimer laser-induced ozone formation in supercritical carbon dioxide

Article abstract of DOI:10.1021/jp9935485

Laser-induced reactions by a pulsed KrF excimer laser were studied using UV absorption spectroscopy in sub- and supercritical O₂/CO₂ mixtures up to the pressure of 15 MPa (corresponding density, 17 mol dm^-3). Although the 248 nm excimer laser photon energy is smaller than the energy required for dissociating O₂, ozone formation was observed in O₂/CO₂ mixtures. Under the laser irradiation, O₃ concentration increased monotonically with the increase of the irradiation time and then stayed constant, which is satisfactorily expressed by the equation d[O₃]/dt = a-b[O₃]. a corresponds to O₃ formation rate and b to O₃ decomposition rate constant. The value of a increased with the increase of CO₂ density up to 3 mol dm^-3 and was then kept almost constant with further increase. O₂ absorbs a photon to yield an oxygen molecule in the Herzberg III state O₂(A′ ³Δ_u), being augmented along with the increase of CO₂ density. In pure O₂, the predominant pathway of O₃ formation is the reaction between excited O₂ in Herzberg states and ground state O₂ to yield O₃ and atomic oxygen. In high-density O₂/CO₂ mixtures, O₃ is considered to be produced through reaction between the Herzberg states O₂ and CO₂. Taking account of the quenching effect for the above reaction together with the augmentation of O₂ absorption of laser light by the high-density CO₂, the behavior of a with respect to CO₂ density was satisfactorily explained. The behavior of b suggested a certain inhibition of O₃ recovery in high-density CO₂ after the photodecomposition of the product O₃, which was ascribed to the formation of CO₃ from the O(¹D) reaction with CO₂. A certain cage effect for the O₃ photodecomposition was also suggested. No specific pressure effect was observed near the critical point.

Full text of DOI:10.1021/jp9935485

3332
J. Phys. Chem. A 2000, 104, 3332-3340
KrF Excimer Laser-induced Ozone Formation in Supercritical Carbon Dioxide
†
Junichiro Otomo, Yoshito Oshima, Akinori Takami, and Seiichiro Koda*
Department of Chemical System Engineering, School of Engineering, The UniVersity of Tokyo, Hongo 7-3-1,
Bunkyo-ku, Tokyo 113-8656, Japan
ReceiVed: October 5, 1999; In Final Form: January 11, 2000
Laser-induced reactions by a pulsed KrF excimer laser were studied using UV absorption spectroscopy in
-
3
sub- and supercritical O
Although the 248 nm excimer laser photon energy is smaller than the energy required for dissociating O
ozone formation was observed in O /CO mixtures. Under the laser irradiation, O concentration increased
monotonically with the increase of the irradiation time and then stayed constant, which is satisfactorily expressed
by the equation d[O ]/dt ) a - b[O ]. a corresponds to O formation rate and b to O decomposition rate
constant. The value of a increased with the increase of CO
almost constant with further increase. O absorbs a photon to yield an oxygen molecule in the Herzberg III
), being augmented along with the increase of CO density. In pure O , the predominant pathway
formation is the reaction between excited O in Herzberg states and ground state O to yield O and
atomic oxygen. In high-density O /CO mixtures, O is considered to be produced through reaction between
the Herzberg states O and CO . Taking account of the quenching effect for the above reaction together with
the augmentation of O absorption of laser light by the high-density CO , the behavior of a with respect to
CO density was satisfactorily explained. The behavior of b suggested a certain inhibition of O recovery in
high-density CO after the photodecomposition of the product O , which was ascribed to the formation of
. A certain cage effect for the O photodecomposition was also suggested.
No specific pressure effect was observed near the critical point.
2 2
/CO mixtures up to the pressure of 15 MPa (corresponding density, 17 mol dm ).
2
,
2
2
3
3
3
3
3
-
3
2
density up to 3 mol dm and was then kept
2
3
state O
of O
2
(A′ ∆
u
2
2
3
2
2
3
2
2
3
2
2
2
2
2
3
2
3
1
CO
3
from the O( D) reaction with CO
2
3
4
-6
6,7
Introduction
critical fluids, such as photodissociation,
recombination,
8
,9
10-13
and isomerization reactions, as well as energy transfer,
In our previous researches,1,2 we carried out a photoinduced
partial oxidation of hydrocarbons (ethane, cyclohexane, and
ethylene) in sub- and supercritical CO2 with irradiation of pulsed
KrF excimer laser at 248 nm. Although these hydrocarbons and
O2 do not absorb KrF excimer laser light under atmospheric
pressure, relatively large amounts of oxidized products were
formed in the mixture of hydrocarbon/O2/CO2 under KrF
excimer laser irradiation. Some active oxygen species should
be involved. At that time, however, the mechanism of the 248
nm photoabsorption was not clear. Later we measured the
absorption cross section of O2 in pressurized CO2 between 230
and 280 nm wavelength, and found that an absorbance of O2
increases proportionally with the increase of the solvent CO2
in order to clarify the solvent density effects on the reactions
from a collisional energy transfer region (gaslike density) to a
diffusion control region (liquidlike density).
To elucidate the laser-induced reactions of O2 in high-density
CO2 should be useful for designing oxidation reactions in
supercritical CO2. Through a preliminary experiment, we have
noticed that an appreciable amount of O3 is produced in
pressurized O2/CO2 mixtures under KrF excimer laser irradia-
tion. Ozone formation is expected to occur through subsequent
3
reaction processes of O (A′ ∆ ). The main objective of the
2
u
present research is to observe in detail O formation with KrF
3
excimer laser irradiation at 248 nm in sub- and supercritical
3
density. The mechanism of the photoabsorption of O2 was
O2/CO2 mixtures, and to understand the mechanism of O3
formation. We discuss relevant elementary reactions of active
oxygen species in CO2, namely, excited oxygen molecules, O3,
oxygen atoms, CO4, and CO3. We also try to clarify the solvent
density effect on the elementary processes.
concluded to be collision-induced symmetry relaxation of the
selection rule for the dipole-forbidden Herzberg III system
3
3
-
(
A′ ∆u r X Σg ). However, subsequent reaction processes of
3
O2(A′ ∆u) have not been clarified.
On the other hand, supercritical CO2 is being given attention
as an alternative solvent for chemical reactions because of its
unique solvent properties and nonignitability. Certain physico-
chemical properties, such as viscosity, diffusion constant,
dielectric constant, and solubility, are related to the fluid density,
which is strongly dependent on pressure and temperature,
particularly near the critical point. Since the 1980s several
researchers have studied chemical reactions in sub- and super-
Experimental Section
Laser irradiation experiments were carried out in a high-
pressure cross-shaped optical cell made from SUS 316 of 5.0
3
cm inner volume, which possesses four sapphire windows
(diameter 1.0 cm), as shown in Figure 1. The optical length for
the laser light in the cell was 3.3 cm and that for a deuterium
lamp light for absorption measurement was 2.3 cm. Temperature
was controlled by four Nichrome line heaters inserted into the
cell block, and monitored by a thermocouple which was settled
*
To whom correspondence should be addressed. Tel: +81-3-5841-7327.
Fax: +81-3-5841-7255. E-mail: koda@chemsys.t.u-tokyo.ac.jp.
†
E-mail: otomo@ecolab.t.u-tokyo.ac.jp.
1
0.1021/jp9935485 CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/24/2000

Ozone Formation in Supercritical Carbon Dioxide
J. Phys. Chem. A, Vol. 104, No. 15, 2000 3333
1
1
Figure 2. Absorption spectrum (B(1 B
2 1
) r X(1 A )) of ozone. Solid
line: experimental profile. Dotted line: spectrum of the absorption cross
1
1
section of ozone, B(1 B
2
) r X(1 A
1
) transition, which is scaled at the
2.94 MPa
2
1.18 mol dm ), total density 12.0 mol dm , CO balance, temperature
experimental peak position. Experimental conditions: O
2
-3
-3
(
-
1
3
08 K, pulse energy 3.8 mJ pulse , laser repetition rate 10 Hz.
Figure 1. Illustration of reaction cell: (a) top view, (b) side view.
We checked the influence of the deuterium lamp light on O3
(A) D
2 4
lamp, (B) CoSO aqueous filter, (C) Nichrome line heater, (D)
pyroelectric joulemeter, (E) sapphire window, (F) thermocouple, (G)
formation and found no detectable influence. We also investi-
gated the spatial distribution of O3 concentration in the cell,
changing the diameter of the spot of deuterium lamp light at
the total pressure of 10.1 MPa. Using a 5 mm and a 10 mm
diameter of the cross section of the deuterium lamp light, the
3
2
stop valve, (H) the cross section of D
.0 cm), (I) magnetic stirrer.
2
lamp light (0.79 cm , diameter
1
inside the cell at the position where direct contact with the fluid
was realized. Fluid mixtures could be stirred by a magnetic
stirrer.
observed O concentrations were the same, which indicated that
the O concentration distribution was almost homogeneous in
3
The experimental procedures were as follows. The optical
cell was evacuated prior to the sample preparation. Then O2
was introduced into the cell up to a given pressure (typical O2
pressure, 1.97 MPa), which was followed by the introduction
of CO2 up to a desired pressure using a HPLC pump (JASCO
Co., PU-980). Pressure was detected by a pressure transmitter
the cell due to the stirring.
The densities of O2/CO2 mixture in gas and supercritical
phases were estimated by the Peng-Robinson equation14
adopting appropriate parameters (CO2: critical temperature (Tc)
304.2 K, critical pressure (Pc) 7.38 MPa, critical density (Fc)
-
3
10.6 mol dm ; and O2: Tc 154.6 K, Pc 5.05 MPa, Fc 13.6 mol
-
3
(KOFLOC, KH15). The cell was maintained at 308 ( 0.1 K.
dm ). As the binary interaction parameter, kij, for O2-CO2,
1
5
The contents were stirred by a magnetic stirrer for 5 min before
irradiation. Carbon dioxide (Suzuki Shokan Co. Ltd.; purity
we employed that for N2-CO2 (kij ) -0.017), because the
former value is not reported. We investigated pressure effects
in O2/CO2 mixtures, keeping the amount of O2 in the cell
constant. Thus, the mole fraction of individual components and
also the critical point of the mixture change continuously with
the increase of CO2 pressure. The estimation method of the
critical point of fluid mixtures was presented by Liu.16 In the
typical mixture with 1.97 MPa O2 at 308 K (the corresponding
>
99.7%) and O2 (Suzuki Shokan Co. Ltd.; purity >99.7%) were
used as received.
Unfocused light pulses at 248 nm from a KrF excimer laser
(Lambda physics, EMG53MSS-1) were introduced along the
3
.3 cm path of the optical cell. The typical pulse energy at the
-
1
entrance window was 3.8 mJ pulse , and the repetition rate
0 Hz. The laser beam cross section, which had an elliptical
-3
O2 density, 0.78 mol dm ), the experimental temperature (308
K) is higher than the critical temperature of the mixture at any
total pressure. The region where the total pressure is smaller
1
shape (major axis ≈ 7 mm, minor axis ≈ 5 mm), was about
2
0
.27 cm as drawn in Figure 1. The laser intensity was monitored
-
3
than 8.0 MPa (the corresponding total density is 5.2 mol dm )
is estimated to be in the gas phase because the total pressure is
smaller than the critical pressure of the mixture. When the total
pressure is larger than 8.0 MPa, the region is regarded to be in
supercritical phase.
with a pyroelectric joulemeter (Gentec, ED-500).
During the KrF excimer laser irradiation, absorbance of O3
was measured at 255 nm through the 2.3 cm path perpendicular
to the irradiated laser beam. A deuterium lamp (Hamamatsu
Photonics, L1626) was used as the light source. The deuterium
lamp light was first passed through a cell of 1 cm thickness
Results
-
3
containing CoSO4 aqueous solution (0.05 mol dm ) to cut the
light shorter than 210 nm. The cross section of the deuterium
lamp light had a circular shape of diameter 10 mm as drawn in
Figure 1. Absorption spectra were dispersed by a 75 cm
monochromator (Jobin-Yvon, HRS2) and detected by a photo-
multiplier (Hamamatsu Photonics, R928; detection range, 185-
Products. A broad absorption between 230 and 300 nm
represented by a solid line in Figure 2 was generated in O2/
CO mixtures with KrF excimer laser irradiation. The spectrum
2
in Figure 2 was already corrected for the contribution of O2
absorption. The noise at about 250 nm is due to the scattered
KrF excimer laser light at 248 nm. The dotted line shows the
absorption spectrum of Hartley band (B(1 B2) r X(1 A1)) of
O3 which is scaled to the experimental value at the experimental
9
00 nm). The output signal was amplified and recorded on a
1
1
personal computer. During the laser irradiation, the contents of
the cell were stirred by the magnetic stirrer.

3334 J. Phys. Chem. A, Vol. 104, No. 15, 2000
Otomo et al.
Figure 3. Typical time profiles of ozone concentration with KrF
excimer laser irradiation. Solid line: observed profile. Dotted line:
simulation from eq 2. Experimental conditions: total density 11.9 mol
Figure 4. Pulse energy dependence of ozone formation rate, a, and
ozone decomposition rate constant, b. Filled circles: ozone formation
rate, a. Open circles: ozone decomposition rate constant, b. Experi-
-
3
-1
dm , CO
laser repetition rate 10 Hz. (A) O
2
balance, temperature 308 K, pulse energy 3.8 mJ pulse
,
-3
2
mental conditions: O 1.97 MPa (0.78 mol dm ), total density 11.8
-
3
2
0.49 MPa (0.19 mol dm ), a )
, b ) 0.10. (B) O 0.98 MPa (0.39 mol
, b ) 0.15. (C) O 1.97 MPa
, b ) 0.15. (D) O
-3
mol dm , CO
2
balance, temperature 308 K, laser repetition rate 10
1
4
-3 -1
1
.1 × 10 molecule cm
s
2
Hz.
-
3
14
-3 -1
dm ), a ) 3.1 × 10 molecule cm
s
2
-
3
14
-3 -1
(
2
0.78 mol dm ), a ) 5.2 × 10 molecule cm
s
2
-
3
14
-3 -1
.94 MPa (1.17 mol dm ), a ) 8.0 × 10 molecule cm
s ,
b ) 0.15.
peak maximum. Both profiles accord with each other, showing
that O3 is generated in O2/CO2 mixtures with KrF excimer laser
irradiation. No other appreciable absorption was detected in the
UV and visible regions.
Ozone concentration was calculated from the absorbance of
O3 at 255 nm, adopting the absorption cross section in the gas
-
17
2
-1 17
phase at 255 nm (1.16 × 10
cm molecule ). Because
1
1
the Hartley band (B(1 B2) r X(1 A1)) of O3 is an allowed
transition, the O3 absorption cross section is expected to be
almost independent of the solvent density. We have indeed
found experimentally that the relative value of the O3 absorption
cross section is kept constant within (10% in the total density
between 1 and 12 mol dm . Troe and co-worker calculated
the O3 concentration in dense N2, Ar, and He media without
taking account of any pressure dependence of the O3 cross
-
3
2
Figure 5. Ozone formation rate, a, as a function of the O concentration
7
at constant total densities. Filled circles: observed values at 5.0 mol
section. They employed an O3 absorption cross section of 9 ×
-
3
dm of the total density. Open circles: observed values at 12.0 mol
-
18
2
-1
1
0
cm molecule at 265 nm that accords with the present
-
3
dm of the total density. Solid line: prediction from eq 18 at 5.0 mol
value at 255 nm, taking wavelength dependence of the absorp-
tion cross section into account.
-3
-21
3
-1
dm of the total density (k17/k16 ) 0.9 × 10 cm molecule ). Dotted
-3
line: prediction from eq 18 at 12.0 mol dm of the total density (k17
k
ature 308 K, CO
rate 10 Hz.
/
-21
3
-1
Time Profile of Ozone Formation. Typical time profiles of
O3 concentration plotted against the irradiation time are
presented by solid lines in Figure 3. The O3 concentration
increased monotonically with the irradiation time, and then it
reached a constant value. Here, it should be noted that the
concentration corresponds to the accumulated O3 concentration
from individual laser pulses during the irradiation time.
We tried to analyze the time dependence of O3 concentration
by the following equation containing two terms: O3 formation
rate, a, and O3 decomposition rate, b[O3]:
16 ) 0.9 × 10 cm molecule ). Experimental conditions: temper-
-1
2
balance, pulse energy 3.8 mJ pulse , laser repetition
determined a and b values, are presented by dotted lines in
Figure 3 that satisfactorily accord with the experimental data.
Equation 2, thus, can explain the time profiles of the O3
concentration adequately. In the following sections, we will
show the determined a and b values as the functions of pulse
energy, O2 concentration and solvent density.
Ozone Formation Rate (a). The O3 formation rate (a) under
a supercritical condition (total density, 11.8 mol dm ) shows
d[O ]/dt ) a - b[O ]
(1)
3
3
-3
a linear dependence on the pulse energy as shown in Figure 4.
Integrating it, we obtain
O ] ) (a/b)(1 - e
The a values are plotted against O2 concentration in gas phase
-bt
-3
(
total density, 5.0 mol dm ) and in supercritical phase (total
[
)
(2)
3
-
3
density, 12.0 mol dm ) in Figure 5. The inserted curves are
estimated according to the reaction model as will be described
later. The present results show that a is almost proportional to
the first order of O2 concentration, independent of the CO2
density.
Experimental time profiles of O3 concentration were fitted to
eq 2 by the least-squares method for nonlinear equations
(
Levenberg-Marquardt method),18 using a and b values as
fitting parameters. The calculated time profiles, adopting the

Ozone Formation in Supercritical Carbon Dioxide
J. Phys. Chem. A, Vol. 104, No. 15, 2000 3335
Figure 6. Ozone formation rate, a, as a function of the total density.
Figure 8. Quantum yield of ozone formation, φ, as a function of the
Filled circles: observed values. Solid line: prediction from eq 18 (k17
/
total density. Filled circles: experimental values. Solid line prediction
-
21
3
-1
-21
3
-1
k
16 ) 0.9 × 10 cm molecule ). Experimental conditions: O
2
1.97
from eq 20 (k /k ) 0.9 × 10
cm molecule ). Experimental
1
7
16
-
3
-3
MPa (0.78 mol dm ), CO
2
balance, temperature 308 K, pulse energy
2 2
conditions: O 1.97 MPa (0.78 mol dm ), CO balance, temperature
-
1
-1
3
.8 mJ pulse , laser repetition rate 10 Hz. The range of the data
308 K, pulse energy 3.8 mJ pulse , laser repetition rate 10 Hz. The
range of the data scattering is shown by the error bar in the figure.
scattering is shown by the error bar in the figure.
to a transition with ∆Λ ) 2 is induced by the collision-induced
relaxation of the angular momentum selection rule, due to the
3
,23
perturbation by O2 and/or by foreign gases.
The Herzberg
III absorption in O2/CO2 mixtures has the density-sensitive cross
section expressed by the following equation:
σtotal ) σ + σ
F
+ σ1(CO2)F
(CO2)
(3)
0
1(O2) (O2)
where σtotal is the total cross section, σ0 is the cross section at
zero pressure, σ1(i) is the density coefficient of component i,
3
and F(i) is the density of component i. The estimated fraction
of 248 nm light extinction from eq 3 is shown as a solid line in
-
3
Figure 7 for the typical experiments with 0.78 mol dm O2,
employing the values of σ0 and σ1(i) from the previous
-
25
2
-1
research: σ0 ) 3 × 10
cm molecule , σ1(O ) ) 3.71 ×
2
-1
-
44
-44
cm molecule at 248 nm.3
5
1
0
, and σ1(CO ) ) 3.45 × 10
2
Figure 7. Fraction of the 248 nm light extinction, (I
against the total density. Filled circles: experimental fraction from
which the contribution of ozone is already subtracted. Solid line fraction
0
0
- I)/I , plotted
The estimated fraction has a good agreement with the experi-
mental fraction.
Thus, we estimated the quantum yield of O3 formation
corresponding to the a value on the basis of the O2 absorption
which was estimated from eq 3. Figure 8 shows the quantum
yield (filled circles), which increases in the low total density
region and then decreases with the increase of the total density.
The curve in the figure is estimated from the reaction model
described later.
of the absorbance of O
2
predicted from eq 3. Experimental conditions:
-
3
O
2
1.97 MPa (0.78 mol dm ), CO balance, temperature 308 K, pulse
2
-
1
energy 3.8 mJ pulse , laser repetition rate 10 Hz.
The dependence of a on the total density at a constant O2
partial pressure is shown by filled circles in Figure 6. The
inserted curve is also estimated from the reaction model
described later. As the CO2 density increases, the value of a
increases monotonically in the region of the total density smaller
Ozone Decomposition Rate Constant (b). The values of b
are already plotted against the laser pulse energy in Figure 4,
which increases proportionally with the increase of laser pulse
energy. On the other hand, the dependence of b on the O2
concentration at two constant total densities (5.0 and 12.0 mol
-
3
than 3 mol dm , and then remains constant in the region of
-
3
the total density larger than 3 mol dm .
Absorption of KrF Excimer Laser Light by O2. We
measured the extinction of the irradiation laser light at 248 nm
as a function of the total density, when the O3 concentration
reached a constant value. In Figure 7, the fraction, (I0 - I)/I0,
from which the contribution of O3 absorption is already
subtracted, is plotted against the total density. It increases with
the increase of the CO2 density.
-
3
dm ) in Figure 9 shows that the value stays constant with
respect to the O2 concentration. The dependence of b on CO2
density is shown by the filled circles in Figure 10a. With the
increase of CO2 density, the value increases in the region of
-
3
the total density smaller than 3 mol dm , reaches a peak at
-
3
around 5 mol dm of the total density, and then slightly
decreases with respect to the CO2 density.
The photoabsorption augmentation of O2 Herzberg bands and
Herzberg continuum by O2 itself and by some foreign gases
Discussion
(
N2, CO2) has been extensively studied in gas and supercritical
3,19-23
phases.
It is concluded that the photoabsorption augmenta-
Ozone Formation Process in Pressurized O2. Several
researchers studied O3 formation with KrF excimer laser
3
3
-
tion of the Herzberg III system (A′ ∆u r X Σg ) corresponding

3336 J. Phys. Chem. A, Vol. 104, No. 15, 2000
Otomo et al.
above reaction. Brown and Vaida27 who studied the photo-
chemistry of (O2)2 dimers in cold jet with resonance-enhanced
3
3
-
3
-
multiphoton ionization claimed that A′ ∆u + X Σg r X Σg
3
-
+
X Σg transition of (O2)2 dimer takes place from 251 to 266
nm, which subsequently yields O3 and atomic oxygen to some
extent.
3
As already mentioned, O2(A′ ∆u) is generated through O2
3
absorption at 248 nm. The fact that O2(A′ ∆u ) is almost
3
+
1
-
isoenergetic with O2(A Σu ) and O2(c Σu ) might cause a rapid
internal conversion and/or intersystem crossing among the
3
+
3
1
-
Herzberg states (A Σu , A′ ∆u , and/or c Σu ). The excited
3
+
3
1
-
O2(A Σu , A′ ∆u , c Σu ) is considered to react with the
3
-
26
ground state O2(X Σg ) according to Shi and Barker, although
we cannot identify which Herzberg state eventually reacts with
O2(X). Thus, the reaction mechanism of O3 formation in gas
phase O2 is represented as follows.
Figure 9. Ozone decomposition rate constant, b, as a function of the
concentration at constant total densities. Filled circles: values at
O (X) + 248 nm f O (A′)
(4)
(5)
2
2
O
2
-
3
5
.0 mol dm of the total density. Open circles: values at 12.0 mol
-
3
O (A′) a O (A ,c)
2 2
dm of the total density. Experimental conditions: temperature 308
-
1
2
K, CO balance, pulse energy 3.8 mJ pulse , laser repetition rate 10
Hz.
3
O (A,A′,c) + O (X) f O + O( P)
(6a)
2
2
3
f O (X) + O (X)
2
2
(6b)
3
Under the present experimental conditions, O( P) generated
through reaction 6a exclusively reacts with O2 to yield O3:
3
O( P) + O (X) + M f O + M
(7)
2
3
Since the O2 absorbance at 248 nm is smaller than 3% in the
present experiments, O2 absorption is approximately represented
2
by σ1(O )[O2] L from eq 3, where L is the optical length (3.3
2
cm) along the laser axis. The contribution of σ0 can be neglected
under the present experimental condition of high-pressure O2.
Thus, the O3 formation rate (a) is expressed as
2
2
Iησ1(O2)[O ] L
2
a )
(8)
V
Here, η is the branching fraction of reaction 6a, V is the cell
-
1
volume, and I is the laser intensity (photon s ) which is
estimated at the center of the cell. The linear energy dependence
of eq 8 is consistent with the result in Figure 4. We estimated
2
the η value by dividing the observed a value by 2Iσ1(O )[O2] L/
2
V according to eq 8, and obtained η ) 0.15 ( 0.01 in pure O2
from 0.98 to 1.97 MPa. Similarly, we estimated the η value
2
6
from the experimental data of Shi and Barker, and obtained η
0.12-0.15 in pure O2 from 0.03 to 0.21 MPa. It is important
)
Figure 10. Ozone decomposition rate constant, b, and γ value as a
function of the total density. (a) Filled circles: observed b values. The
range of the data scattering is shown by the error bar. (b) Filled
circles: observed γ values. Solid line: prediction from eq 30 (k27/k28
to note that the values from the present work and Shi and
Barker’s experiments are very close, which guarantees that the
principal O3 formation mechanism is the same in pure O2.
Slanger and Shi also discussed an autocatalytic mechanism
21
-3
)
3 × 10 molecule cm ). Experimental conditions: temperature 308
-1
2
25,26
†
3
-
K, CO
Hz.
balance, pulse energy 3.8 mJ pulse , laser repetition rate 10
of the O3 formation
via vibrationally excited O2 (X Σg )
produced through subsequent photolysis of the product O3 within
the same laser pulse.
irradiation from O2 in gas phase. It is important to note that the
-
1
energy of 248 nm light is smaller by 0.12 eV (11.1 kJ mol )
24
1
1
than the dissociation threshold of O2 at 242.4 nm. Slanger et
O + 248 nm f O( D) + O (a ∆ )
(9)
3
2
g
25
al. first found that O3 is produced from unfocused KrF excimer
26
3
†
3
-
laser irradiation to pure O2. Afterwards, Shi and Barker studied
the reaction between electronically excited O2 in the Herzberg
f O( P) + O (X Σ )
(10)
2
g
3
+
3
1
-
states (A Σu , A′ ∆u, c Σu ) and ground state O2(X), and
†
3
-
3
O (X) + 248 nm f O (B Σ ) f 2O( P)
(11)
2
2
u
claimed that O3 and atomic oxygen are formed through the

Ozone Formation in Supercritical Carbon Dioxide
J. Phys. Chem. A, Vol. 104, No. 15, 2000 3337
†
2
-3
of 10 mol dm . Also eq 14 cannot reproduce the first order
O (X) + M f O (X) + M
(12)
(7)
2
O2 concentration dependence of a shown in Figure 5, because
eq 14 implies almost the second order of O2 concentration
dependence of a. According to the above discussion, it is
suggested that some additional mechanism is present to produce
atomic oxygen or O3 from O2(A,A′,c).
3
O( P) + O (X) + M f O + M
2
3
We estimated the contribution of the above autocatalytic
mechanism for the O3 formation rate (a) under the present
experimental conditions as discussed below. First, the amount
We will consider the possibility of O formation via CO4
intermediate species from excited O and CO . Recently, CO
4
molecule was studied by ab initio method by several research-
The following processes on singlet and triplet potential
energy surfaces are plausible.
3
†
of O2 (X) obtained from reaction 10 was estimated, using the
2
2
-
17
2
absorption cross section of O3 at 248 nm (1.07 × 10
cm
-
1 17
molecule ), the branching fraction of reaction 10 (φ ≈
38-40
ers.
2
5,28
0
.1),
and the employed laser power. Second, the amount of
regenerated O3 through reactions 11, 12, and 7 was estimated.
As the absorption cross section for reaction 11, we employed
1
-
1
+
1
-
18
2
-1
O (c Σ ) + CO (X Σ ) f CO (1 A )
(15a)
the empirical value, 1.5 × 10
cm molecule , from the
2
u
2
g
4
1
2
6
literature. The quenching rate constants by O2 or CO2 for
-
13
-15
3
-1
reaction 12 are reported to be 10 -10
cm molecule
3
+
3
1
+
3
O (A Σ , A′ ∆ ) + CO (X Σ ) f CO (1 B ) (15b)
-
1 29-31
2
u
u
2
g
4
1
s .
Finally, we found the contribution of the autocatalytic
mechanism for a is smaller than 10% in pure O2 at the present
3
typical condition (density 0.78 mol/dm ). It becomes smaller
1
1
1
+
3
CO (1 A ) f O (X A ) + CO(X Σ )
(16a)
(16b)
than 1% when the total density is larger than 5 mol/dm in O2/
4
1
3
1
g
CO2 mixtures.
†
In addition, while highly vibrationally excited O2 (X) from
3
1
3
CO (1 B ) f CO (X A ) + O( P)
4
1
3
1
reaction 10 was expected to react with ground state O2(X) to
yield O3 and atomic oxygen,32 recent ab initio calculation has
According to Averyanov’s calculation,38,39 reaction 15a is
energetically possible under the 248 nm irradiation (exothermic
by 0.56 eV), while the required energy for reaction 15b is larger
by 0.56 eV than the energy of 248 nm light. However, taking
the uncertainties in ab initio calculations into account, both
singlet and triplet reaction paths cannot be denied at present.
3
3
denied any important contribution of the above reaction. In
conclusion, the autocatalytic reaction through the production
of vibrationally excited O2 (X) is not important under the present
†
experimental conditions.
Ozone Formation Process in O2/CO2 Mixtures. In O2/CO2
mixtures, O2 photoabsorption is augmented by CO2 as well as
by O2 itself. It is important to investigate whether CO2 also
plays the role of producing O3 or not. At first, we will assume
that CO2 only acts as a collisional quencher of excited O2 in
Herzberg states. The relevant processes are
1
Indeed, the energy level of CO4(1 A1) by ab initio second-order
Mφller-Plesset (MP2) level from several researchers3 scatters
8,40
in the range of 0.4 eV.
Reactions 16a and 16b are considered to be exothermic
1
reactions, although the energy level of CO3(X A1,C2V) by MP2
O (X) + 248 nm f O (A′)
(4)
(5)
2
2
38,41-43
3
level
scatters in the range of 2.5 eV. The produced O( P)
from reaction 16b exclusively converts to O3 under the present
condition, and thus the discrimination between reaction 16a and
O (A′) a O (A,c)
2
2
1
6b is impossible so long as only O3 formation is concerned.
O (A,A′,c) + CO f O (X) + CO
2
(13)
2
2
2
In order to check the possibility of CO formation, we tried to
detect CO production in O2/CO2 mixture gas-chromatographi-
cally (Shimadzu, GC-8APT; column, Unibeads C) after the KrF
excimer laser irradiation. We, however, failed to observe CO
formation, which indicates that the relative contribution of
reaction 16a to 16b should be lower than 2%, considering the
detection limit of CO by the employed gas-chromatographic
measurement. The plausible reaction, thus, proceeds through
reactions 15b and 16b.
3
O (A,A′,c) + O (X) f O + O( P)
(6a)
2
2
3
f O (X) + O (X)
2
2
(6b)
Thus, the O3 formation rate (a) is expressed as
2
V
IL
2
a )
(ησ1(O2)[O ] + ησ
[CO ] ×
2
1(CO2)
2
k [O ]
Taking the decrease of the quantum yield of O3 formation
with the increase of solvent CO2 density into consideration, the
quenching process of CO4 may occur in parallel with reaction
6
2
[
O ])
(14)
2
k [O ] + k [CO ]
6
2
13
2
1
6, as presented by reaction 17.
The meanings of V, L, I, η, and σ1 in eq 14 are the same as
those in eq 8.
We tried to estimate the value of a by eq 14. Because the
collisional removal rate constant of O2(A′) has not been
CO + M f CO
4
2
+ O
2
+ M
(17)
3
4-37
reported,
we employed the collisional removal rate con-
From the above discussion, the O3 formation rate (a) is
expressed as
stants of O2(A,V)7) as k6 and k13,36 considering that the energy
difference between O2(A,V)7) and O2(X,V)0) is nearly equal
to the 248 nm photon energy. Equation 14 then predicts that
most excited O2(A,A′,c) molecules are electronically quenched
by CO2, because k13[CO2] is much larger than k6[O2] under the
present conditions. The predicted O3 yield is, therefore, only
ILY
a )
(18)
V
1
0% of the experimental value under the typical total density
where

3338 J. Phys. Chem. A, Vol. 104, No. 15, 2000
Otomo et al.
k [O ]
consistent with this proposed relationship. Provided that γ is
not dependent on O2 concentration, b is also independent of it.
Indeed, it is found in Figure 9 that b is kept constant with respect
to O2 concentration. Concerning the dependence of γ on the
total density, experimentally determined γ values from Figure
2
6
2
Y ) 2η(σ1(O2)[O ] + σ
[CO ][O ])
+
2
1(CO2)
2
2
k [O ] + k [CO ]
6
2
15
2
2
(
σ1(O2)[O ] + σ
[CO ] ×
2
1(CO2)
2
k [CO ]
k16
1
5
2
1
0a and eq 22 are plotted in Figure 10b. Under the typical
[
O ])
(19)
2
k [O ] + k [CO ] k [M] + k
16
-1
3
6
2
15
2
17
16
condition (φ ) 1, I ≈ 4.0 × 10 photon s , V ) 5.0 cm , σ
-17 2 -1
)
1.07 × 10
cm molecule , L ) 3.3 cm), φIσL/V is
Here, we will predict the a value by eq 18. We employ the
collisional removal rate constants of O2(A,V)7) as k6 and k15
calculated to be 0.28.
We will consider the dependence of γ value on the total
density based on a possible reaction mechanism. In gas phase,
-
11
3
-1 -1
-11
3
(
k6 ) 3.5 × 10 cm molecule s , k15 ) 7.2 × 10
cm
-
1 -1 36
molecule s ) as previously. Figure 6 shows that a successful
agreement between the experimental data and the prediction
1
it is reported that O( D) generated by photodissociation of O3
45
is deactivated through collision with CO2.
-
21
3
from eq 18 is obtained when we employ 0.9 × 10
cm
-
1
1
3
molecule as the value of k17/k16. As shown in Figure 5, the
O2 concentration dependence of a is also explained well by eq
O( D) + CO f O( P) + CO
(23)
2
2
-
11
3
-1 -1
1
8. When k17 is assumed to be 10
cm molecule s , k16
3
Subsequent reactions of O( P) are considered to regenerate O3.
10
-1
should be on the order of 10 s , which is within a plausible
44
3
range as a unimolecular decomposition rate constant. However,
further quantitative analysis for the validity of the k17/k16 value
may be needed.
O( P) + O (X) + M f O + M
(7)
2
3
When reaction 23 proceeds exclusively, the value of γ is
The quantum yield of O3 formation is then expressed by
expected to be 0, because the same amount of O regenerates
3
through reaction 7. In the present experiments where pure O2
2
-
3
φ ) Y/(σ1(O2)[O ] + σ
[CO ][O ])
(20)
(the total density is the same as [O2] of 0.78 mol dm ) is
irradiated, the experimental b and γ values are actually very
small (b ) 0.0009, γ ) 0.003) as shown in Figure 10a,b. The
value of b, however, has been found to increase with the increase
of CO2 density. In the region where the total density is larger
2
1(CO2)
2
2
The prediction from eq 20 shows a good agreement with the
experimental data as shown in Figure 8. In conclusion, reaction
between excited O2 and CO2 should be responsible for the
formation of O3 in high-density O2/CO2 mixtures, although CO2
is generally considered as an inactive solvent.
Ozone Photolysis and Subsequent Processes. As already
mentioned, when O3 is photodissociated in the Hartley band
under the 248 nm irradiation, a singlet channel and a triplet
channel are obeyed as shown by the following equations.
-
3
than 3 mol dm , b has the value of around 0.15. The
corresponding value of γ is approximately 0.5 as shown in
Figure 10b, which indicates that there should be another reaction
1
3
mechanism in which O( D) or O( P) is trapped so as to suppress
the regeneration of O3.
4
1
Froese et al. calculated the reaction pathway on a singlet
1
1
+
energy surface of O( D) + CO2( Σg ) and a triplet energy
1
1
3
1
+
O + 248 nm f O( D) + O (a ∆ )
(9)
surface of O( P) + CO2( Σg ) by ab initio method. They claimed
that the intersystem crossing from the singlet surface to the
triplet surface may occur through the formation of CO3 and that
the reaction pathway on the singlet surface leads to a bound
region of the singlet surface where CO3 intermediates of C2V
and D3h geometry exist. Their energies are lower by 163 kJ
3
2
u
3
†
2
3
-
g
f O( P) + O (X Σ )
(10)
The quantum yield is reported to be 0.85-0.9 into the singlet
channel, and 0.15-0.1 into the triplet channel at 248 nm.
Because the O3 absorbance at 248 nm is smaller than 20% in
the present experiments, O3 absorption can be approximately
represented by σ[O3]L. Therefore, the rate of the O3 photodis-
sociation is expressed by the following equation:
2
5,28
-
1
-1
1
mol
(C2V) and by 146 kJ mol
CO2( Σg ), respectively.
(D3h) than O( D) +
1
+
Experimental evidence for the existence of CO3 in condensed
CO2 was given in previous researches. Several researchers4
observed the formation of CO3 in CO2 matrix through the
6-48
1
49
reaction of CO2 and O( D). DeMore et al. investigated the
253.7 nm photolysis of O3 in liquid CO2 (228 K, density: 25.8
d[O3]
φIσL
V
)
-
[O3]
(21)
dt
3
mol/dm ). They discussed that the quantum yield of O3
disappearance is controlled by the following reactions.
where φ is the total quantum yield of eqs 9 and 10, which is
equal to 1, V is the cell volume, σ is the absorption cross section
of O3 at 248 nm, and L is the optical length. I is the laser
intensity expressed by photon number per second and is
estimated at the center of the cell.
1
O( D) + CO + M f CO + M
(24)
(25)
2
3
CO + CO f 2CO + O
2
3
3
2
There is some possibility that O3 is regenerated through
subsequent processes. Thus, the O3 decomposition rate, b[O3],
in eq 1 should be expressed by the following equation.
In the present experiments, a similar mechanism should be
obeyed. However, reaction 25 may not be an elementary
reaction, and the initial process may proceed as follows:
φIσL
V
1
1
1
1
+
b[O ] ) γ
[O3]
(22)
3
CO (X A ) + CO (X A ) f CO (1 A ) + CO (X Σ )
3 1 3 1 4 1 2 g
(
25a)
Here, γ empirically represents the efficiency of the decomposi-
tion of O3, which is defined as (photodissociated O3 -
regenerated O3)/photodissociated O3. Equation 22 implies b ∝
I, and the result of the pulse energy dependence in Figure 4 is
because the above spin-allowed reaction 25a is exothermic
according to Averyanov’s ab initio calculation. Most of
produced CO4(1 A1) are likely to decompose into CO2 and O2
38
1

Ozone Formation in Supercritical Carbon Dioxide
J. Phys. Chem. A, Vol. 104, No. 15, 2000 3339
through eq 17 as discussed later, and thus O3 is not expected to
be regenerated to a meaningful amount through the CO3
reactions. It is, thus, plausible that the disappearance of oxygen
atom in the present O2/CO2 mixture is caused by reactions 24
and 25.
the shortage of reliable rate constants of corresponding elemen-
tary reactions. For further analysis, we need to develop a
technique that realizes the direct observation of intermediate
species such as CO4 and CO3.
Reaction 24 is considered to be consisted of the following
competing processes.
Concluding Remarks
The following conclusions are obtained from the KrF excimer
laser photoinduced O3 formation reaction in O2/CO2 mixtures.
1
O( D) + CO f CO *
(26)
2
3
1
. The predominant pathway of O3 formation in a O2/CO2
3
+
3
mixture is the reaction between the excited O2(A Σu , A′ ∆u
3
CO * f CO + O( P)
(27)
(28)
1
-
3
2
, c Σu ) and ground state CO2 to form CO4 intermediate species,
and then O3 is produced from the subsequent reactions.
2. The solvent density effects on the O3 formation are
explained by two processes that have inverse dependence on
the solvent density. With the increase of the solvent density,
the absorption cross section of Herzberg III system of O2 is
enhanced. On the other hand, the collisional quenching of the
CO4 intermediate species generated from the reaction of
O2(A,A′,c) + CO2 causes the decrease of O3 formation with
the increase of the solvent density.
CO * + M f CO + M
3
3
1
3
O( D) + O f O( P) + O
(29)
2
2
The γ value is, thus, represented by
[
CO2]
[M]
γ )
(30)
k29
k27
k28
[
O ] + [CO ]
+ [M]
3. During the KrF excimer laser irradiation, the O3 concentra-
tion is kept at a relatively low concentration. The regeneration
of O3 after the photolysis of O3 may be suppressed due to the
2
2
k26
-
10
-11
3
-1
The values of 1.28 × 10
and 4.2 × 10
cm molecule
1
reaction between O( D) and CO2 to yield CO3. The cage effect
-1
s
are employed for k26 and k29 from the quenching rate
may also suppress the O3 photolysis to some extent.
5
0
constant data in gas phase. The predicted curve from eq 30 is
4
. Any specific CO2 pressure effect for the O3 formation and
shown in Figure 10b, when the value of k27/k28 is selected to
2
1
-3
photodecomposition is not observed in the near-critical region
be 3 × 10 molecule cm . Provided that the quenching rate
-
3
(
total density ≈ 5 mol dm ).
constant, k28, is around the same as the values of k26 and k29,
10
11 -1
the value of k27 should be equal to 10 -10 s , which is in
a plausible range as a unimolecular decomposition rate constant.
Although eq 30 qualitatively expresses the behavior of γ value
as a function of the total density, the quantitative agreement
between the prediction and the experimental result is not good,
Acknowledgment. This work is partly supported by Re-
search for the Future Program of the Japan Society for the
Promotion of Science (96P00401), which is greatly appreciated.
References and Notes
-
3
particularly, in the density region larger than 5 mol dm as
shown in Figure 10b. This deviation might be due to following
three mechanisms, namely, (1) the decomposition of CO4
generated through reaction 25a, (2) the decomposition of ground
state CO3 in the subsequent processes, and/or (3) the cage effect
of O3 photolysis in dense CO2 media. First, the influence of
decomposition of CO4 generated through reaction 25a to yield
atomic oxygen on the γ value is evaluated. When the branching
fraction of reaction 16 to 17 as a function of the total density
is taken to be the same as the previous value in eq 19, it is
found that most of CO4 decomposes into CO2 and O2 through
eq 17, and the contribution of CO4 decomposition for the γ
value is smaller than 14% in the total density larger than 5 mol
(1) Koda, S.; Oshima, Y.; Otomo, J.; Ebukuro, T. Process Technol.
Proc., High-Press. Chem. Eng. 1996, 12, 97.
(2) Koda, S.; Ebukuro, T.; Otomo, J.; Tsuruno, T.; Oshima, Y. J.
Photochem. Photobiol A: Chem. 1998, 115, 7.
(3) Oshima, Y.; Okamoto, Y.; Koda. S. J. Phys. Chem. 1995, 99, 11830.
(4) Combes, J. R.; Johnston, K. P.; O′Shea, K. E.; Fox, M. A. ACS
Symp. Ser. 1992, 488, 31.
(
(
5) Lienau, C.; Zewail, A. H. J. Phys. Chem. 1996, 100, 18629.
6) Otto, B.; Schroeder, J.; Troe, J. J. Chem. Phys. 1984, 81, 202.
(7) Hippler, H.; Rahn, R.; Troe, J. J. Chem. Phys. 1990, 93, 6560.
8) Maneke, G.; Schroeder, J.; Troe, J.; Voâ, F. Ber. Bunsen-Ges. Phys.
Chem. 1985, 89, 896.
9) Hara, K.; Kiyotani, H.; Kajimoto, O. J. Chem. Phys. 1995, 103,
5548.
(10) Roberts, C. B.; Zhang, J.; Chateauneuf, J. E.; Brennecke, J. F. J.
Am. Chem. Soc. 1995, 117, 6553.
11) Worrall, D. R.; Wilkinson, F. J. Chem. Soc., Faraday Trans. 1996,
(
(
-
3
dm . Second, CO3 molecule in the ground state needs about
(
3
1
25 kJ/mol to decompose into CO2 and O( P) according to ab
9
2, 1467.
41
initio calculation by Froese. Thus, the contribution of the
reaction, CO3 f CO2 + O( P), is also expected to be relatively
(12) Urdahl, R. S.; Myers, D. J.; Rector, K. D.; Davis, P. H.; Cherayil,
3
B. J.; Fayer, M. D. J. Chem. Phys. 1997, 107, 3747.
13) Myers, D. J.; Chen, S.; Shigeiwa, M.; Cherayil, B. J.; Fayer, M.
D. J. Chem. Phys. 1998, 109, 5971.
14) Peng, D. Y.; Robinson, D. B. AIChE J. 1977, 23, 137.
(
small in the present experiment at 308 K, once CO3* is stabilized
into the ground state. Therefore, the decomposition of CO4 and/
or CO3 is not a principal reason for the deviation. Next, we
will consider the third possibility.
(
(15) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases
and Liquid, 4th ed.; McGraw-Hill: New York, 1987; p 83.
(
(
(
16) Liu, Z.-Y. AIChE J. 1998, 44, 1709.
17) Molina, L. T.; Molina, M. J. J. Geophys. Res. 1986, 91, 14501.
18) Press, W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. P.
Troe and co-workers once investigated I2 photodissociation
6
in sub- and supercritical CO2 and C2H6. They found that the
quantum yield of I2 photodisocciation gradually decreased in
-
3
the high solvent density region from 1 through 20 mol dm ,
which was considered to be due to the cage effect. Similarly,
the present deviation between the experimental value and the
estimation might be partly ascribed to the cage effect for O3
photodissociation. However, we will only suggest the possible
contribution of the cage effect, because the more quantitative
analysis of relevant mechanisms is a formidable task due to

3340 J. Phys. Chem. A, Vol. 104, No. 15, 2000
Otomo et al.
(
24) Pernot, C.; Durup, J.; Ozenne, J. B.; Beswick, J. A.; Cosby, P. C.;
Moseley, J. T. J. Chem. Phys. 1979, 71, 2387.
25) Slanger, T. G.; Jusinski, L. E.; Black, G.; Gadd, G. E. Science 1988,
41, 945.
(39) Averyanov, A. S.; Khait, Yu. G.; Puzanov, Yu. V. J. Mol. Struct.
(Theochem.) 1999, 459, 95.
(
(
40) Song, J.; Khait, Yu. G.; Hoffmann, M. R.J. Phys. Chem. A 1999,
2
1
03, 521.
(
(
(
26) Shi, J.; Barker, J. R. J. Geophys Res. 1992, 97, 13039.
27) Brown, L.; Vaida, V. J. Phys. Chem. 1996, 100, 7849.
28) Amimoto, S. T.; Force, A. P.; Wiesenfeld, J. R.; Young, R. H. J.
(
(
41) Froese, R. D. J.; Goddard, J. D. J. Phys. Chem. 1993, 97, 7484.
42) Ferreira, E.; Gardiol, P.; Sosa, R. M.; Ventura, O. N. J. Mol. Struct.
(Theochem.) 1995, 335, 63.
Chem. Phys. 1980, 73, 1244.
29) Mack, J. A.; Mikulecky, K.; Wodtke, A. M.J. Chem. Phys. 1996,
05, 4105.
30) Hickson, K. M.; Sharkey, P.; Smith, I. W. M.; Symonds, A. C.;
Tuckett, R. P.; Ward, G. N. J. Chem. Soc., Faraday Trans. 1998, 94, 533.
31) Klatt, M.; Smith, I. W. M.; Symonds, A. C.; Tuckett, R. P.; Ward,
G. N. J. Chem. Soc., Faraday Trans. 1996, 92, 193.
32) Miller, R. L.; Suits, A. G.; Houston, P. L.; Toumi, R.; Mack, J. A.;
Wodtke. A. M. Science 1994, 265, 1831.
(
(43) Castro, M. A. Canuto, S. Chem. Phys. Lett. 1991, 177, 98.
1
(
44) Benson, S. W. Thermochemical kinetics; Wiley & Sons: New York,
(
1
968; Chapter 3. Steinfeld, J. I.; Francisco, J. S.; Hase, W. L. Chemical
kinetics and dynamics; Prentice Hall: New Jersey, 1989; Chapter 11.
(
(45) Sedlacek, A. J.; Harding, D. R.; Weston, Jr. R. E.; Kreutz, T. G.;
Flynn, G. W. J. Chem. Phys. 1989, 91, 7550.
(46) Moll, N. G.; Clutter, D. R.; Thompson, W. E. J. Chem. Phys. 1966,
45, 4469.
(
(33) Lauvergnat, D.; Clary, D. C. J. Chem. Phys. 1998, 108, 3566.
(34) Kenner, R. D.; Ogryzlo, E. A. Int. J. Kinet. 1980, 12, 501.
(35) Kenner, R. D.; Ogryzlo, E. A. Can. J. Chem. 1983, 61, 921.
(36) Knutsen, K.; Dyer, M. J.; Copeland, R. A. J. Chem. Phys. 1994,
(
47) Weissberger, E.; Breckenridge, W. H.; Taube, H. J. Chem. Phys.
1
967, 47, 1764.
(
(
48) Jacox, M. E.; Milligan, D. E. J. Chem. Phys. 1971, 54, 919.
49) DeMore, W. B.; Jacobsen, C. W. J. Phys. Chem. 1969, 73, 2935.
1
01, 7415.
(
(
37) Wildt, J.; Bednarek, G.; Fink, E. H. Chem. Phys. 1991, 156, 497.
38) Averyanov, A. S.; Khait, Yu. G.; Puzanov, Yu. V. J. Mol. Struct.
(50) Amimoto, S. T.; Force, A. P.; Gulotty, R. G.; Wisenfeld, J. R. J.
Chem. Phys. 1979, 71, 3640.
(
Theochem.) 1996, 367, 87.