Spin-orbit transitions (2P1/2←2P3/2) of iodine and bromine atoms in solid rare-gases

Article abstract of DOI:10.1016/S0009-2614(97)01305-5

Infrared absorptions of iodine and bromine atoms in solid rare-gases are presented. The isolated atoms are produced by UV-photolysis of HI or HBr in solid Ar, Kr and Xe. The absorptions are characterised by sharp zero-phonon lines and broad structured phonon side bands. Some of the zero-phonon lines are resolved and split into two components, separated by a few wavenumbers. The production of the atoms follow second-order kinetics indicating that the primary hydrogen atom undergoes a secondary reaction with the hydrogen halide producing a halogen atom and a hydrogen molecule. This is supported by the observation of the infrared absorptions of hydrogen molecules in concentrated matrices after photolysis.

Full text of DOI:10.1016/S0009-2614(97)01305-5

30 January 1998
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Chemical Physics Letters 283 1998 1–6
2
2
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Spin–orbit transitions P1r2 § P3r2 of iodine and bromine atoms
in solid rare-gases
)
1
Mika Pettersson , Janne Nieminen
Laboratory of Physical Chemistry, UniÕersity of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland
Received 1 September 1997
Abstract
Infrared absorptions of iodine and bromine atoms in solid rare-gases are presented. The isolated atoms are produced by
UV-photolysis of HI or HBr in solid Ar, Kr and Xe. The absorptions are characterised by sharp zero-phonon lines and broad
structured phonon side bands. Some of the zero-phonon lines are resolved and split into two components, separated by a few
wavenumbers. The production of the atoms follow second-order kinetics indicating that the primary hydrogen atom
undergoes a secondary reaction with the hydrogen halide producing a halogen atom and a hydrogen molecule. This is
supported by the observation of the infrared absorptions of hydrogen molecules in concentrated matrices after photolysis.
q 1998 Elsevier Science B.V.
1. Introduction
The spin–orbit transition of the iodine atom has
been observed in emission in rare-gas matrices by
w
x
Halogen atoms have been extensively studied in
rare-gas matrices. Production of the halogen atoms
can be achieved by photolysing suitable precursor
molecules in the matrix or by depositing a gas
mixture through a discharge or a high temperature
many authors 9–12 . Despite the fact that matrix
isolation combined with IR-absorption measurements
is a common method we are not aware of infrared
absorption detection of halogen atoms in rare-gas
matrices. In the gas phase these transitions, which
are strictly forbidden within the electric dipole selec-
tion rules, have been extensively studied and they
have been found to be pure magnetic transitions
w
x
oven 1–3 . Halogen atoms have been directly ob-
served by using laser induced fluorescence, ESR, or
w
x
Mossbauer techniques. 1,4,5 . Engdahl and Nelander
¨
w
x
indirectly observed iodine and bromine atoms com-
plexed with different molecules, by recording the
infrared absorptions of the complexed molecules
13,14 . For iodine atoms, the integrated absorption
y23
Ž
.
cross section was found to be 1.05 "0.25 =10
cm2 13 .
During resent years, we have conducted a series
of experiments with rare-gas matrices doped with
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x
w
x
3,6–8 .
w
x
hydrogen halides 15–20 . UV-photolysis of such
matrices leads to the dissociation of the precursor
and subsequent trapping of the neutral atoms. More-
over, several different molecular ions have been
)
Corresponding author. E-mail: mika.pettersson@csc.fi.
1 Present address: Customs Laboratory, Tekniikantie 13, P.O.
Box 53, 02150 Espoo, Finland.
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
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PII S0009-2614 97 01305-5

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M. Pettersson, J. NieminenrChemical Physics Letters 283 1998 1–6
observed as a consequence of the excitation of
charge-transfer and trapping of the positive and neg-
w
x
ative charges 21,22 . In these experiments our mainv
detection scheme has been FTIR-spectroscopy and in
the course of these experiments we have frequently
found absorptions, which can be directly assigned to
halogen atoms.
In this Letter, the infrared absorptions of iodine
and bromine atoms in Ar, Kr, and Xe matrices are
presented. The bromine absorptions and the absorp-
tions of iodine atom in Ar and Kr are reported.
Iodine atom absorption in Xe was reported earlier in
connection with our studies of the mechanism of
w
x
formation of HXel 20 .
2. Experimental
Hydrogen iodide was synthesized from iodine, red
w
x
phosporous and water 23 and purified by low tem-
Ž
.
perature distillation. HBr of 99% purity Matheson
was used without further purification. The purities of
Ž
.
Ž
the rare-gases were Ar 99.999% , Kr 99.997%, CF4
.
Ž
.
free and Xe 99.997%, CF4 free and they were not
further purified. The gas mixtures were deposited
onto a CsI window through a 1r16 in stainless steel
Fig. 1. Iodine atom absorptions appearing after UV-photolysis of
HI doped rare-gas matrices.
Ž
capillar. Closed cycle helium cryostats AirProducts,
.
DE-202, HS-4 were used in the experiments.
The FTIR spectra were recorded with a Nicolet 60
SX FTIR spectrometer with a resolution of 1.0 cmy1
.
going from Xe to Ar. Also, the width of the side
band increases from Xe to Ar. The corresponding
absorptions of the bromine atom recorded at 15 K
are presented in Fig. 2 and they resemble the iodine
absorptions. Especially, comparison of the spectra of
different halogen atoms in the same rare-gas envi-
ronment recorded at the same temperature look strik-
ingly similar. The positions of the absorptions are
collected in Table 1.
In the mid infrared region a KBr beamsplitter and
MCT detector were used and in the near-infrared
region a Quartz beamsplitter and InSb detector were
used. The samples were photolysed with an excimer
Ž
.
laser ELI-76, Estonian Academy of Sciences , oper-
ating at 193 or at 248 nm, pulse energy varying
between 1–50 mJ.
The observed absorptions can be assigned to iso-
lated atoms by following arguments. The absorptions
are produced in matrices with a wide range of dop-
3. Results and discussion
Ž
.
The infrared absorptions resulting from the pho-
tolysis of HI-doped Ar, Kr and Xe matrices at 7 K
are presented in Fig. 1. The main features consist of
the sharp peak and the broad structured side band on
the blue side of the sharp absorption. The absorp-
tions shift to higher energy and the difference be-
tween the sharp peak and the side band increases on
ing 1:150–1:3000 were studied . They are only
halogen dependent, since deuterated hydrogen halides
produce the same absorption as hydrogen forms.
There are no hydrogen halide absorptions, which
would correspond to the atomic absorptions, which
excludes the possibility of absorptions being due to a
hydrogen halide–halogen atom-complex. One more

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M. Pettersson, J. NieminenrChemical Physics Letters 283 1998 1–6
3
where mostly monomers and some dimers of the
hydrogen halides are visible. The heavy halogen
atoms do not diffuse at the temperatures used, which
means that the formation of such a complex would
require total photolysis of a trimer.
The thermal behaviour of the iodine atom absorp-
tion in xenon is presented in Fig. 3. The main effect
is the broadening and a blue shift on going to higher
temperatures, the effect being reversible. Also, at
higher temperatures, there is intensity growth on the
red side of the sharp peak. This thermal behaviour
applies to both halogen atoms in different matrices.
All the spectral features can be explained if the sharp
peak is assigned to an inhomogeniously broadened
Ž
.
zero-phonon line ZPL and the broad features to
Ž
.
phonon side bands PSB . The positions of the ZPLs
are red-shifted from the gas phase positions, except
in argon where the ZPL exceeds the gas-phase value
by a few wavenumbers. The relatively large intensity
in the PSB indicates substantial electron–phonon
coupling.
Fig. 2. Bromine atom absorptions appearing after UV-photolysis
of HBr doped rare-gas matrices. An absorption belonging to a
water molecule is marked with an asterix.
possibility is a complex between a molecular halo-
gen and a halogen atom but this can be excluded,
since the absorptions appear in diluted matrices,
Table 1
Observed wavenumbers of iodine and bromine atoms in Ar, Kr
and Xe. ZPLszero-phonon line, PSBsphonon side band. The
wavenumbers of the phonon side bands are the resolved maxima
of the broad bands. Iodine atom absorptions are recorded at 7 K
and bromine atom absorptions at 15 K
y1
y1
y1
y1
Ž
.
Ž
.
Ž
.
Ž
.
Ar cm
Kr cm
Xe cm
Gas phase cm
7603a
Ž
.
I ZPL 7620.1
7551.5
7548
7456.6
7615
Ž
.
I PSB 7670
7585
7483
7598
3662.7
7497
3623.1
Ž
.
Br ZPL 3699.3
3685b
3694.5
Ž
.
Br PSB 3743
3696
3648
Fig. 3. Temperature dependence of the iodine atom absorptions in
Xe. The changes are reversible.
a
b
w
x
w x
Ref. 13 . Ref. 14 .

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M. Pettersson, J. NieminenrChemical Physics Letters 283 1998 1–6
Table 2
The growth curves of the iodine and bromine
UV-photolysis-induced absorptions of hydrogen molecule in con-
centrated hydrogen halide doped rare-gas matrices
absorptions as a function of the number of photolysis
pulses are presented in Fig. 4. It is to be noted that
the curves cannot be fitted by first-order expression
but rather by a second-order expression. The same
observation was made by LaBrake et al. when they
studied the photogeneration of bromine atoms in
solid xenon by observing the fluorescence from the
y1
y1
Ž
.
Ž
.
Matrix
Ar
This work cm
Previous work cm
4124.4
4131.6
4135.8
4113.0
4119.4
4096.0
4098.2
4102.6
4104.9
4130a
4137a
4138b
4130b
Kr
4121b
w
x
Xe-Br charge–transfer states 24 . They suggested
that the primary hydrogen atoms formed in the pho-
tolysis migrate extensive distances in the lattice due
to their excess kinetic energy and react subsequently
with HBr molecules producing H2 and Br atoms.
More evidence from this reaction is obtained from
this study, because in some concentrated matrices
Xe
a
b
w
x
w x
Ref. 25 . Ref. 26 .
Ž
.
1r300–1r500 the formation of hydrogen
molecules was observed during photolysis.
photolysis of HBrrAr three bands at 4315.8, 4131.6
and 4124.4 cmy1. The first two correspond with the
4137 and 4130 cmy1 bands observed by Smith et al.
The IR and Raman spectra of hydrogen molecules
w
x
in rare-gases are known 25–27 . We observed after
w
x
25 . They assigned these bands to the Q-branches of
para and ortho hydrogen, respectively. However, we
would expect to observe mainly the para-form be-
cause of the presence of a paramagnetic halogen
atom, which should enhance the ortho-para conver-
sion as was observed in the presence of oxygen
w
x
26,27 . The third band was not seen by Smith et al.
w
x
25 and it possibly belongs to complexed hydrogen
molecules. In heavier rare-gases our results differ
somewhat from the previously reported Raman val-
ues as can be seen from Table 2. If the Kr and Xe
w
x
bands reported by Prochaska and Andrews 26 be-
long to isolated H2 , then our hydrogen molecules
must be perturbed by complexation. Careful compar-
ison of a large numbers of spectra have shown that
H2-absorpotions a re not correlated with the halogen
atom absorptions. Thus, it seems that molecular
halogens are the best candidates for the complexing
partners, because they should be formed in concen-
trated matrices from multimers. It is not possible to
say, whether the production of hydrogen molecules
is due to the reaction between two hydrogen atoms,
or reaction between a hydrogen atom and hydrogen
halide. Most probably, both contribute, but further
experiments are needed in order to solve this prob-
lem with confidence. An interesting observation con-
cerning the hydrogen molecules was made. In an-
nealing experiments, they were stable up to 40 K in
argon, which is a surprisingly high temperature com-
Fig. 4. Kinetics of the photoproduction of bromine and iodine
atoms in Xe. Bromine data is from a HBrrXes1r150 matrix
photolysed by 248 nm irradiation. Iodine data is from a HIrXEs
1r500 matrix photolysed by 193 nm irradiation.

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M. Pettersson, J. NieminenrChemical Physics Letters 283 1998 1–6
5
pared to the boiling point of hydrogen. Clearly, the
4. Conclusions
interactions between the argon lattice and hydrogen
molecules are significant.
The spin–orbit transitions of iodine and bromine
atoms in Ar, Kr and Xe matrices have been observed
in absorption. These absorptions, which lie in the
near infrared and mid-infrared regions consist of
inhomogeniously broadened zero-phonon lines and
of phonon side-bands. Some of the zero-phonon lines
are split and the phonon side bands show structures
Splitting of the zero-phonon lines in IrKr, IrAr
and BrrAr matrices was observed. It is possible that
the other zero-phonon lines are also split but they are
not resolved. The observed splittings were only visi-
ble at the lowest recorded temperatures and at higher
temperatures the lines broadened and coalesced into
one band. There are three possible explanations for
the splitting. One component could be due to com-
plexation of the halogen atom with, for example, a
hydrogen molecule, but this alternative was rejected
above. Another possiblity is the splitting of the
fourfold degenerate ground-state due to lattice vibra-
tions. In the perfect octahedral substitutional site the
ground state belongs to the fourfold degenerate G
species and the excited state to the doubly degener-
Ž
.
at the lowest temperatures used 7 K . The splitting
is attributed to the atoms being in slightly different
environments. The atoms are generated by UV-
irradiation of hydrogen halide doped rare-gas matri-
ces. In concentrated matrices hydrogen also molecule
absorptions were observed.
Acknowledgements
w
x
ate E species 28 . Asymmetric motion breaks the
w
x
degeneracy of the ground state into two states 12 .
Quite recently, Krylov et al. investigated the nonadi-
abatic dynamics of chlorine atom in solid Ar by
M. Rasanen is thanked for the careful reading of
¨ ¨
the manuscript and for valuable comments. R. Timo-
nen is thanked for the lone of the excimer laser.
w
x
computer simulations 29 . They found that the IR-
absorption spectrum of the chlorine atom consists of
a doublet because of the splitting of the ground state.
However, our splittings are only a couple of
wavenumbers, while the simulations of Krylov et al.
on chlorine give 32 cmy1 for the splitting. Even
more importantly, the intensity ratios of the two
components did not obey the Boltzman populations
of two levels separated by an energy difference
between the two components. Unless the two levels
have different absorption coefficients this possibility
has to be rejected. The third possibility is the trap-
ping of the atoms in slightly different, but well
defined environments. This is supported by the ob-
servation that especially the higher energy compo-
nent became substantially narrower after an anneal-
ing cycle, indicating strong thermal relaxation of the
lattice around the halogen atom in this site. The
changes, however were not so large indicating a
more relaxed site even before annealing.
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