Molecular beam studies of Cl_2O photolysis at 248 and 308 nm have been repeated and the analysis refined. At 248 nm, three distinct dissociation pathways that led to Cl+ClO products were resolved. At 308 nm, the angular distribution was slightly more isotropic than previously reported, leaving open the possibility that Cl_2O excited at 308 nm lives longer than a rotational period

Minton, Timothy K.

Moore, Teresa A.

Okumura, Mitchio

English

Caltech Authors - Main

NOTES
Photodissociation of Cl2O at 248 and 308 nm
Teresa A. Moore and Mitchio Okumuraa)
Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena,
California 91125
Timothy K. Mintona)
Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717
~Received 14 March 1997; accepted 19 May 1997!
@S0021-9606~97!00632-6#INTRODUCTION
Studies of the photodissociation dynamics of Cl2O
~dichlorine monoxide! can aid in understanding the photo-
chemistry of the more complex chlorine oxides which play a
major role in stratospheric ozone depletion. As we reported
in a previous publication,1 the formation of Cl1ClO prod-
ucts ~ DH0
o533.8 kcal/mol!2 is the only channel observed for
Cl2O photolysis at 308 nm and is the major channel at 248
nm. We have repeated the Cl2O photolysis experiment at 248
and 308 nm and present here a refinement of the analysis for
the Cl1ClO channel. At 248 nm, three distinct dissociation
pathways that led to Cl1ClO products were resolved. At 308
nm, the angular distribution is slightly more isotropic than
previously reported.
The experimental method and apparatus are described
elsewhere.3 Briefly, a molecular beam was crossed with an
excimer laser beam at the center of rotation of a rotatable
mass spectrometer detector. Collisionless conditions allowed
for the detection of primary dissociation products recoiling
away from the beam direction. Photoproduct time-of-flight
~TOF! distributions were measured as a function of the de-
tector angle.
The new data were recorded on the same apparatus un-
der similar conditions that differed primarily in source con-
ditions and laser polarization. For the new experiment, a con-
tinuous molecular beam of Cl2O was formed by passing a
gas mixture of 2% SF6 in helium over the surface of liquid
Cl2O at 273 °C and expanding the mixture through a 0.1
mm quartz nozzle held at or below 260 °C with a stagnation
pressure of 190 Torr. In all experiments, the laser beam was
nearly randomly polarized in a plane perpendicular to the
scattering plane and parallel to the molecular beam axis. In
the current experiment, there was a slight net polarization
~;58%! perpendicular to the scattering plane, whereas pre-
viously there was a slight net polarization parallel to that
plane. The differences in the beam conditions and the laser
polarization were taken into account in the analysis.
Analysis of product TOF and angular distributions were
carried out by the forward convolution technique used pre-
viously. For each channel, center of mass ~c.m.! translational
energy P(ET) and angular w(u) distributions were trans-
formed to the LAB frame and adjusted iteratively until the
predicted LAB TOF distributions agreed with the observedJ. Chem. Phys. 107 (8), 22 August 1997 0021-9606/97/107(8)/3
Downloaded 07 Sep 2006 to 131.215.225.158. Redistribution subject distributions at all LAB angles. The c.m. angular distribu-
tions were fit to w(u) } 1 1 bP2(cos u), where the anisot-
ropy parameter b varies from 21 to 2 for dissociation with
the recoil vector perpendicular and parallel to the transition
dipole, respectively.
248 nm
Representative TOF distributions of products from 248
nm photolysis are shown in Fig. 1. The m/z551 ~ClO1) data
could not be fit at all angles with a single P(ET) distribution
as was done in the previous work. Instead, the three observed
peaks at m/z551 were fit with three separate components in
the P(ET) distribution ~Fig. 2!, each with different anisot-
ropy parameters. The fastest component had a large transla-
tional energy release with a maximum at 49 kcal/mol
~Eint533 kcal/mol!. The angular distribution was fit by
b50.760.2, indicating a prompt, parallel dissociation. The
middle component had an average translational energy re-
lease of ^ET&536 kcal/mol (^Eint&546 kcal/mol! and anisot-
ropy of b51.560.2. A fit to the slowest component gave
b51.260.2 and a P(ET) distribution with a sharp cutoff
FIG. 1. TOF number density distribution N(t) from Cl2O photolysis at 248
nm for ~A! Cl1 products collected at a detector angle of 30°, ~B! ClO1
products at 30°, ~C! Cl1 products at 50°, and ~D! ClO1 products at 50°. The
solid line is the overall fit to the data. The dashed, dotted, and dot–dashed
lines correspond to the three components in the c.m. P(ET) distribution
~Fig. 2!. The long-dashed line in ~A! and ~C! is a hypothetical fit based on
photolysis of Cl2O clusters in the beam. The dot–dot–dashed line in ~A! and
~C! corresponds to the spontaneous secondary dissociation of ClO. Stimu-
lated secondary dissociation of ClO is not included.3337337/2/$10.00 © 1997 American Institute of Physics
to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
3338 Letters to the Editorwith a minimum ET of 1461 kcal/mol. The cutoff was due
to opening of the Cl2O!Cl1Cl1O channel, because the in-
ternal energy of the ClO product exceeded the bond disso-
ciation energy ~D0563.4 kcal/mol! for products with
ET.19.0 kcal/mol. The 5 kcal/mol discrepancy could be at-
tributed to ~a! spin–orbit excitation ~2.5 kcal/mol! of one or
both Cl atom products,4 ~b! a centrifugal barrier to dissocia-
tion of rotationally excited ClO products ~in the rigid radical
approximation,5 the ClO centrifugal barrier could be as high
as 7.3 kcal/mol!, or ~c! a combination of these causes.
The m/z535 signal ~Figs. 1A, C! comprised direct Cl
photoproducts, Cl atoms from secondary dissociation of ClO,
ClO fragments which cracked in the ionizer, and background
from cluster photolysis. In order to fit the Cl counterfrag-
ment, we extended the slowest P(ET) distribution to lower
translational energies. We concluded that '4% of the ClO
underwent spontaneous dissociation. The secondary Cl sig-
nal from this dissociation was modeled and included in the
fit. Contributions from primary ClO products were fit assum-
ing 32%, 37%, and 49% of the ClO fragmented to Cl1 in the
ionizer with the extent of fragmentation increasingly linearly
with ClO internal energy. Photodissociation of ClO was also
observed at higher laser power, but was minimized by keep-
ing laser fluences low. A small underlying signal remained,
which we assigned to products from photolysis of Cl2O clus-
ters.
FIG. 2. Optimized c.m. P(ET) distributions for the Cl1ClO channel from
Cl2O photolysis at 248 nm. The light-dashed line in the slowest component
shows the cutoff from the m/z551 data, corresponding to internal energies
beyond the ClO dissociation limit. The anisotropy parameters and average
translational energies for each component are dashed line, b51.260.2,
^ET&517.8 kcal/mol; dotted line, b51.560.2, ^ET&535.9 kcal/mol; and
dot–dashed line, b50.760.2, ^ET&551.1 kcal/mol. The scaling factor for
each component is independent of angle.J. Chem. Phys., Vol. 107,
Downloaded 07 Sep 2006 to 131.215.225.158. Redistribution subject In summary, we have now resolved three components in
the P(ET) distribution with differing anisotropies, each lead-
ing to Cl1ClO products. These results support our conjec-
ture that the formation of Cl1ClO products involves several
excited state potential energy surfaces.
308 nm
At 308 nm, the m/z551 ~ClO1! TOF spectra were fit by
a P(ET) distribution very similar to the one published pre-
viously, with a peak at 33 kcal/mol and a slow shoulder
extending down to 6.5 kcal/mol. The angular distribution, fit
with b50.460.3, was slightly less anisotropic than previ-
ously reported.
A long-lived excited state of Cl2O was recently proposed
by Nickolaisen et al.6 to explain quenching at high pressures
of the ClO yield from Cl2O photolysis at l.300 nm in flow
tube studies. They postulated a metastable excited state
which must live longer than the mean collision time at 5 Torr
of ;50 ns. Such a lifetime would lead to complete rotational
averaging of the anisotropic angular distribution, resulting in
b<0.19 if Cl2O dissociates from its ground state geometry.7
This value of b is half of the observed value, but within our
uncertainty. Our results thus indicate that t ,9 ps at 308 nm,
but our uncertainties preclude us from ruling out a longer-
lived state.
ACKNOWLEDGMENTS
We acknowledge support of NASA Grant No. NAGW-
3893, Montana Space Grant Consortium Federal Grant
No. NGT-40041-2, and assistance from C. M. Nelson and
J. W. Seale.
a!Authors to whom correspondence should be addressed.
1 C. M. Nelson, T. A. Moore, M. Okumura, and T. K. Minton, J. Chem.
Phys. 100, 8055 ~1994!.
2 R. P. Thorn, Jr., L. J. Stief, S.-C. Kuo, and R. B. Klemm, J. Phys. Chem.
100, 14178 ~1996!.
3 Y. T. Lee, J. D. McDonald, P. R. LeBreton, and D. R. Herschbach, Rev.
Sci. Instrum. 40, 1402 ~1969!; M. J. O’Laughlin, B. P. Reid, and R. K.
Sparks, J. Chem. Phys. 83, 5647 ~1985!.
4 Cl* products were observed with 18% yield from Cl2O photolysis at 248
nm by A. J. Chichinin, Chem. Phys. Lett. 209, 459 ~1993!.
5 G. E. Busch and K. R. Wilson, J. Chem. Phys. 56, 3626 ~1972!.
6 S. L. Nickolaisen, C. E. Miller, S. P. Sander, M. R. Hand, I. H. Williams,
and J. S. Francisco, J. Chem. Phys. 104, 2857 ~1996!.
7 S.-C. Yang and R. Bersohn, J. Chem. Phys. 61, 4400 ~1974!.No. 8, 22 August 1997
to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp


Photodissociation of Cl_2O at 248 and 308 nm

NOTES
Photodissociation of Cl 2O at 248 and 308 nm
Teresa A. Moore and Mitchio Okumuraa)
Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena,
California 91125
Timothy K. Mintona)
Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717
~Received 14 March 1997; accepted 19 May 1997!
@S0021-9606~97!00632-6#o-
y a
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timu-INTRODUCTION
Studies of the photodissociation dynamics of Cl2O
~dichlorine monoxide! can aid in understanding the phot
chemistry of the more complex chlorine oxides which pla
major role in stratospheric ozone depletion. As we repor
in a previous publication,1 the formation of Cl1ClO prod-
ucts~ DH0
o533.8 kcal/mol!2 is the only channel observed fo
Cl2O photolysis at 308 nm and is the major channel at 2
nm. We have repeated the Cl2O photolysis experiment at 24
and 308 nm and present here a refinement of the analysi
the Cl1ClO channel. At 248 nm, three distinct dissociati
pathways that led to Cl1ClO products were resolved. At 30
nm, the angular distribution is slightly more isotropic th
previously reported.
The experimental method and apparatus are descr
elsewhere.3 Briefly, a molecular beam was crossed with
excimer laser beam at the center of rotation of a rotata
mass spectrometer detector. Collisionless conditions allo
for the detection of primary dissociation products recoili
away from the beam direction. Photoproduct time-of-flig
~TOF! distributions were measured as a function of the
tector angle.
The new data were recorded on the same apparatus
der similar conditions that differed primarily in source co
ditions and laser polarization. For the new experiment, a c
tinuous molecular beam of Cl2O was formed by passing
gas mixture of 2% SF6 in helium over the surface of liquid
Cl2O at 273 °C and expanding the mixture through a 0
mm quartz nozzle held at or below260 °C with a stagnation
pressure of 190 Torr. In all experiments, the laser beam
nearly randomly polarized in a plane perpendicular to
scattering plane and parallel to the molecular beam axis
the current experiment, there was a slight net polariza
~;58%! perpendicular to the scattering plane, whereas p
viously there was a slight net polarization parallel to th
plane. The differences in the beam conditions and the la
polarization were taken into account in the analysis.
Analysis of product TOF and angular distributions we
carried out by the forward convolution technique used p
viously. For each channel, center of mass~c.m.! translational
energy P(ET) and angularw(u) distributions were trans
formed to the LAB frame and adjusted iteratively until th
predicted LAB TOF distributions agreed with the observJ. Chem. Phys. 107 (8), 22 August 1997 0021-9606/97/107(8)/3
Downloaded 07 Sep 2006 to 131.215.225.158. Redistribution subject to Ad
8
for
ed
le
ed
t
-
n-
n-
as
e
In
n
e-
t
er
-
distributions at all LAB angles. The c.m. angular distrib
tions were fit tow(u) } 1 1 bP2(cosu), where the anisot-
ropy parameterb varies from21 to 2 for dissociation with
the recoil vector perpendicular and parallel to the transit
dipole, respectively.
248 nm
Representative TOF distributions of products from 2
nm photolysis are shown in Fig. 1. Them/z551 ~ClO1) data
could not be fit at all angles with a singleP(ET) distribution
as was done in the previous work. Instead, the three obse
peaks atm/z551 were fit with three separate components
the P(ET) distribution ~Fig. 2!, each with different anisot-
ropy parameters. The fastest component had a large tra
tional energy release with a maximum at 49 kcal/m
~Eint533 kcal/mol!. The angular distribution was fit by
b50.760.2, indicating a prompt, parallel dissociation. Th
middle component had an average translational energy
lease of̂ ET&536 kcal/mol (̂ Eint&546 kcal/mol! and anisot-
ropy of b51.560.2. A fit to the slowest component gav
b51.260.2 and aP(ET) distribution with a sharp cutoff
FIG. 1. TOF number density distributionN(t) from Cl2O photolysis at 248
nm for ~A! Cl1 products collected at a detector angle of 30°,~B! ClO1
products at 30°,~C! Cl1 products at 50°, and~D! ClO1 products at 50°. The
solid line is the overall fit to the data. The dashed, dotted, and dot–da
lines correspond to the three components in the c.m.P(ET) distribution
~Fig. 2!. The long-dashed line in~A! and ~C! is a hypothetical fit based on
photolysis of Cl2O clusters in the beam. The dot–dot–dashed line in~A! and
~C! corresponds to the spontaneous secondary dissociation of ClO. S
lated secondary dissociation of ClO is not included.3337337/2/$10.00 © 1997 American Institute of Physics
IP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
-
s
at
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3338 Letters to the Editorwith a minimumET of 1461 kcal/mol. The cutoff was due
to opening of the Cl2O→Cl1Cl1O channel, because the in
ternal energy of the ClO product exceeded the bond dis
ciation energy ~D0563.4 kcal/mol! for products with
ET.19.0 kcal/mol. The 5 kcal/mol discrepancy could be
tributed to~a! spin–orbit excitation~2.5 kcal/mol! of one or
both Cl atom products,4 ~b! a centrifugal barrier to dissocia
tion of rotationally excited ClO products~in the rigid radical
approximation,5 the ClO centrifugal barrier could be as hig
as 7.3 kcal/mol!, or ~c! a combination of these causes.
The m/z535 signal~Figs. 1A, C! comprised direct Cl
photoproducts, Cl atoms from secondary dissociation of C
ClO fragments which cracked in the ionizer, and backgrou
from cluster photolysis. In order to fit the Cl counterfra
ment, we extended the slowestP(ET) distribution to lower
translational energies. We concluded that'4% of the ClO
underwent spontaneous dissociation. The secondary Cl
nal from this dissociation was modeled and included in
fit. Contributions from primary ClO products were fit assum
ing 32%, 37%, and 49% of the ClO fragmented to Cl1 in the
ionizer with the extent of fragmentation increasingly linea
with ClO internal energy. Photodissociation of ClO was a
observed at higher laser power, but was minimized by ke
ing laser fluences low. A small underlying signal remain
which we assigned to products from photolysis of Cl2O clus-
ters.
FIG. 2. Optimized c.m.P(ET) distributions for the Cl1ClO channel from
Cl2O photolysis at 248 nm. The light-dashed line in the slowest compon
shows the cutoff from them/z551 data, corresponding to internal energi
beyond the ClO dissociation limit. The anisotropy parameters and ave
translational energies for each component are dashed line,b51.260.2,
^ET&517.8 kcal/mol; dotted line,b51.560.2, ^ET&535.9 kcal/mol; and
dot–dashed line,b50.760.2, ^ET&551.1 kcal/mol. The scaling factor fo
each component is independent of angle.J. Chem. Phys., Vol. 107,
Downloaded 07 Sep 2006 to 131.215.225.158. Redistribution subject to Ao-
-
,
d
ig-
e
-
o
p-
,
In summary, we have now resolved three component
theP(ET) distribution with differing anisotropies, each lead
ing to Cl1ClO products. These results support our conje
ture that the formation of Cl1ClO products involves severa
excited state potential energy surfaces.
308 nm
At 308 nm, them/z551 ~ClO1! TOF spectra were fit by
a P(ET) distribution very similar to the one published pr
viously, with a peak at 33 kcal/mol and a slow should
extending down to 6.5 kcal/mol. The angular distribution,
with b50.460.3, was slightly less anisotropic than prev
ously reported.
A long-lived excited state of Cl2O was recently proposed
by Nickolaisenet al.6 to explain quenching at high pressur
of the ClO yield from Cl2O photolysis atl.300 nm in flow
tube studies. They postulated a metastable excited s
which must live longer than the mean collision time at 5 To
of ;50 ns. Such a lifetime would lead to complete rotation
averaging of the anisotropic angular distribution, resulting
b<0.19 if Cl2O dissociates from its ground state geometr
7
This value ofb is half of the observed value, but within ou
uncertainty. Our results thus indicate thatt ,9 ps at 308 nm,
but our uncertainties preclude us from ruling out a long
lived state.
ACKNOWLEDGMENTS
We acknowledge support of NASA Grant No. NAGW
3893, Montana Space Grant Consortium Federal Gr
No. NGT-40041-2, and assistance from C. M. Nelson a
J. W. Seale.
a!Authors to whom correspondence should be addressed.
1C. M. Nelson, T. A. Moore, M. Okumura, and T. K. Minton, J. Chem
Phys.100, 8055~1994!.
2R. P. Thorn, Jr., L. J. Stief, S.-C. Kuo, and R. B. Klemm, J. Phys. Ch
100, 14178~1996!.
3Y. T. Lee, J. D. McDonald, P. R. LeBreton, and D. R. Herschbach, R
Sci. Instrum.40, 1402 ~1969!; M. J. O’Laughlin, B. P. Reid, and R. K.
Sparks, J. Chem. Phys.83, 5647~1985!.
4Cl* products were observed with 18% yield from Cl2O photolysis at 248
nm by A. J. Chichinin, Chem. Phys. Lett.209, 459 ~1993!.
5G. E. Busch and K. R. Wilson, J. Chem. Phys.56, 3626~1972!.
6S. L. Nickolaisen, C. E. Miller, S. P. Sander, M. R. Hand, I. H. William
and J. S. Francisco, J. Chem. Phys.104, 2857~1996!.
7S.-C. Yang and R. Bersohn, J. Chem. Phys.61, 4400~1974!.
nt
geNo. 8, 22 August 1997
IP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp


http://authors.library.caltech.edu/29014/1/MOOjcp97.pdf

Photodissociation of Cl_2O at 248 and 308 nm

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