Electronic spectrum of atomic sulfur in argon matrices in the vacuum ultraviolet region

Article abstract of DOI:10.1016/S0009-2614(01)00803-X

S atoms have been generated through vacuum ultraviolet photolysis of carbonyl sulfide (OCS) in Ar matrices. Excitation spectra measured by monitoring the ¹S→¹D emission of S at 784 nm are assigned to Ar⁺S^-←ArS c

Full text of DOI:10.1016/S0009-2614(01)00803-X

3
1 August 2001
Chemical Physics Letters 344 &2001) 479±487
www.elsevier.com/locate/cplett
Electronic spectrum of atomic sulfur in argon matrices in the
vacuum ultraviolet region
*
Murthy S. Gudipati , Andreas Klein
Institut f u r Physikalische Chemie, Universit a t zu K o ln, Luxemburger Strasse 116, D-50939 K o ln, Germany
Received 8 May 2001; in ®nal form 28 June 2001
Abstract
S atoms have been generated through vacuum ultraviolet photolysis of carbonyl sul®de &OCS) in Ar matrices.
‡
À
1
1
Excitation spectra measured by monitoring the S ! D emission of S at 784 nm are assigned to Ar S
ArS charge-
transfer &CT) &135 nm, substitutional site &SS) and 159 nm, interstitial site &IS)) and S P Rydberg &160.8 nm)
3
3
‡
À
‡
À
transitions. Empirically derived potential curves of Rg S and Rg O species predict the transition energies with
reasonable accuracy. S atoms are found to be more photomobile than O atoms in Ar matrices. Ó 2001 Elsevier Science
B.V. All rights reserved.
1
. Introduction
problem to describe the nature and composition
of the excited species, especially of charge-
transfer &CT) nature.
The recently emerging emphasis of matrix
isolation as a means to study and understand
many body interactions in condensed media [1]
has furthered the conventional matrix isolation, a
means to stabilize otherwise unstable species in
low-temperature inert media, to utilize rare gases
themselves as reactive media. Thus inert gas as
synonym to rare gas has been shown to be invalid
at least in the case of Ar, Kr and Xe. With do-
pant atoms like hydrogen, halogen, oxygen, sul-
fur etc., rare gases form locally generated stable
or metastable ground-state molecules [2,3] or ex-
cited species [1,4]. Due to the presence of several
equivalent rare gas atoms surrounding the dopant
atoms, it still remains a challenging theoretical
In our earlier Letter on O doped Ar and Kr
matrices [4], we could understand to a large extent
the CT and Rydberg transitions of O in rare gas
matrices. We have shown that the empirical po-
tential energy curves of the CT states, after taking
into account the stabilization through host matrix
polarizability, could be used to predict the excita-
tion energies with a reasonable accuracy. To the
best of our knowledge, so far there has been no
literature data on the absorption or emission
spectra involving ArS or KrS CT states either in
the gas-phase or in rare gas matrices. On the other
hand, emission spectra of XeS involving the CT
states have been measured both in the gas-phase
[
5] and in Xe matrices [6]. Ab initio potential en-
ergy curves of KrS [7] and XeS [8] CT states have
been reported, but not of ArS. In the present
communication we report the ®rst observation of
*
Corresponding author. Fax: +49-221-470-5144.
E-mail address: murthy.gudipati@uni-koeln.de &M.S. Gudi-
3
3
pati).
ArS CT transitions and the SꢀS
P†atomic
0
009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0009 - 2 6 1 4 & 01 ) 008 03 - X

480
M.S. Gudipati, A. Klein / Chemical Physics Letters 344 ꢀ2001) 479±487
transition in a rare-gas matrix. The experimental
results are compared with empirical predictions.
The present work may be a useful reference point
to both theoretical and experimental investigations
on RgS in the future.
3. Results and discussion
3.1. Spectral analysis
Excitation spectra measured while monitoring
1
1
the S ! D emission of S at 784 nm from a
freshly prepared and during the photolysis of 0.1%
OCS in an Ar matrix are shown in Fig. 1. The
2
. Experimental
1
dissociative excitation of OCS to produce S& S)
1
‡
The experiments have been carried out at the
and COꢀR †is well studied in the gas-phase [10].
In Ar matrices Taylor et al. [11] have reported a
similar spectrum with an irregularly spaced vib-
rational progression. The longer wavelength part
between 145 and 175 nm consists of transitions
into two upper valence states as shown in Fig. 1.
synchrotron radiation facility BESSY-I in Berlin
using a 3 m normal incidence &3m-NIM-1)
monochromator. Emission spectra were measured
using a 0.5 m monochromator &Acton Research,
SpectraPro-500). Transmission spectra were si-
multaneously measured with an InGaAs photodi-
ode &Hamamatsu) when recording the excitation
spectra. Due to the fact that the horizontal and
vertical foci of the 3m-NIM-1 monochromator do
not spatially coincide with each other, the irradi-
ated spot on the matrix window &FWHM) was
1
1
‡
The unresolved shoulder due to the P
X R
transition at 165 nm is a clearly resolved broad
band in the gas-phase absorption spectrum with
the maximum at 167 nm [12]. The absorption
1
‡
1
‡
maximum of the R
X R transition at 152 nm
is in good agreement with the well-resolved regular
2
approximately 4ꢀH†Â2ꢀV†mm , consisting of a
vibronic structure with a vibrational spacing of
À1
central high-¯ux region and a peripheral low-¯ux
region. While measuring the transmission spectra,
all the transmitted light has been detected on the
ꢁ800 cm centered at 152 nm [12]. The sharp
band at 145 nm is characteristic of transition into
the upper state of Rydberg character. Keeping in
mind the general tendency of Rydberg states being
shifted to higher energies in rare-gas matrices [13],
we assign this band at 145 nm to the
2
5
Â5 mm photodiode. In order to measure
the emission and excitation spectra, the spot on the
matrix window was imaged on to the entrance slits
1
1
‡
&
opened to 1 mm) of the emission monochroma-
PꢀR†
X R
&3p !nsr series, 3p !11r)
tor. Hence, only the central 1 mm &H) stripe of the
irradiated matrix receiving the maximum amount
of photon ¯ux was interrogated through the
emission and excitation spectra, whereas the
transmission measurements covered a wider area
on the matrix. Thus, subtle changes in the excita-
tion spectra may not be detected in the transmis-
sion spectra and a quantitative comparison
between the excitation and transmission spectra
cannot be carried out. All the experimental details
are given in earlier publications [4,9]. Sulfur atoms
were produced through photolysis of 0.1% OCS in
Ar matrices while measuring excitation and emis-
sion spectra at excitation wavelengths between 200
and 100 nm. A premixed 5.76% OCS in Ar was
further diluted with Ar &both from Linde) to
achieve the desired concentration. The spectra
were recorded at the lowest temperature &21 K)
reachable with our cryostat.
Fig. 1. Excitation spectra of OCS in Ar matrices with in-
1
1
creasing photolysis &monitoring the S ! D emission of S at
84 nm). Dispersed S ! D emission of S during the photol-
ysis is shown in the insert &excitation at 149 nm).
1
1
7

M.S. Gudipati, A. Klein / Chemical Physics Letters 344 ꢀ2001) 479±487
481
transition observed at 158 nm in the gas-phase
12]. All the other Rydberg transitions occur at
[
shorter wavelengths than 145 nm in the gas-phase.
As can be seen in Fig. 1, OCS is eciently
photolyzed up to >90% during the measurements
with monochromatic synchrotron light in the re-
gion between 200 and 100 nm. The spectral in-
tensity decreases more or less uniformly upon
photolysis throughout, except for the region be-
tween 130and 140nm, enlarged and marked with
an arrow in Fig. 1. In this region where the ab-
sorption of OCS is almost negligible, instead of
decreasing, the spectral intensity increases slightly
upon photolysis, which can be attributed to the S
atoms that are formed during the photolysis.
We have also followed the generation of the
photoproducts in the transmission spectra. These
spectra, displaced vertically to gain clarity, are
summarized in Fig. 2. It should be noted that light
scattering is even more a serious problem in the
vaccum ultraviolet &VUV) region compared to the
UV and visible region. Thus, it is very dicult to
di€erentiate between scattering and absorption
when the transitions involve broad and weak ab-
sorption. Further, the optical quality of the ma-
trices with respect to the VUV light change
signi®cantly upon photolysis. Keeping these as-
pects in mind, we did not over interpret the broad
continua. Excitation spectroscopy is much more
sensitive than the absorption spectroscopy to
measure weak and broad bands in the VUV region
Fig. 2. &a) Transmission spectra measured during the photol-
ysis of OCS, displaced vertically to enhance the clarity. The Ar
matrices with 0.1% OCS are virtually non-transparent below
135 nm. &b) Spectra from part a converted to absorbance units,
by taking the ®rst measured transmission spectrum as the ref-
erence spectrum. The spectra were corrected to the background
continuum. Second spectrum from the bottom has been shown
again under the top spectrum as dashed curve to compare the
relative intensities of the bands of CO and S that are produced
with increasing photolysis of OCS. &c) Absorption spectra
measured after 90% photolysis of OCS. Before &solid line) and
after &dashed line) irradiation with undispersed synchrotron
light. The di€erence spectrum &after ) before) enlarged ®ve
times is also shown &dotted line).
&
vide infra). In the transmission spectra shown in
Fig. 2a, one can clearly notice an increase in the
several absorption bands below 157 nm due to CO
and the band at 160.8 nm due to the
3
2
3
3
2
4
Sꢀ3s 3p 4s†
Pꢀ3s 3p †atomic transition of S
with increasing photolysis, indicating the genera-
tion of CO and S photoproducts upon photodis-
sociation of OCS.
These transmission spectra were converted to
absorption spectra by taking the ®rst transmission
spectrum during the photolysis as the reference
spectrum. These spectra after removing the con-
tinua are shown in Fig. 2b. If we assume that the
oscillator strengths measured in the gas-phase for
the corresponding transitions to hold good also in
the matrix environment, then the integrated in-
[14]) should be 5.74 times stronger than the
A±X&0-0) band of CO &f ˆ0:0162 [15]) at 156.3
nm and 2.65 times stronger than the A±X&1-0)
band of CO&f ˆ0:0351 [15]) at 152.8 nm. The
respective integrated intensity ratios, being 4.3 and
2.2 at the initial stages of the photolysis, decrease
with increasing photolysis to ®nal values of 1.8
and 1.1, respectively. When the matrices were
3
3
tensity of the S
P transition of S &f ˆ0:093

482
M.S. Gudipati, A. Klein / Chemical Physics Letters 344 ꢀ2001) 479±487
irradiated with undispersed synchrotron light after
about 90% photolysis of OCS, the absorption
bands of CO increase whereas the absorption band
due to S decreases, as shown in Fig. 2c. Hence, we
conclude that VUV photolysis of OCS renders CO
and S species in Ar matrices and S atoms are si-
multaneously photomobilized to undergo recom-
2
bination to form S or react with O atoms to form
SO under the present experimental conditions.
Prolonged photolysis thus depletes almost 50% of
S atoms generated through photodissociation of
OCS. Further evidence for the photoinduced mo-
bility of the S atoms will be presented below.
In Fig. 3a excitation spectra during the pho-
tolysis and at the end of the photolysis and a dif-
ference spectrum after rescaling the 145 nm bands
in both the spectra are shown. The di€erence
spectrum, enlarged and ®tted with two Gaussian
curves is shown in Fig. 3b. These spectra are
reminiscent of the spectra of O atoms in Ar ma-
trices obtained through photolysis of N O pre-
2
cursor [16], where the precursor spectrum strongly
overlapping with the product spectrum. We may
note that these excitation spectra were measured
1
1
by monitoring the S± D-emission of S. Popula-
1
tion of S& S) can only be achieved either through
photodissociation of the OCS precursor or exci-
tation of S in Ar lattice. The other minor photo-
products CS and SO cannot be dissociated in this
Fig. 3. &a) Excitation spectra during the photolysis, marked
with &1); at the end of photolysis, marked with &2); and sub-
tracted spectrum after scaling the intensity at 145 nm, marked
with &3). Emission of S was monitored at 784 nm. &b) Sub-
tracted spectrum from &a) marked with &3) ®tted with two
Gaussian curves &dotted lines). Absorption bands of ArS CT
excitations in SS and IS as well as the S Rydberg transition are
marked accordingly. Several bands between 138 and 158 nm are
1
spectral region to generate S& S). Hence, the dif-
ference spectrum must belong to S in Ar matrix.
Further, the excitation spectra of CS and SO di€er
signi®cantly from the spectrum shown in Fig. 3b
&
to be published elsewhere). Several interesting
1
1
‡
features can be detected in the subtracted spec-
trum. The sharp band at 160.8 nm is clearly the
dominant feature that is due to the
due to CO A P
X R vibronic transitions.
3
2
3
3
2
4
Sꢀ3s 3p 4s†
Pꢀ3s 3p †atomic transition in S.
of S the electron is promoted into the 4s orbital
2
3
As noted earlier, matrix destabilization of the
Rydberg states of small molecules in rare-gas
matrices is a well-known fact [13]. In the gas-phase
&con®guration: 3s 3p 4s), which is larger than the
3
3s orbital as in the case of O, that result in the S
state. Thus the electrostatic repulsion with the
matrix surrounding is larger for S than for O
Rydberg states. The present results are in full
agreement with the results of Chergui et al. on NO
in Ne matrices [13], where the matrix destabiliza-
tion of 4s Rydberg states is larger than that of 3s
Rydberg states compared to the corresponding
energies in the gas-phase. The next feature of the
3
3
this S
The blue-shift of around 20nm, corresponding to
.84 eV for S in Ar matrices is larger than the blue-
shift of 0.69 eV for the same transition in O in Ar
P transition occurs at 180.7 nm [17].
0
3
matrices [4]. Greater destabilization of the
S
Rydberg state of S compared to the same state of
O in Ar matrices is due to the fact that in the case

M.S. Gudipati, A. Klein / Chemical Physics Letters 344 ꢀ2001) 479±487
483
spectrum in Fig. 3b is the presence of two broad
absorption bands, of which one centered around
SS and IS, respectively. This assignment is further
supported by the empirical calculations &vide in-
1
1
1
35 nm and the other around 159 nm, the later
fra). S ! D emission of S in SS and IS of Ar and
Kr lattices has been well studied by Zoval and
Apkarian [18]. In the present study the detection of
the CT transitions of S in Ar lattice in both these
sites complements the studies of these authors. The
fact that OCS could not be completely photolyzed
with monochromatic synchrotron light and that
the absorption spectra of S and OCS strongly
overlap with each other, hindered us to undertake
further detailed study on the site-selective spec-
troscopy of S in Ar matrices. The last feature of
interest is the region between 140and 160nm,
consisting several vibronic bands. Though rela-
tively weak, these bands clearly correspond to the
being more intense and broader. These two bands
resemble the CT absorption bands of O in Ar
matrices centered around 132 and 145 nm &Fig.
‡
À
4
b), which we assigned to Ar O
ArO CT
transitions in the substitutional site &SS) and in-
terstitial site &IS), respectively [4]. Based on this
similarity we assign the 135 and 159 nm bands in
‡
À
Fig. 3b to the Ar S
ArS CT transitions in the
1
1
‡
fourth positive system of CO &A P
transition). It may be noted that these spectra were
X R
1
1
recorded by monitoring the S ! D emission of S
at 784 nm, which is shown in the insert of Fig. 1.
Presence of the CO vibronic bands in the excita-
tion spectrum shown in Fig. 3b may be due to
photochemically induced energy transfer in
SÁÁÁCO van der Waals complexes, similar to the
energy transfer in OÁÁÁCO van der Waals com-
plexes generated through VUV photolysis of CO
in Ar matrices [19].
2
If we compare the excitation spectra shown in
Fig. 3b with the absorption spectra in Fig. 2, it
becomes immediately evident that the strong
atomic absorption of S at 160.8 nm in the ab-
sorption spectra &Fig. 2b) appears as a relatively
weak feature in the excitation spectra &Fig. 3b). On
‡
À
the other hand, the Ar S CT bands, which may
be buried under the scattering continuum in the
absorption spectra &Fig. 2a), are clearly seen in
the excitation spectra &Fig. 3b). By noting that the
excitation spectra were measured by monitoring
1
1
the atomic transition S ! D in S, we conclude
that the molecular CT states undergo internal
conversion more eciently than the atomic Ryd-
Fig. 4. &a) Excitation spectra at the end of photolysis of OCS
with monochromatic synchrotron light, but before photolyzing
with undispersed light &solid line), after 10min photolysis with
undispersed, zero-order synchrotron light &dotted line) and
subtracted spectrum &before ) after) photolysis with undi-
spersed light. Emission was monitored at 784 nm. &b) Excitation
spectra of O in Ar matrices before &solid line) and after &dotted
line) photolysis with undispersed zero-order synchrotron light.
3
1
‡
berg state SꢀS†to populate the molecular 2 R
1
state corresponding to the atomic SꢀS†. The
Rydberg state may directly undergo radiative re-
3
laxation to the ground-state SꢀP†due to strongly
allowed nature of this transition, bypassing the
1
3
3
1
1
SꢀS† state. However, the & S ! P emission,
S ! D emission of O at 560nm was monitored. The assign-
ment of the bands is shown accordingly.
which may occur in the VUV region around

484
M.S. Gudipati, A. Klein / Chemical Physics Letters 344 ꢀ2001) 479±487
1
60±180 nm, cannot be reached with the present
2
through photolysis of 0.1% O in Ar) with poly-
detection system &detection limit >200 nm). We
have attempted to measure the CT emission in the
UV region between 200 and 400 nm, but without
any success. Most likely the CT states undergo
predominantly non-radiative relaxation to the
chromatic radiation rather increases the O atom
population &Fig. 4b). This is a clear demonstration
of more ecient photomobility of S atoms com-
pared to O atoms in Ar matrices. If the photo-
1
mobility of the S atoms should occur in the D
1
1
‡
2
R
S state of S.
covalent state of ArS, corresponding to the
state [1], then the lifetime of the SꢀD†must be
1
1
longer than that of OꢀD†in Ar matrices or that
Photoinduced mobility of S atoms in rare-gas
matrices has been well documented [1,20]. Due to
the attractive or nearly non-repulsive nature of the
the collisional momentum transfer from the heav-
ier S atoms to Ar should be more ecient than the
lighter O atoms to Ar resulting in easier cage-exit
of S atoms compared to O atoms. On the other
hand, the photomobility by irradiating with VUV
light, as in the present case, can also occur after the
initial expansion and subsequent rearrangement of
the ®rst shell around the Rydberg S atoms.
1
1
ground-state Rg ꢀS†and O [21] or S [22] in the D
state, mobility of O and S is proposed to occur in
1
the D state in Rg matrices [1]. However, it was a
surprising observation for us that after near to 90%
photolysis of the parent OCS molecules with
monochromatic synchrotron radiation between
2
00 and 100 nm, when the matrices were irradiated
3.2. Potential energy curves of the CT states
with polychromatic &undispersed, zero-order grat-
ing position) light, the spectral features corre-
sponding to S disappear to a great extent &Fig. 4a).
The spectrum obtained after subtracting the exci-
tation spectrum measured after photolysis with
undispersed synchrotron light from the excitation
spectrum measured at the end of photolysis with
monochromatic synchrotron light, marked as &be-
fore ) after) in Fig. 4a, is identical to the spectrum
Successful prediction of the CT excitation en-
ergies of O in Ar and Kr matrices using empirical
Rittner potentials, a combination of Coulomb at-
tractive and Born±Mayer repulsive terms, and the
stabilization due to the induced polarization of the
matrix material [4], has encouraged us to use these
empirical potential energy curves also to describe
‡
À
the Ar S CT states. In order to test the generality
of this empirical deduction we have computed the
&
3) shown in Fig. 3b. This clearly indicates
‡
À
‡
À
photoinduced mobility of S atoms in Ar matrices
caused by undispersed synchrotron radiation. The
characteristic re¯ected wavelengths of the grating
used at the 3m-NIM-1 monochromator spans the
range between ꢁ50and 450nm. On the other
hand, the absorption spectra measured simulta-
neously during the measurement of the excitation
spectra, shown in Fig. 2c, show only a small de-
crease in the 160.8 nm band of S. As we noted in
the experimental section, the excitation spectra
interrogate only the central part of the irradiated
matrix receiving maximum photon ¯ux, whereas
the absorption spectra cover the whole irradiated
region. Consequently, the S atoms must have
moved away from the central region of the matrix
and to a small extent they must have reacted with
potential energy curves for Rg O and Rg S
&Rg ˆAr, Kr and Xe) and then compared with
available ab initio curves and experimental data.
These results are discussed below:
ꢀ
ꢁ
2
e
R
CT
Rg
ꢀgas†ˆ E ÀE
X
EA
E
À
IP
R
Â
À
ÁÃ
‡ ARg
‡
À
exp Àb
‡ À
Rg X
R ;
ꢀ1†
X
ꢂ
ꢃ
2
e À1
e
R
CT
R
CT
E
ꢀmatrix†ˆE ꢀgas†À
:
ꢀ2†
2
e ‡1
R
‡
À
Potential energy curves of the CT states of Rg X
&X ˆO or S) in the gas-phase using the Rittner
potentials &Eq. &1)) are shown in Fig. 5a. Only the
lower j ˆ3=2 state that results from spin±orbit
‡
coupling in Rg is shown in each case. The upper
each other to form S , whose photolysis with the
2
j ˆ1=2 state runs parallel to the lower state with
undispersed synchrotron light does not generate
isolated S atoms back. Under similar conditions,
irradiation of Ar matrices containing O &generated
the corresponding di€erence of the spin±orbit
) states. Similarly,
‡
2
splitting energy of the Rg ꢀP
j
À
2
due to small spin±orbit splitting energy of X ꢀP
),
j

M.S. Gudipati, A. Klein / Chemical Physics Letters 344 ꢀ2001) 479±487
485
which six singlet and six triplet multiplicities with
R; P and symmetries result from
D
‡
2
À
2
Rg ꢀP†X ꢀP†con®guration that results from the
lowest ionized state of Rg. Thus, the empirical
potential energy curve represents an average of
these 12 states, which run close to each other at
larger interionic separation but may diverge at
shorter interionic separations due to correlation
e€ect as well as mixing of ionic states with covalent
states. For this reason and due to the fact that di-
atomic CT species may develop to polyatomic CT
species when the same Rg acts as both the counter
positive ion and the matrix material, we have
purposefully omitted the CT emission of RgX in
Rg matrices when comparing the empirically de-
rived values with the experimental data. The pa-
rameters used to derive the empirical potential
energy curves are collected in Table 1 and the
present results are compared with experimental as
well as ab initio results reported in the literature in
Table 2. In the case of CT emission, the ground-
3
state ꢀP†energies at the corresponding inter-
atomic distances, taken from ab initio data &ArO
and KrO [21]; XeO and XeS [8]; ArS [22]), were
subtracted to derive the vertical transition energies.
As can be seen in Table 2, the di€erence between
the experimental band maxima and the empirically
derived vertical energies of transitions involving
the CT states of several RgX species is within a
maximum of Æ0.4 eV. Similarly, the agreement
between the energy minima of the potential curves
derived from ab initio and empirical methods is
also very good, as shown in the last three rows of
Table 2. Keeping in mind the simplicity of the
empirical computations, the overall predictions of
the vertical transition energies is excellent. After
taking the stabilization due to induced polarization
of the matrix medium &Eq. &2)), the empirical po-
tential curves of the CT states of ArO and ArS are
Fig. 5. &a) Empirical gas-phase potential energy curves of
‡
À
Rg X &Rg ˆAr, Kr and Xe; X ˆO and S) derived using Eq.
‡
À
&1) and parameters given in Table 1. Solid line: Ar O and
Ar S , dashed line: Kr O and Kr S , dotted line: Xe O
‡
À
‡
À
‡
À
‡
À
‡
À
and Xe S . An interatomic distance at the energy minimum of
each curve is given in parentheses. &b) Empirical potential en-
‡
À
‡
À
ergy curves of Ar O and Ar S in Ar matrices derived using
Eqs. &1) and &2) and parameters given in Table 1. Solid line:
‡
À
‡
À
Ar S and dashed line: Ar O . Solid and dashed arrowhead
lines pointing to each other locate vertical excitation energies in
the SSs. Horizontal arrowhead lines point the regions of vertical
excitations in the ISs.
spin±orbit coupling is neglected for the negative
ions. It is important to note here that even after
neglecting the spin±orbit coupling, 12 CT states of
Table 1
Parameters used in Eqs. &1) and &2) to derive empirical potential energy curves of the CT states of RgX &X ˆO,S)
Parameter
Ar
Kr
Xe
O
S
Rg
X
EA
EIP =E &eV)
15.759
6411.8 &Cl)
3.63681 &Cl)
13.999
18297 &Br)
3.54319 &Br)
12.13
33501 &I)
3.51532
1.46
2619.4 &F)
3.78383 &F)
2.07
6411.8 &Cl)
3.63681 &Cl)
ARg
bRg
‡
=AXÀ &eV)
=bXÀ &A)
ꢀ
‡
&
I)
e &dielectric constant of the matrix)
1.66
1.86
2
The Born±Mayer interatomic potentials for the isoelectronic halogen atoms are taken from Abrahamson [26]. e =R ˆ14:41 eV.

486
M.S. Gudipati, A. Klein / Chemical Physics Letters 344 ꢀ2001) 479±487
Table 2
Empirically derived transition energies and minima of the potential energy curves of the CT states of RgX &X ˆO,S) compared with
the experimental and ab initio results. For the last three rows Eexp should be read as Eab inito
Species
Medium or
method
Transition or
location
E
exp
&eV)
E
calc
&eV)
DE
&exp ) calc)
Ref.
3
ArO
ArO
KrO
KrO
XeO
ArS
XeS
KrS
XeO
XeS
Gas-phase
Ar matrix &SS)
Gas-phase
Kr matrix &SS)
Gas-phase
Ar matrix &SS)
Gas-phase
Ab initio
CT ! P
8.27
9.4
8.54
9.5
)0.27
)0.1
)0.38
)0.04
)0.23
)0.14
0.33
[27]
[4]
3
CT
P
3
CT ! P
6.89
7.75
5.3
7.27
7.79
5.53
9.32
5.13
[27]
[4]
3
CT
P
3
CT ! P
[27]
Present work
3
CT
P
9.18
5.46
3
CT ! P
[5]
[7]
[8]
[8]
ꢀ
ꢀ
CT minimum
CT minimum
CT minimum
ꢁ7.5 &3.1 A)
7.25 &2.77 A)
0.25
0.3
ꢀ
ꢀ
ꢀ
Ab initio
Ab initio
ꢁ6.1 &2.8 A)
5.8 &2.67 A)
ꢀ
5.55 &2.89 A)
ꢁ5.9 &3.3 A)
0.35
shown in Fig. 5b. In the SS with a nearest neigh-
ꢀ
bour distance of 3.75 A in an unperturbed lattice,
ArO exists as a van der Waals molecule in the P
ꢀ
ground-state at an interatomic separation of 3.47 A
hole is localized on only one Ar atom [4]. Irre-
spective of the nature of the upper CT state, we
3
n
expect destabilization of the ground-state of Ar S
&n ˆ1±6) in the IS due to the repulsive nature of the
3
[
23], whereas due to the van der Waals minimum of
3
ArS P ground-state at shorter ArÁÁÁS interatomic
ꢀ
the P ground-state of ArS at 3.79 A [24], the S
atoms occupy perfectly the substitutional cavity.
distances [22], which should be taken into account
to compute the CT vertical excitation energies.
Considering all these factors and noting that the
ꢀ
The vertical transition energies of ArO &at 3.47 A)
ꢀ
and ArS &at 3.79 A) in the SS, as shown in Fig. 5,
di€er minimally by ca. 0.2 eV, which is in excellent
agreement with the experimental observation: band
maxima of the CT transitions in ArO &132 nm) and
ArS &135 nm) in Ar matrices. On the other hand, in
3
ꢀ
ground-state ꢀP†energy of ArS is 0.14 eV at 2.9 A
[
22], the predicted vertical CT transition wave-
length of 155 nm in the IS is in good agreement
with the experimental value of 159 nm.
ꢀ
the IS, with the cavity radius of 2.65 A in an un-
perturbed Ar lattice, both O as well as S cannot be
accommodated without distorting the surround-
ing. Thus, in the case of O in Ar, we have shown
4. Conclusions
CT transitions of ArS in the substitutional and
interstitial sites and Rydberg excitation of S have
been observed for the ®rst time from matrix iso-
lated S atoms that have been generated through
VUV photolysis of OCS in Ar matrices. Empiri-
cally predicted excitation energies of the transi-
tions from the repulsive ground-state to the CT
states are in good agreement with the experimental
data. S atoms are found to be more photomobile
than O atoms in Ar matrices.
ꢀ
that by taking the 0.38 A expansion of the axial Ar
atoms in the ®rst shell of the IS [25], the empirical
ꢀ
vertical excitation energy of ArO at 2.83 A repro-
duces the experimental data well. In the case of S in
IS of Ar stronger distortion of the cavity is to be
expected due to larger van der Waals radius of S.
Accordingly, the restructuring of the IS site should
occur more often with S than O, especially in defect
surroundings. Thus, we expect a wide distribution
of ArS distances in the IS, as marked in Fig. 5b,
which is also re¯ected in the much broader ab-
sorptions band at 159 nm compared to the one at
Acknowledgements
1
35 nm. Further, the CT excitation may involve
delocalization of the positive charge &hole) on all
the six surrounding Ar atoms, something like for-
mation of an excited supermolecule, or that the
A part of the present work is ®nanced by DFG
&Grant No. GU 413/3). Travel support by DESY
through BESSY is also acknowledged gratefully.

M.S. Gudipati, A. Klein / Chemical Physics Letters 344 ꢀ2001) 479±487
487
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