Infrared spectroscopy and 266 nm photolysis of H2S2 in solid Ar

Article abstract of DOI:10.1016/S0009-2614(99)00756-3

The infrared spectra of H₂S₂ and its 266 nm photolysis products are studied in an Ar matrix at 7.5 K. The antisymmetric and symmetric HS-stretching, and the antisymmetric bending of H₂S₂ are assigned at 2556.6, 2553.8 and 880.3 cm^-1, respectively. The absorptions appearing during the photolysis at 2463, 2460 and 903 cm^-1 are tentatively assigned to the HS₂ radical. The two main dissociation channels are: H₂S₂+hν → HS₂+H and H₂S₂+hν → S₂+H₂, with a branching ratio close to 1.

Full text of DOI:10.1016/S0009-2614(99)00756-3

1
7 September 1999
Chemical Physics Letters 311 Ž1999. 47–54
www.elsevier.nlrlocatercplett
Infrared spectroscopy and 266 nm photolysis of H S in solid Ar
2
2
)
Esa Isoniemi , Leonid Khriachtchev, Mika Pettersson, Markku R a¨ s a¨ nen
Laboratory of Physical Chemistry, UniÕersity of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland
Received 4 May 1999
Abstract
The infrared spectra of H S and its 266 nm photolysis products are studied in an Ar matrix at 7.5 K. The antisymmetric
2
2
y
1
and symmetric HS-stretching, and the antisymmetric bending of H2S2 are assigned at 2556.6, 2553.8 and 880.3 cm
respectively. The absorptions appearing during the photolysis at 2463, 2460 and 903 cm are tentatively assigned to the
,
y
1
HS2 radical. The two main dissociation channels are: H S qhn ™ HS qH and H S qhn ™ S qH , with a
2
2
2
2
2
2
2
branching ratio close to 1. q 1999 Elsevier Science B.V. All rights reserved.
1
. Introduction
were suggested to be the primary photolysis products
w2,3x. The UV Ž300–380 nm. absorption spectrum of
Hydrogen disulfide ŽH S . is the simplest
2
2
HS was first observed in a flash photolysis study of
2
molecule with a single S–S bond and has therefore
been the subject of extensive theoretical and experi-
mental studies w1–17x. In the atmosphere, H S can
be formed by a self-reaction of SH radicals, which
are produced via oxidation of hydrogen sulfide ŽH S.
w11x. The thermochemistry of sulfanes ŽH S . has
been studied by ab initio and experimental methods
w4,14,16,17x. The gas-phase rotational w10x and vibra-
tional–rotational spectra w8x of H S were reported,
and the ground-state vibration wavenumbers are
2
2
H S by Porter w18x, and it was later confirmed by
2
Strausz et al. w2x and Gosavi et al. w3x. The excited-
2
2
state SH-stretching Žn ., bending Žn . and SS-
1
2
stretching Žn . vibrations were determined to be at
3
2
y1
2
500, 900 and 600 cm , respectively w3x. For the
electronic ground state, Holstein et al. derived the
values of 904 cm Žn . and 595 cm Žn . from a
2
n
y1
y1
2
3
2
X
2
Y
chemiluminescence spectrum of the A A–X A tran-
sition w5x.
2
2
Barnes et al. w19x observed three IR absorption
555.8 Žn ., 883.0 Žn ., 515.9 Žn ., 417.5 Žn .,
1 2 3 4
y1
bands at 2513, 912 and 864 cm after annealing a
y1
558.6 Žn . and 880.0 cm
Žn .. The ultraviolet
6
5
photolysed H S matrix, and they assigned these
2
absorption of H S begins at ;400 nm and has a
maximum near 266 nm w1x.
2
2
bands to H S . To the best of our knowledge, these
2
2
are the only data related to H S in solid matrices,
2
2
In the gas phase, flash photolysis of H S is
2
2
and no data on HS are available.
2
known to produce SH, HS and S ; and SH radicals
2
2
In the present Letter, the IR spectra and 266 nm
photolysis of H S are studied in solid Ar at 7.5 K.
2
2
)
The IR spectrum of gas-phase deposited H S is
Corresponding author. Fax: q358-9-19140279; e-mail:
isoniemi@csc.fi
2
2
reassigned in an Ar matrix. The 266 nm photolysis
0
009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž99.00756-3

4
8
E. Isoniemi et al.rChemical Physics Letters 311 (1999) 47–54
kinetics is studied, and the IR absorption bands
appearing under photolysis are tentatively assigned
order ŽMP2. and coupled cluster methods, including
single, double and perturbative triple excitations
ŽCCSDŽT.., were used with the standard split va-
lence 6-311qqGŽ2d, 2p. basis set on all atoms.
to an HS radical. The other photolysis products are
2
also discussed.
The calculations were carried out on SGI Power
Challenge and SGI Origin 2000 computers at the
CSC-Center of Scientific Computing ŽEspoo, Fin-
land..
2
. Experimental and computational details
H S was synthesized by cracking raw sulfane
2
2
according to the instructions of Feh e´ r et al. w20x, and
it was purified by low-pressure and low-temperature
distillation. The H S sample was stored at liquid-
3. IR absorption spectra
2
2
nitrogen temperature, and H S was further purified
3.1. H S
2
2
2
from H S by distillation at y708C immediately
2
prior to the preparation of a gas mixture. Mixtures of
In the ArrH S matrices, there is always some
2 2
2
2
H S with Ar Ž99.9999%, Aga. were prepared by
H S as an impurity, because H S is the only ther-
2
2
standard manometric procedures in various propor-
modynamically stable species of H S w24x. The
2
n
tions ŽArrH S from 300 to 2000.. The gas mix-
absorption spectrum of H S in an Ar matrix is
2
2
2
tures were sprayed onto a CsI substrate through a
shown in trace Ža. of Fig. 1. Monomeric H S has
2
y1
1
r16 in Ž;1.6 mm. capillary. Despite the purifica-
vibrational–rotational structure at 2650–2615 cm
,
tion of H S , H S was always present in the matri-
and ŽH S. , H S PPP H O and ŽH S. absorb at
2
2
2
2
2
2
2
2
3
y1
ces as an impurity. During deposition, the substrate
temperature was typically 7.5 K, provided by
closed-cycle helium cryostat ŽDE-202A, APD.. The
temperature was measured by a silicon diode Žaccu-
racy 0.5 K., and a resistive heater was used to
2582, 2586 and 2577 cm , respectively w22x. In
various experiments, our ArrH S matrices con-
2
2
tained 10–50% of H S relative to the amount of
2
H S . This was estimated by comparing the absorp-
2
2
tion of H S in H S rAr and H SrAr matrices.
2
2
2
2
control temperatures.
Without current facilities, the direct synthesis of
D S is not possible. For deuteration, we passivated
3.2. H S
2 2
2
2
the deposition line by D O, but the D–H exchange
In the ArrH S matrices, a number of new ab-
2
2
2
during the deposition did not exceed 20%. Because
the H S absorptions are typically intrinsically weak,
sorption bands appear in addition to the H S bands
2
as demonstrated in traces Žb. and Žc. of Fig. 1. At
higher ArrH S ratios, the absorptions at 2556.6,
2
2
we could not obtain reliable data on the deuterated
species.
2
2
y1
2553.8 and 880.3 cm are the only observed bands
In order to photodissociate H S , the fourth har-
of H S Žsee Fig. 1b.. At higher H S concentra-
2
2
2
2
2
2
y1
monic of a Nd:YAG laser Ž266 nm, Powerlite, Con-
tions, the absorptions at 2507 and 2499 cm
with
tinuum. with a pulse duration of ;7 ns and pulse
side bands appear Žsee Fig. 1c.. The relative intensi-
2
y1
energies of ;10 mJ at the matrix area of 1 cm was
ties of the 2507 and 2499 cm
absorptions com-
y1
used. Photolysis products were monitored by measur-
ing IR absorption spectra and laser-induced fluores-
cence ŽLIF.. The IR spectra were used for the detec-
pared to the 2556.6 and 880.3 cm bands, respec-
tively, increase with the H S concentration. The
2
2
y1
shapes of the 2556.6, 2553.8 and 880.3 cm
ab-
tion of H S, H S and HS , and the luminescence
sorption bands are independent of the ArrH S
2
2
2
2
2
2
spectra allowed us to follow SH, S and S . Details of
ratios.
2
the experimental arrangement can be found else-
The assignment of the absorption bands is
straightforward on the basis of comparison with the
where w21,22x.
The ab initio calculations were performed using
gas-phase values and the ab initio calculated values
y1
the GAUSSIAN 98 packages of computer codes w23x.
Møller–Plesset perturbation theory to the second
Žsee Table 1.. The 2556.6 cm
absorption is as-
signed to the antisymmetric HS-stretching Žn . of
5

E. Isoniemi et al.rChemical Physics Letters 311 (1999) 47–54
49
Ref. w19x, and the lines observed there did not appear
in our study. We consider our assignment as reliable
because H S was deposited directly from the gas
2
2
phase.
3
.3. Photolysis products
Irradiation at 266 nm decreases the absorptions of
H S , and an absorption doublet at 2463 and 2460
2
2
y1
y1
cm and a weak band at 903 cm appear Žsee Fig.
d.. The intensity ratio of the doublet is constant at
1
different stages of the photolysis, but it changes
slightly upon annealing of the matrix up to 30 K.
This suggests that the doublet structure possibly
originates from different sites of the same species.
Photodecomposition of the H S impurity is about
2
two orders of magnitude slower than the decrease of
H S . In addition, we observed rising LIF from S
2
2
q
atoms, SH radicals, S2 molecules, and S2 cations
w28x.
In the 266 nm photolysis, the energetically possi-
ble photolysis products are: ŽI. H qS , ŽII. H SS,
2
2
2
3
ŽIII. H SqSŽ P., ŽIV. HSqSH, ŽV. HS qH, and
2
2
1
ŽVI. H SqSŽ D., the photolysis products being
2
listed with increasing energy w4x. In the solid envi-
ronment, channels ŽIII. and ŽVI. can form HSH PPP S
complexes. Channels ŽIV. and ŽV. can lead to the
formation of H2 and S by the cage induced reac-
tion. Also, a self-reaction of two SH radicals, formed
2
Fig. 1. The infrared spectra of H S and H S in an Ar matrix at
2
2
2
7
3
.5 K. Trace Ža. presents the IR spectrum of H S ŽArrH S s
2
2
2
00.. Traces Žb. and Žc. are the IR spectra of H S with the
via channel ŽIV., can restore H S or create
2
2
2
2
ArrH S ratios of 1000 and 300, respectively. Trace Žd. presents
V can also be formed via
Ž .
2
2
HSH PPP S. Product
the difference spectrum of 266 nm photolysis of H S Žtotal
2
2
decomposition of the H SS intermediate produced in
2
irradiation time 20 min with repetition rate 10 Hz, the ArrH S
2
2
channel ŽII.. H S might be recovered from H SS
ratio is 1000..
2
2
2
ŽII. by a hydrogen shift, in a similar way as sug-
gested for H OO in Ref. w29x. The possible products
2
y1
H S . The band at 2553.8 cm
is assigned to the
and the reaction channels are presented in Scheme 1.
2
2
symmetric HS-stretching Žn . of H S , which is
The LIF spectra of SH and S appearing during
1
2
2
2
based on the similar difference between n and n1
the UV photolysis of the ArrH S Ž266 nm. and
5
2
2
y1
in the gas phase Ž3 cm . w8x. The absorption at
ArrH S Ž193 nm. matrices are compared in Fig. 2.
2
y1
8
80.3 cm
is assigned to the antisymmetric HSS-
In the 266 nm photolysis of H S , the luminescence
2
2
bending Žn . of H S . The absorption of n has
either too low intensity to be observed, or it is
of S2 molecules is seen as a progression in the
330–500 nm region w30x, and the luminescence of
SH radicals is a broad band around 350 nm Žsee Fig.
6
2
2
2
overlapped by n . The SS-stretching Žn . and torsion
6
3
Žn . modes are out of our spectral region. The 2507
2a.. In the 193 nm photolysis of H S Žsee Fig. 2b.,
4
2
y1
and 2499 cm absorptions most probably belong to
the luminescence of SH radicals is two orders of
magnitude stronger than in the 266 nm photolysis of
H S , but S is relatively weak Žsee Fig. 2b, note
dimeric H S . It should be mentioned that these
2
2
results are in disagreement with the assignment of
2
2
2

5
0
E. Isoniemi et al.rChemical Physics Letters 311 (1999) 47–54
Table 1
The IR absorptions of H S , H S, HS and H SS
2
2
2
2
2
Gas Žcmy1
.
Ara Žcmy1
.
Ab initio Žcmy1.
a
Mode
MP2
CCSDŽT.
b
H S
n1Žsym. HS-str..
n2Žsym. HSS-bend..
n3ŽSS-str..
n4ŽSS-torsion.
n5Žantisym. HS-str..
2555.78
883.0
515.92
417.48
2553.8Žw.
2730.8 Ž-1.
899.2 Ž-1.
522.4 Ž-1.
436.0 Ž14.
2731.4 Ž-1.
893.2 Ž7.
2657.3
886.9
500.8
412.9
2658.3
882.3
2
2
b
b
b
b
2558.63
2556.6Žs.
880.3Žs.
b
n6Žantisym. HSS-bend..
880.0
complexes
2516, 2509, 2507, 2501, 2499
d
d
H S
n1 Žsym. HS-str..
n2 Žbending.
2614.4c
2624.6
1180
2757.8 Ž-1.
1229.1 Ž3.
2775.8 Ž-1.
2724.1 Ž53.
2714.8 Ž98.
2690.3
1222.8
2706.1
2
e
d
1182.7
c
n3 Žantisym. HS-str..
H S dim. n Žacceptor.
2628.5
2648.5
d
2582
2586
2
1
d
H S PPP H O n Žacceptor.
2
2
1
SH
n ŽHS-str..
2591.8f
2607d
2740.1Ž-1.
2675.6 Ž-1.
2660.0
2595.0
HS2
n1 ŽHS-str..
2463
2
903
460
g
n2 Žbending.
n3 ŽSS-str..
904
595
913.0 Ž-1.
592.5 Ž10.
899.1
562.3
g
H SS
n1Žsym. HS-str..
n2ŽHSH-bend..
n3Žsym. HSS-bend..
n4ŽSS-str..
n5Žantisym. HS-str..
n6Žantisym. HSS-bend..
2578.3 Ž54.
1258.2 Ž44.
855.2 Ž27.
548.3 Ž43.
2574.5 Ž1.
832.5 Ž3.
2497.2
1236.6
841.3
511.4
2490.8
793.7
2
a
Our data. Ab initio calculated intensities Žin km moly1 . are in parentheses and the 6-311qqGŽ2d, 2p. basis set was used in all
calculations.
b
Ref. w8x and refs. therein.
c
Ref. w25x.
d
Ref. w22x.
e
Ref. w26x.
f
Ref. w27x.
g
Ref. w5x.
the different gate width.. Furthermore, in the 266 nm
H S , H qS or H S PPP S. It is interesting to
2
2
2
2
2
photolysis of H S , no evidence of SH radical ab-
compare that, in the 266 nm photolysis of hydrogen
2
2
sorption is seen in the IR spectra, in contrast with the
93 nm photolysis of H S where the maximum SH
1
2
concentration is about a half of the initial concentra-
tion of H S w21,22x. On this basis, we estimate that
2
the observed minor amount of SH radicals Žseen in
Fig. 2a. might be formed by the slow 266 nm
photolysis of H S. Thus, we have no evidence that
2
isolated SH radicals are formed in 266 nm photolysis
of H S in solid argon. This observation indicates
2
2
that cage exit of SH is not probable with the excess
energy of ;0.9 eV, and the primarily formed SH
radicals react in the cage creating in some proportion
Scheme 1.

E. Isoniemi et al.rChemical Physics Letters 311 (1999) 47–54
51
photolysis of H S is much smaller compared to that
2
2
in the 193 nm photolysis of H S. Similarly to the
2
origin of SH radicals, the formation of S atoms
might be explained by the photodissociation of H S,
2
although only the short-range separation of geminate
S atoms contributes to some extent to the observed
signal of S atoms. Thus, S seems to be the natural
2
major product of the 266 nm photolysis of H S in
2
2
solid Ar.
In the following, the assignment of the absorp-
y1
tions at 2463 and 2460 cm appearing in the pho-
tolysis of H S is discussed. The possible IR active
2
2
photolysis products of H S are SH, HSH PPP S,
2
2
H SS and HS . SH radicals cannot be responsible
2
2
for the observed absorption because they are known
y1
to absorb in solid argon at 2607 cm
w22x. Also,
HSH PPP S is excluded because of the large shift of
y1
the 2463 and 2460 cm
absorptions from
y1
monomeric H S Žby ;90 cm .. In analogy to the
2
case of HOH PPP O and ŽH O. , the n shift in the
2
2
SH
H S PPP S complex from the monomer wavenumber
2
can be expected to be much smaller than that for
y1
ŽH S. , Ž43 cm . w32x. In the case of HOH PPP O,
2
2
y1
the shift of the water stretching bands is ;5 cm
Fig. 2. The laser-induced luminescence of S2 and SH in an Ar
and we expect a similar shift for HSH PPP S, if it
exists.
matrix at 7.5 K: Ža. the 266 nm photolysis of H S2 and Žb. the
2
1
93 nm photolysis of H S. The excitation wavelength and time
2
delay in both traces are 311 nm and 30 ns, respectively. The gate
widths in traces Ža. and Žb. are 1000 and 5 ns, respectively. The
S2 and SH emissions have similar lifetimes of 200 and 400 ns,
respectively w21,30x.
The HS
2
radical and H
2
SS are the probable
y1
candidates for the 2463, 2460 and 903 cm absorp-
tions. Nevertheless, we suppose that these absorp-
tions do not belong to H SS. According to our ab
2
initio calculations, H SS has a characteristic IR
2
peroxide, the OH radicals formed have enough ki-
netic energy Ž;1.4 eV. to exit the cage with a
probability of 17% w29x. In the 300 nm photolysis of
spectrum with four fairly intense absorptions Žsee
Table 1., but no signs of the calculated absorptions
y1
y1
n2 Ž1236.6 cm ., n3 Ž841.3 cm . and n Ž793.7
6
y1
H O , corresponding to an OH excess energy of
cm . of H SS are observed in the spectrum. On the
2
2
2
;
0.9 eV, the cage exit probability is still ;10%.
other hand, the gas-phase vibrational wavenumbers
This presents a difference between the cage exit
probabilities of SH and OH.
of n and n3 of ground-state HS are 904 and 595
2
2
y1
y1
cm , respectively w5x. Thus our 903 cm
band
Fig. 2 demonstrates that S molecules are formed
goes well with n of gas-phase HS . It should be
2
2
2
q
y1
in large amounts in the 266 nm photolysis of H S .
noted that Ar H is known to absorb at 903 cm
2
2
2
It is known that the decomposition of S2 in an Ar
matrix is prevented by the cage-effect w30x. Recently,
we have suggested a local Žshort-range. separation of
and we cannot definitely exclude the presence of the
ionic species in our matrices w33x. Under the assign-
y1
ment of the 2463 and 2460 cm
doublet to HS2 ,
two S atoms in the UV photolysis of S molecules,
the shift between the calculated CCSDŽT. and exper-
2
y1
but the amount of these photodecomposed S2
imental SH stretching wavenumbers is ;130 cm
,
molecules is minor w31x. In addition, another minor
and this is near the corresponding value for H S
2
2
q
y1
channel deals with the formation of S2 cations w28x.
Ž;100 cm .. The shift between the calculated
y1
Furthermore, the LIF signal from S atoms in the
H SS absorption and the doublet is ;30 cm
,
2

5
2
E. Isoniemi et al.rChemical Physics Letters 311 (1999) 47–54
which compares less favorably with H S . Thus, we
2
2
y1
suggest that the bands at 2463, 2460 and 903 cm
are most probably from HS2 radicals. This assign-
ment corresponds to the gas-phase data on the UV
photolysis of H S where HS2 radicals were ob-
2
2
served to appear w2,3x.
4
. Photolysis kinetics
We discuss a simplified photolysis scheme assum-
ing negligible amounts of SH, HSH PPP S and H SS
2
y1
species, and connecting the 2463 and 2460 cm
doublet to HS radicals. The time evolution of H S ,
2
2
2
S2 and HS2 in the 266 nm photolysis is shown in
Fig. 3. Qualitatively, the concentration of H S de-
2
2
creases Žsee Fig. 3a. and the concentration of S2
increases Žsee Fig. 3b., monotonically. The concen-
tration of HS radicals increases in the beginning of
2
the photolysis, reaches its maximum value, and then
decreases slowly Žsee Fig. 3c.. The detectable photo-
stable photolysis products are S molecules, in addi-
2
tion to S atoms and SH radicals that are minor in the
2
66 nm photolysis.
In the framework of the considerations given
above, and neglecting the minor channels, we con-
sider the following part of Scheme 1:
k1
H S qhn ™ HS qH ,
Ž
Ž
Ž
1
2
3
.
.
.
2
2
2
k2
H S qhn ™ S qH ,
2
2
2
2
k3
Fig. 3. The kinetics of the 266 nm photolysis of H S in an Ar
2 2
HS qhn ™ S qH ,
matrix at 7.5 K: Ža. H S ; Žb. S ; and Žc. HS . The experimental
2
2
2 2 2 2
data are fitted by Eqs. Ž4.–Ž6.. See text for details.
where k denotes the corresponding rate constants.
i
Reactions Ž1. and Ž2. are direct or cage-induced, and
they can also occur via photodecomposition of inter-
The concentrations of H S , HS and S are mutu-
ally normalized, and constitute a closed photolysis
scheme, and the concentrations at infinite irradiation
mediates such as H SS.
2
2
2
2
2
Solving the rate equations of reactions Ž1.–Ž3.
with the initial conditions wH S x s1, wHS x s
2
2
0
2
0
time are wH S x s0, wHS x s0 and wS x s1.
wS x s0, we obtain the concentrations
2
2 `
2 `
2 `
2
0
The fit of Eqs. Ž4.–Ž6. to the 266 nm photolysis
wH S xsexp y
Ž
k1 qk
.
t ,
Ž
4
.
2
2
2
data is shown in Fig. 3. The rate constants were
obtained in the following procedure. At the begin-
ning of photolysis, HS2 is formed via reaction Ž1.
wHS xs k r
Ž . Ä Ž .
k yk yk exp y k1 qk2 t
2
1
3
1
2
and the formation of S is mainly due to reaction Ž2..
yexp
Ž
yk3 t
.
4
,
Ž
Ž
5
6
.
.
2
Thus, the slopes of the HS2 and S2 concentration
curves at the beginning of the photolysis estimate the
wS xs1y w H S xy w HS x.
2
2
2
2

E. Isoniemi et al.rChemical Physics Letters 311 (1999) 47–54
53
y1
rate constants k s0.041 min
and k s0.035
absorptions most probably originate from HS radi-
cals. The 2463 and 2460 cm absorptions are tenta-
1
2
2
y1
y1
min , respectively. Fitting of Eq. Ž6. to the experi-
mental data of S Žsee Fig. 3b., with the constants
tively assigned to the HS-stretching mode Žn . of
2
1
y1
y1
y1
k s0.041 min
and k s0.035 min , gives k3
HS in different sites, and the 903 cm absorption
1
2
2
y1
s0.006 min . The simulation of wHS x with Eq.
is tentatively assigned to the HSS-bending mode
2
Ž5. and the rate constants given above is shown in
Žn .. A simplified scheme of the 266 nm photolysis
2
Fig. 3c, demonstrating a good agreement with the
experimental data, and this confirms the reasonable
validity of the constructed kinetic scheme.
is built, and H S is found to have two nearly
2
2
equally contributing dissociation channels: H S q
2
2
hn ™ HS qH and H S qhn ™ S qH . The
2
2
2
2
2
The H S decay is not one exponential Žsee Fig.
HS radical undergoes sequential photodissociation
2
2
2
3
a., but it is more complex. The deviation from the
to a S molecule and an H atom.
2
simplest photolysis scheme Ž1.–Ž3. can be explained
by several reasons. First, in the photolysis of H S ,
secondary reactions of escaping H atoms with close
2
2
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2
2
2
2
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2
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. Conclusions
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w23x
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6
2
2
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2
2

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