Reflection-absorption IR spectroscopic investigation of the photolysis of thin films of dichlorine monoxide and chlorine dioxide

Article abstract of DOI:10.1039/a701379e

Reflection-absorption infrared (RAIR) spectroscopy and mass spectrometry have been employed in order to investigate the low-temperature photochemistry of thin films of chlorine dioxide, OClO, and dichlorine monoxide, Cl₂O, grown on a gold foil in an ultra-high vacuum system. Photolysis of a neat film of OClO (λ > 300 nm, 90-110 K) produces chloryl chloride, ClClO₂. Irradiation of a co-deposited OClO/H₂O film also produces chlorine superoxide, ClOO, which suggests that OClO isomerisation is the first step in the reaction producing ClClO₂. Photolysis of Cl₂O (300 < λ < 515 nm, 90-110 K) is shown to produce OClO, initially, which is subsequently converted to ClClO₂. Analysis of the observed IR band intensities and consideration of the metal surface selection (MSS) rule indicates that the photochemically produced OClO intermediate aligns perpendicular to the gold substrate. Mechanistic details and the atmospheric implications of the chemistry are discussed.

Full text of DOI:10.1039/a701379e

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Reflection–absorption IR spectroscopic investigation of the photolysis
of thin Ðlms of dichlorine monoxide and chlorine dioxide
Matt P. Gane, Neil A. Williams and John R. Sodeau*
School of Chemical Sciences, University of East Anglia, Norwich, UK NR4 7T J
ReÑectionÈabsorption infrared (RAIR) spectroscopy and mass spectrometry have been employed in order to investigate the
low-temperature photochemistry of thin Ðlms of chlorine dioxide, OClO, and dichlorine monoxide, Cl O, grown on a gold foil in
2
an ultra-high vacuum system. Photolysis of a neat Ðlm of OClO (j [ 300 nm, 90È110 K) produces chloryl chloride, ClClO .
2
Irradiation of a co-deposited OClO/H O Ðlm also produces chlorine superoxide, ClOO, which suggests that OClO isomerisation
2
is the Ðrst step in the reaction producing ClClO . Photolysis of Cl O (300 \ j \ 515 nm, 90È110 K) is shown to produce OClO,
2
2
initially, which is subsequently converted to ClClO . Analysis of the observed IR band intensities and consideration of the metal
2
surface selection (MSS) rule indicates that the photochemically produced OClO intermediate aligns perpendicular to the gold
substrate. Mechanistic details and the atmospheric implications of the chemistry are discussed.
Chlorine oxides play a crucial role in the catalytic destruction
of polar stratospheric ozone; a consequence of their chemistry
is the formation of the ozone “holeÏ over Antarctica in the
early austral spring. Ozone destruction occurs principally via
a ClO dimer mechanism, but other chlorine oxides species can
also potentially a†ect polar stratospheric chemistry by both
thermal and photochemical mechanisms.1
Atmospheric chlorine dioxide, OClO, was Ðrst observed
above Antarctica in the austral spring of 1986 by Solomon et
al.,2 thereby conclusively linking OClO to the polar strato-
spheric ozone depletion phenomenon. High OClO concentra-
tions have since been measured at sunset and night,
particularly within the perturbed ozone “holeÏ region,3,4 but
daytime OClO concentrations are found to be low owing to
rapid photolysis.5 Currently, the only established scheme for
the production of OClO in the stratosphere is the gas-phase
ClO and BrO coupling reaction, e.g. ref. 6 and 7:
for the production of atomic Cl range from 0.15 to 5 ] 10~4
and are dependent on the wavelength of radiation;11h14
hence, the branching ratio for reaction (2a) vs. (2b ] 2c) is still
a matter of some controversy.10
OClO ] hl ] Cl ] O
(2b)
(2c)
2
OClO ] hl ] ClOO ] Cl ] O
2
Several observations suggest that the production of Cl atoms
by photolysis of OClO will be signiÐcant for heterogeneous
chemistry involving water-ice. First, atomic chlorine pro-
duction from the photolysis of OClO is enhanced when it is
dissolved in polar solutions, e.g. Cl quantum yields of 0.1 have
been measured on irradiating an aqueous solution of OClO at
355 nm.15 For such solutions, reaction (2c) has been shown to
be dominant over reaction (2b).16 In addition, a Cl atom pro-
duction quantum yield of 0.8 has been determined for the
photolysis of OClO adsorbed on a polycrystalline platinum
surface.17
ClO ] BrO ] OClO ] Br
(1)
Gas-phase-only models can accurately predict stratospheric
OClO concentrations at mid-latitudes; however, they fail to
predict the considerably higher OClO concentrations found in
ozone-depleted polar regions.8 This discrepancy (of up to two
orders of magnitude) is presumably due to heterogeneous pro-
cessing, which controls much of the chemistry in the perturbed
polar stratosphere. Few reactions which could lead to direct
OClO production [other than reaction (1)] have been studied
in the laboratory, although the decomposition of chloryl chlo-
The photolysis of matrix-isolated OClO has also been
studied. OClO undergoes quantitative conversion to ClOO on
irradiation; it is assumed that ClOO is stabilised by the
matrix-cage e†ect. ClOO is slightly more thermodynamically
stable than OClO18,19 but is kinetically unstable and nor-
mally decomposes rapidly to form Cl and O .20 Pursell et
2
al.21 have reported that photolysis (at 360 nm) of OClO in
amorphous water-ice, maintained at 80 K, also results in
quantitative conversion to the ClOO isomer. Di†erent results
were observed when similar photolysis experiments were
carried out at higher temperatures, 140È185 K;22 chloryl chlo-
ride, ClClO , a structural isomer of (ClO) , was identiÐed by
ride, ClClO , has been shown to produce OClO.9
2
The photochemistry of OClO has been extensively studied
and many of the results have been summarised in a recent
article.10 The principal gas-phase photolysis pathway of
OClO produces ClO and an O atom:
2
2
IR spectroscopy as the main product.
In contrast, the role of dichlorine monoxide, Cl O, in atmo-
2
spheric chemistry has received scant attention. This is partly
OClO ] hl ] ClO ] O
(2a)
due to the absence of observations or measurements of Cl O
2
Overall, reaction (2a) does not lead to depletion of ozone
levels because, although ClO coupling with BrO or another
ClO radical produces Br or Cl which destroy ozone, the
photo-produced oxygen atom generates ozone by reaction
in the stratosphere or troposphere. However, it is in equi-
librium with HOCl, which itself plays a role in stratospheric
ozone-destruction chemistry:
2HOCl(g)
% H O ] Cl O
2 (g) 2 (g)
(3)
with O . An alternative photolysis pathway of OClO pro-
2
2
duces O and a Cl atom, which may proceed directly by bond
The HOCl equilibrium at 298 K has been studied by several
groups, who have reported the equilibrium constant to lie
between 4 and 12.23h25 HOCl is formed in the stratosphere by
Ðssion [reaction (2b)] or via the chlorine superoxide, ClOO,
intermediate [reaction (2c)]. This process would lead to a
reduction in ozone levels as the Cl atom can participate in
ozone-destroying reactions. The relative importance of the
alternative pathway is debatable as quantum yields measured
the gas-phase reaction of ClO and HO radicals:
2
ClO ] HO ] HOCl ] O
(4)
2
2
J. Chem. Soc., Faraday T rans., 1997, 93(16), 2747È2753
2747

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More pertinent to polar stratospheric-ozone depletion is the
heterogeneous decomposition of chlorine nitrate on PSC par-
ticles to form HOCl, which is one of the key processes in the
release of active chlorine from reservoir molecules:
Preparation of OClO
Chlorine dioxide is highly explosive in both the gas phase and
condensed phase. To minimise the handling risk, OClO was
produced in an aqueous solution by the method of Pursell et
al.22 Sodium chlorite (Lancaster, 80%) and sodium persulfate
ClONO2(ads)
] H O
2 (ads)
] HOCl ] HNO
3(ads)
(5)
(
Sigma, 98]%) were added to water in a darkened vessel,
HOCl is not considered an important chlorine reservoir in the
stratosphere owing to rapid photolysis below 290 nm.24
During the Antarctic ozone-depletion period, the HOCl
column abundance has been observed to be around 10% of
producing OClO:
2NaClO2(aq)
] Na S O
2 2 8(aq) ] 2OClO(aq)
] 2Na SO
4(aq) (9)
2
The majority of OClO produced remained dissolved in the
water. On completion of the reaction, the reaction vessel was
evacuated to 40 mbar to remove the majority of atmospheric
gases. Subsequently, IR spectroscopy revealed that the vapour
above the solution was principally OClO. The primary con-
taminant was carbon dioxide. OClO was dried before use by
the concurrent abundances of ClONO and HCl;26 although
2
it should be noted that HOCl concentrations have not been
measured at night when accumulation may occur. The HOCl
equilibrium with Cl O may also occur heterogeneously, hence,
2
adsorbed HOCl may directly produce adsorbed Cl O. In a
2
recent study, however, dosing HOCl onto ice surfaces at
passing the vapour through anhydrous P O (Aldrich, 97%).
stratospherically relevant temperatures was shown not to
2 5
produce Cl O.27 Overall, despite the lack of Cl O Ðeld
2
2
Apparatus
observations, it is possible that it does exist in signiÐcant con-
centrations during the polar night.
The experimental arrangement used in this study has been
detailed elsewhere.35 It consists of a temperature-controlled
gold foil substrate mounted in an ultra-high vacuum (UHV)
chamber. Background pressures of the di†erentially pumped
vacuum chamber were typically 1 ] 10~9 mbar. RAIR spec-
troscopy was employed for the analysis of thin substrate Ðlms
deposited in vacuo. The IR beam emitted from a Digilab
FTS-60 spectrometer was reÑected o† the gold substrate at a
high angle of incidence to the surface normal (ca. 75¡) and
detected by an external liquid-nitrogen-cooled MCT detector.
RAIR spectra were recorded at 4 cm~1 resolution with 256
scans. Based on exposure (1 ] 10~6 mbar s~1 \ 1
Langmuir B 0.5 monolayer s~1), the layers were estimated to
be ca. 500 nm thick. The UHV chamber housed a Hiden EQP
quadrupole mass spectrometer allowing temperature-
programmed desorption (TPD) experiments to be carried out.
Photolysis was performed using a 75 W xenon arc lamp;
wavelengths below 300 nm were removed using a 5 mm thick
Pyrex glass Ðlter.
Cl O absorbs light throughout the UV and visible
regions28,29 and is readily photolysed. Gas-phase photolysis
can follow two routes:29
2
ClOCl ] hl ] ClO ] Cl
(6a)
(6b)
ClOCl ] hl ] O ] Cl
2
Reaction (6a) is the primary mechanism, although at short
wavelengths reaction (6b) has been observed.29,30 Both chan-
nels release active chlorine and are therefore potentially ozone
destructive.
Irradiation of Cl O trapped in cryogenic matrices results in
2
isomerisation to form ClClO, which is presumed to be stabil-
ised by the matrix-cage e†ect.31h33 The ClO radical and chlo-
rine chlorite, ClOClO, are observed as minor side
products31,32 It is to be expected that if such species are
formed by heterogeneous reactions and then released into the
gas phase, they would rapidly decompose to ClO or Cl2 and
an O atom.
ClOClO ] hl ] 2ClO
(7)
(8a)
(8b)
Results and Discussion
Deposition of OClO Ðlms
ClClO ] hl ] ClO ] Cl
ClClO ] hl ] Cl ] O
Thin solid Ðlms of OClO were deposited onto the gold sub-
strate at 80 K in order to make comparisons with solid-state
and gas-phase spectra. The experiments also served as a check
on the purity of the samples. Fig. 1(a) corresponds to a rela-
tively thick Ðlm of OClO (ca. 2000 nm) produced by dosing at
a pressure of 3 ] 10~7 mbar for 71 min. Fig. 1(b) corresponds
2
As both OClO and Cl O are potentially linked to ozone-
2
destruction cycles, we have undertaken a laboratory study to
elaborate on their heterogeneous, photochemical behaviour in
the condensed phase, using FT-RAIR spectroscopy and mass
spectrometry for analysis. RAIR spectroscopy o†ers an advan-
tage over transmission IR spectroscopy as it can provide
information on the orientation of molecules with respect to
the metal surface. This is a result of the MSS rule which arises
from the metal surface imposing a vertical polarisation of the
surface standing wave of the incident radiation. As a result,
only vibrations with a dipole moment change perpendicular
to the metal surface are seen. This phenomenon applies to
Ðlms of up to ca. 1 lm in thickness and means that only vibra-
tions with a dipole moment change perpendicular to the
surface can be detected.
Experimental
Preparation of Cl O
2
Dichlorine monoxide was produced by the standard method
of passing dry chlorine through a column of yellow mercury
oxide (Aldrich, 99%).34 Cl O was collected (at 195 K) and
Fig. 1 RAIR spectra of (a) neat Ðlm of OClO, ca. 2000 nm, (b) neat
2
puriÐed by repeated freezeÈpumpÈthaw cycles. The puriÐed
Ðlm of OClO, ca. 300 nm and (c) co-deposited Ðlm of H O and OClO,
2
Cl O was stored at 78 K to minimise decomposition.
ca. 500 nm
2
2748
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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to a thin Ðlm of OClO (ca. 300 nm) produced by dosing at a
pressure of 3 ] 10~7 mbar for 10 min. The study of the
thicker Ðlms allows closer comparison with previous studies
and also identiÐcation of IR combination bands.
cm~1 to the asymmetric stretch of OClO clusters, (OClO) .42
n
For the co-deposited Ðlms used in the present study, clus-
tering may occur and, therefore, be responsible for the
observed frequency shift.
IR data for the active vibrational modes of OClO under a
variety of conditions are shown in Table 1. The frequencies for
the RAIR spectra represent a blue shift compared to the
normal incidence transmission spectra of a thin Ðlm solid. We
have previously observed similar shifts for RAIR spectra of
N O and N O thin Ðlms.39 They can be attributed to the
Photolysis of thin Ðlms of OClO
On photolysing (j [ 300 nm, 90È110 K) thin Ðlms (ca. 300
nm) of neat OClO or OClO deposited onto a layer of water-
ice, complete conversion to chloryl chloride, ClClO , was
observed. Fig. 2 summarises a typical experiment, showing a
series of di†erence spectra recorded during the photolysis of a
thin Ðlm of OClO (ca. 300 nm) deposited onto a layer of
water-ice (negative peaks indicate species lost and positive
peaks species formed). Decreases in intensity at 1103, 943 and
2
2
4
2 5
coupling of p-polarised radiation with a longitudinal optical
LO) mode of the thin Ðlm in the RAIR spectra rather than
(
the more usual transverse optical (TO) mode excited in trans-
mission experiments. In RAIR experiments the MSS rule
means that no coupling with a TO mode can occur, whilst LO
modes cannot be excited in transmission experiments at
normal incidence because of the transverse nature of electro-
magnetic radiation. The di†erence in frequency between the
two is known as LydaneÈSachsÈTeller (LST) splitting and is
proportional to the absorption strength.
For a thin Ðlm of OClO (ca. 300 nm) the IR spectrum
reveals two major peaks in the asymmetric stretch region of
OClO; the sharp peak at 1103 cm~1 can be assigned to crys-
talline OClO and the broader peak at 1086 cm~1 to an
amorphous form. The asymmetric region is more complicated
for much thicker Ðlms (ca. 2000 nm), which is partially a result
of the breakdown of the MSS rule (valid for Ðlms up to ca. 1
lm), thereby allowing TO modes to be measured. The new
sharp peak at 1064 cm~1 is very close to the value observed
for OClO in a transmission spectrum.36 Further splitting in
this region may be due to increased specular reÑectance in
thicker Ðlms.
9
20 cm~1 indicate the disappearance of OClO whilst increases
at 1204 and 1040 cm~1 indicate the formation of ClClO .
2
Unlike OClO, the adsorbed ClClO RAIR spectra are similar
2
to the gas-phase and matrix-isolation (transmission) vibra-
tional spectra reported in the literature.9,43,44 Table 2 sum-
marises the vibrational frequencies of the key chlorine oxide
species observed in this study and previous publications.
Chloryl chloride was Ðrst synthesised by MuŽ ller and
Willner, who identiÐed it in both cryogenic matrices and the
gas phase by IR spectroscopy.9 They originally synthesised
ClClO by halogen exchange between FClO and AlCl .
2
2
3
However, more recently, several groups have produced
ClClO by the direct reaction of OClO with Cl.43,44 In the
2
gas phase, ClClO is estimated to have a lifetime of 1 min,
2
before decomposing to OClO and Cl.9 Irradiation of matrix-
isolated ClClO yields two other ClO dimers, ClOOCl and
the less stable ClOClO.43
On photolysing thin Ðlms of OClO co-deposited with H O
2
A comparable dependence of RAIR spectra peak proÐles on
the thickness of the Ðlms is observed for water-ice.40 Analysis
using classical optics revealed that the structure in the OH
stretching bands of thick Ðlms arises owing to a combination
of an increased specularly reÑected component, an s-polarised
component and a relaxation of the MSS rule.
2
(j [ 300 nm, 90È110 K), chlorine superoxide, ClOO (l
\
O~O
In order to make a better comparison with previous work
and to give a more appropriate approximation of strato-
spheric mixtures, thin Ðlms of OClO in a water-ice matrix
were prepared by co-depositing OClO and H O onto the gold
2
substrate. A typical RAIR spectrum of OClO in water-ice (ca.
500 nm) is shown in Fig. 1(c). This thin Ðlm was prepared by
the co-deposition of OClO and H O at 6 ] 10~7 mbar for 10
2
min. The asymmetric stretch is observed at 1095 cm~1, a
small shift from the comparable absorption band at 1086
cm~1 which is observed for neat OClO deposition. This shift
may be due to complexation of OClO with neighbouring
water molecules as it is close to the perturbed OClO asym-
metric stretch, observed at 1090.5 cm~1 by Johnsson et al.41
in their studies of low-temperature matrix-isolated
Fig. 2 RAIR di†erence spectra showing the photolysis (j [ 300 nm,
05 K) of OClO deposited onto a thin layer of water-ice. Only OClO
1
loss bands (l \ 1103, 1086 cm~1, 2d \ 943 cm~1, l \ 920 cm~1)
as
s
OClOwH O complexes. In a related transmission IR study of
and ClClO formation bands (l
\ 1204 cm~1, l
\ 1040
2
2
as(OClO) s(OClO)
OClO in ice matrices, Pursell has attributed a peak at 1086
cm~1) are observed.
Table 1 Vibrational frequencies of OClO
wavenumber/cm~1
adsorbed co-
deposition OClO/H Oa
normal mode
adsorbed neat OClOa
solid phase at 77 Kb
gas phasec
neon matrixd,e
2
l (A1)
s
920 (910)
920
908
472
1062
944
1854
1987
È
943.2
445
1110.5
È
1888
2040
2215
944.8
448.7
1107.6
È
1881
2036
2204
d(A1)
È
1087 (1103, 1064)
943
1854
1987 (1971, 2006)
2150
È
l (B2)
1095 (1101)
944
1854
1994 (2010)
È
as
2
d(A1)
2
l (A1)
s
l ] l (B2)
s
as
l (A1)
as
2
a This study. b Ref. 36. c Ref. 37. d Ref. 38. e O35ClO isotopic vibrational frequencies.
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
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Table 2 Principle vibrational frequencies for several key chlorine
oxide species relevant to this study
matrix, allowing reactions with OClO to produce ClClO , or
to preferential ClOO desorption at higher temperatures. As
2
the total amount of ClClO formed at higher temperatures
shows no increase, preferential ClOO desorption must be the
compound
wavenumber/cm~1
assignment
2
Cl Oa
671
l
reason for the observations.
2
as
6
29
1086
20
1210
040
l
Pursell et al.21,22 have studied the photolysis of dilute
OClO in ice matrices under similar temperature conditions to
our experiments. They observed the quantitative conversion of
s
OClOa
l
as
9
l
s
ClClO b,c
l
2
as(OClO)
OClO to ClClO on irradiation at temperatures in the range
1
l
2
s(OClO)
1
40È185 K. However, at a lower temperature (80 K), a di†er-
5
4
20
40
d
(
OClO)
l
ent product was observed, namely ClOO. It was assumed, rea-
sonably, that at higher temperatures the photolysis reaction
proceeds through the chlorine peroxide intermediate before
(
ClCl)
ClOClOc,d
994.5
l
for ClOCl@O@
for ClOCl@O@
for ClOCl@O@
d
(
Cl{O{)
695.9
440.3
338.7
l
ClO)
l
(
OCl{)
forming ClClO . One possible mechanism may be sum-
2
(
ClOCl)
marised by the following two reactions:
ClOOClc,d
ClOOc,e
754
l
(
OO)
6
5
48
43
l
as(ClO)
OClO ] hl ] [ClOO] ] Cl ] O
(2c)
(10)
l
2
s(ClO)
1440
l
l
Cl ] OClO ] ClClO
(
OO)
4
2
10
00
d
2
The preference for reaction (2c) occurring over reaction (2a)
(
ClO)
ClClOc,f
961.9
l
(the production of ClO) is conÐrmed by the high quantum
as
3
2
74.2
40.2
l
s
yields recently found for Cl production from the photolysis of
adsorbed OClO.17 In further conÐrmation the ClO radical (or
the expected product dimers, ClOClO and ClOOCl) are not
observed in our study.
The principal di†erence between PursellÏs study and the
present experiments is that, here, ClOO was produced simul-
d
ClOc,g
850
2710
410
l
HOClh,i
l
(
HO)
1
d
721
l
ClO)
a This study. b Ref. 9. c Matrix isolation. d Ref. 43. e Ref. 38. f Ref. 33.
g Ref. 32. h Ref. 45. i Thin adsorbed layer.
taneously with ClClO and was found to be measurable up to
2
1
60 K. These facts do not exclude the possibility of ClOO
acting as a precursor to ClClO ; the observed ClOO may
2
simply be isolated and unable to further react, whilst non-
1
439 cm~1)9,38,46 and ClClO are produced simultaneously.
isolated ClOO may react rapidly producing ClClO . If this is
the case then it is no surprise that ClOO was not observed in
the study of neat Ðlms of OClO; in the absence of an inert
matrix it may be too short-lived to be observable.
Pursell has previously suggested that clustering of OClO
may be the key to the production of ClClO .22 Recently, it
has been reported that OClO forms a loosely bonded dimer in
the condensed phase36 and possibly in the gas phase.48 Using
the crystallographic data of Rehr and Jansen,36 mechanistic
Scheme 1 is suggested.
2
2
ClOO forms at a comparable rate and in competition with
ClClO , as illustrated in Fig. 3. It follows, therefore, that the
observed ClOO is not an intermediate to ClClO production.
2
2
The ClOO : ClClO2 ratio increases with lower initial OClO
concentration. The rate of loss of OClO does not vary accord-
ing to initial concentration and therefore is not a second-order
process.
2
In order to make further comparison with the previously
published studies, OClO trapped in water-ice was photolysed
at temperatures higher than 110 K. OClO trapped in a water-
ice matrix does not desorb until ice sublimation47 and, there-
fore, co-deposited OClO/H O can be photolysed up to 160 K,
without signiÐcant OClO loss. It was found that ClOO is still
produced simultaneously with ClClO up to 160 K, although
loss of both species is signiÐcant owing to desorption. As the
temperature at which photolysis was performed is increased
Overall, Scheme 1 gives:
2 OClO ] hl ] ClClO ] O
(11)
2
2
2
It is clear that, if the rate of loss of OClO is independent of the
OClO concentration, the absorption of the light must be the
rate-determining step.
2
from 110 to 160 K, more ClClO is observed relative to
2
ClOO. This increased ClClO : ClOO ratio may be due to
2
Deposition of Cl O Ðlms
2
enhanced migration of ClOO (or Cl) through the warmer ice
Dichlorine monoxide has C symmetry, hence all three fun-
2
v
damental vibrations are observable by IR spectroscopy. Its
gas-phase and solid-state IR spectra were Ðrst correctly inter-
preted in 1965 by Rochkind and Pimentel49 who also report-
ed the Ðrst matrix-isolation spectrum.31 Relative to the gas
phase the solid-state spectrum shows red-shifted peaks whilst
the matrix-isolation spectrum is very similar, as conÐrmed
later by Chi and Andrews.32 Cl O vibrational frequencies,
2
recorded in di†erent states, are shown in Table 3.
Cl O was dosed at a pressure of 1 ] 10~6 mbar for 5 min
2
onto the gold substrate at 80 K, giving the RAIR spectrum
shown in Fig. 4. Cl O is characterised by the asymmetric
2
stretch at 671 cm~1 and the symmetric stretch at 629 cm~1.
Table 3 shows a full listing of the fundamental, combination
and overtone vibrational bands observed. Minor impurities
are also observed, namely OClO (1086 l and 918 cm~1 l )
Fig. 3 RAIR di†erence spectra showing the photolysis (j [ 300 nm,
as
s
and ClClO (1221 l
and 1039 cm~1 l
).
1
05 K) of OClO co-deposited with water-ice. OClO loss bands are
2
as(OClO)
s(OClO)
Thin Cl O Ðlms (ca. 500 nm) on or under a Ðlm of water-ice
observed (l \ 1103, 1086 cm~1, 2d \ 943 cm~1, l \ 920 cm~1) as
2
as
s
well as ClClO formation bands (l
\ 1204 cm~1, l
\
and Cl O co-deposited with water were also studied; such
2
as(OClO)
s(OClO)
2
1
040 cm~1) and a ClOO formation band (l
\ 1439 cm~1).
spectra have been previously published by Banham et al.45
(
OO)
2750
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Fig. 4 RAIR spectra of a neat thin Ðlm (ca. 500 nm) of Cl O
2
dangling stretch) and 1640 cm~1 (OH deformation), which
both arise from Cl OÈH O interactions.
2
2
Photolysis of Cl O Ðlms
2
Fig. 5 shows a series of di†erence spectra recorded during the
photolysis (j [ 300 nm, 105 K) of a thin layer of Cl O (ca. 500
2
nm) adsorbed onto a layer of water-ice. The consumption of
Cl O is revealed by the decrease in intensity at 671 and 629
2
cm~1. Within several seconds of photolysis, a species is
formed which absorbs, principally, at 906 cm~1. It is seen to
maximise in intensity and then fall as the Ðnal product
ClClO is formed (1204 and 1040 cm~1). Peaks previously
2
assigned to OClO are also observed (l \ 1086 and 2d \ 940
as
cm~1) within a few minutes of photolysis. It is suggested that
the intense 906 cm~1 absorption band is due to the symmetric
stretch of OClO, as will be discussed below.
The large discrepancy in the relative intensities of the peaks
assigned to OClO formed during the photolysis of Cl O and
2
those observed for the deposition of OClO (Fig. 1) is, at Ðrst
sight, surprising. The relative intensities of peaks observed in
IR spectra of very dilute OClO in water matrices and of
OClO impurities in Cl O Ðlms (Fig. 4) are comparable with
2
those seen for neat OClO. This suggests that the relative
change in intensity is not due to weak molecular or depolar-
isation e†ects. An alternative possibility is that it is a result of
orientation e†ects, the MSS rule states that if the OClO is
aligned perpendicular to the surface, the asymmetric stretch is
formally forbidden while the symmetric stretch (906 cm~1)
and bending (940 cm~1) vibrational mode absorption inten-
sities are enhanced.
Clearly, this explanation demands that the OClO aligns
perpendicular to the surface when it is formed during photoly-
sis, but shows no alignment e†ects when it is dosed directly
onto the gold foil under carefully controlled conditions. Such
Scheme 1 Illustration of OClO dimers forming ClClO ] O
2
2
When Cl O and H O are deposited as separate layers,
2
2
absorptions are observed for Cl O and also for amorphous
2
water (800 and 3400 cm~1). When Cl O and H O are co-
deposited, additional features are observed at 3650 cm~1 (OH
behaviour would require that a speciÐc conÐguration of Cl O
2
2
2
molecules photoreacts to produce “perpendicularÏ OClO. A
Table 3 Vibrational frequencies of Cl O
2
wavenumber/cm~1
normal mode
adsorbed Cl Oa
solid phase at 77 Kb
gas phaseb
argon matrixc
2
l
s
629
È
671
935
970
1260
1298
1331
1755
1819
1917
1979
630.7
296.4
670.8
È
971.4
È
1299.1
1331.6
È
640
300
686
È
972
È
1320
1360
È
639
288
677.1
È
È
È
È
È
È
È
d
l
as
l ] d
s
l
] d
as
2
l
s
l ] l
s
as
2
3
l
as
l
s
2
l ] l
È
È
1979.4
È
È
1945
s
as
as
l ] 2l
È
È
s
3
l
as
a This study. b Ref. 48. c Ref. 32.
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
2751

View Article Online
possible set of structures to clarify such a process is shown in
Scheme 2 below.
In an attempt to isolate systematically the spectral features
Fig. 7 shows a TPD proÐle of a Cl O Ðlm that has been
2
irradiated at j [ 300 nm for 5 min (cf. Fig. 5) and annealed at
140 K for a further 15 min to remove the majority of the Cl O
2
of the photolysis products of Cl O, low-pass cut-o† Ðlters
starting material. The weak OClO signal (m/z \ 67) suggests
2
(
300 \ j \ 515 nm) were employed. The photolysis of Cl O to
that only a small amount of OClO is produced by the pho-
2
produce ClClO proceeded throughout this range, though the
rate of the reaction slowed as higher wavelength cut-o† Ðlters
tolysis of adsorbed Cl O, notwithstanding the strong RAIR
2
2
absorption at 906 cm~1. Peaks corresponding to the Cl O
2
were used. The decrease in reaction rate is predictable and is
due both to a reduction in light intensity and the lower
(m/z \ 86) starting material and Cl (m/z \ 70) impurities are
2
also observed.
absorption cross-sections of Cl O and OClO at longer wave-
lengths.
Further evidence for the proposed mechanism of the pho-
The production of OClO from the photolysis of Cl O may
occur via a clustering or non-clustering mechanism. The non-
clustering mechanism can be represented as follows:
2
2
tolysis of Cl O can be found by studying the concentration
proÐles of species against time. Fig. 6 shows a timeÈ
absorbance plot of the main absorbing species present during
the photolysis of adsorbed Cl O . The plot illustrates the rela-
2
ClOCl ] hl ] O ] Cl
(6b)
(12)
2
O ] ClOCl ] OClO ] Cl
2
Initial photodissociation of Cl O produces an oxygen atom
tive rates of the loss of Cl O and the formation of ClClO .
2
2
2
which is able to migrate and react with another Cl O mol-
Two intermediate peaks, the enhanced OClO stretch at 906
cm~1 and the enhanced overtone of the OClO deformation
mode at 940 cm~1, are shown to form quickly and then decay
during the photolysis. The absorption proÐle for the OClO
deformation mode at 940 cm~1 is o†set from the 906 cm~1
proÐle due to interference from an underlying peak at 935
cm~1, the l ] d mode of Cl O. However, of most interest, is
2
ecule. The production of atomic oxygen from the photolysis of
Cl O is unusual in the gas phase; ClO and Cl being prefer-
2
entially produced [reaction (6a)].29 No sign of the ClO
radical, or the expected product dimers ClOClO and ClOOCl,
are observed in our study, which suggests that reaction (6a) is
not dominant over reaction (6b). Reaction (6b) may occur for
adsorbed Cl O after the isomerisation of Cl O to ClClO; such
s
2
the behaviour of the OClO asymmetric stretch absorption at
2
2
an isomerisation has been readily observed in the photolysis
1
086 cm~1 which is seen to decrease in intensity initially
of matrix-isolated Cl O.31h33
before rising to its maximum. This observation can be attrib-
uted to the fact that an impurity of unaligned OClO, which
2
ClOCl ] hl ] [ClClO] ] O ] Cl
(13)
absorbs principally at 1086 cm~1, is deposited with the Cl O
2
2
Ðlms. Such OClO photolyses rapidly, forming ClClO . As
Reaction (12) results in the production of atomic chlorine
which, if it does not produce molecular chlorine, is free to
migrate and react with a lone OClO molecule to produce the
2
observed in Fig. 1(c) the 2d and l modes for dilute OClO are
s
relatively weak and, because of the rapid formation of aligned
OClO, no initial loss is apparent. An increase in intensity at
Ðnal product, ClClO via reaction (10). In this case, stopping
photolysis after 15 min, when the OClO concentration is at a
maximum, should result in continued loss of OClO and pro-
2
1086 cm~1 is observed eventually, as this mode is not com-
pletely forbidden; presumably due to a lack of perfect align-
ment of the photochemically produced OClO molecules.
duction of ClClO . However, this behaviour is not in accord
2
Scheme 2 Illustration of photolysis of Cl O with an example orientation which would produce aligned OClO
2
2752
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

View Article Online
migrating O atom would be able to react with an isolated
Cl O molecule [reaction (12)] from any direction. Therefore,
2
possible clustering mechanisms have been considered to
account for the reaction. A reaction, analogous to Scheme 1,
by which two adjacent Cl O molecules produce OClO would
2
also result in Cl atom production. It may be possible for
several Cl O molecules to align in a suitably speciÐc manner
2
to allow for the production of aligned OClO, without produc-
ing a Cl atom. As mentioned above, Scheme 2 illustrates a
system that would allow the production of aligned OClO,
which is able to photoreact further to produce ClClO .
2
Atmospheric implications
The heterogeneous photochemical reactions performed in this
study demonstrate that both Cl O and OClO can be con-
2
verted to chloryl chloride on water-ice by photolysis with
both near-UV or visible light. Chloryl chloride released into
the gas phase readily decomposes to OClO and a Cl atom
which contributes to ozone destruction.9 Our postulated
mechanism requires clusters of Cl O and OClO, or at least
2
neighbouring molecules, for conversion to ClClO . Therefore,
2
the relevance of this route to stratospheric or tropospheric
ozone depletion depends on whether the concentrations of
Cl O, OClO and HOCl are high enough to allow this to
2
occur. A recent study has shown that the surface coverage of
OClO on polar stratospheric clouds (PSCs) at stratospheric
conditions is too low for signiÐcant clustering to occur.50
HOCl, however, readily adsorbs to laboratory PSC mimics at
stratospheric temperatures and pressures51 and its surface
coverage is likely to be high.52 If we consider the chlorine
reactivating process:
Fig. 5 RAIR di†erence spectra showing the photolysis (j [ 300 nm,
T \ 105 K) of Cl O on a layer of water-ice. Spectra are o†set for
2
clarity.
with the results, because on terminating irradiation no further
reaction is observed to take place. Therefore slow migration of
atomic chlorine is not signiÐcant in our experiments.
The non-clustering mechanism also does not support the
postulate that OClO is produced in an aligned form; a free,
ClONO2(ads)
] H O
2 (ads)
] HOCl ] HNO
(5)
3(ads)
and that HOCl is in equilibrium with Cl O then it is possible
2
that surface concentrations of Cl O may be sufficiently high
for stratospheric OClO and ClClO production to occur.
Should the heterogeneous photolysis of Cl O be signiÐcant
then it also provides an alternative mechanism for the pro-
2
2
2
duction of stratospheric OClO. Observed OClO concentra-
tions are currently used to estimate BrO concentrations
following the argument that the BrO ] ClO coupling reaction
is the sole source of stratospheric OClO.53,54 If signiÐcant
OClO concentrations are also formed by the photolysis of
adsorbed Cl O then current estimates of stratospheric BrO
2
concentrations may be too high.
This work has been carried out with support from the NERC
LSAC Programme and the CEC-Environment Programme.
We would also like to thank Professor Norman Sheppard
FRS, Dr. Thomas Koch and Nick Holmes for several useful
discussions about this work.
Fig. 6 TimeÈabsorbance plot for peak heights observed during the
photolysis of a Ðlm of Cl O. All absorbances are shown as a fraction
2
of their maximum value.
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J. Chem. Soc., Faraday T rans., 1997, V ol. 93