Femtosecond photodissociation dynamics of excited-state SO2

Article abstract of DOI:10.1021/jp020535+

Photodissociation dynamics of SO₂ were investigated employing a multiphoton pump-probe excitation technique (312 nm pump/624 nm probe), with evidence supporting the processes being initiated through the E state. The pump-probe optical transient for the SO₂ species was fit to a single-exponential decay, yielding a time of 271 ± 8 fs. Subsequent decay products of SO and S were also investigated and mathematically fit to determine formation and/or decay time dynamics.

Full text of DOI:10.1021/jp020535+

J. Phys. Chem. A 2002, 106, 10843-10848
10843
†
Femtosecond Photodissociation Dynamics of Excited-State SO2
Eric S. Wisniewski‡ and A. Welford Castleman, Jr.*,‡,§
Departments of Chemistry and Physics, The PennsylVania State UniVersity,
UniVersity Park, PennsylVania 16802
ReceiVed: February 26, 2002
Photodissociation dynamics of SO2 were investigated employing a multiphoton pump-probe excitation
technique (312 nm pump/624 nm probe), with evidence supporting the processes being initiated through the
E˜ state. The pump-probe optical transient for the SO2 species was fit to a single-exponential decay, yielding
a time of 271 ( 8 fs. Subsequent decay products of SO and S were also investigated and mathematically fit
to determine formation and/or decay time dynamics.
Introduction
by SO2. Furthermore, [SO(SO2)n]+ clusters similarly contained
an SO+ ion core with solvation by SO2 molecules.
In view of its ubiquitous nature, interest in the photochemistry
of sulfur dioxide (SO2) is wide ranging. SO2 is produced in an
industrial setting mainly (∼98%) for use as a precursor to
sulfuric acid (H2SO4).1 In this context, it is produced by the
combustion of elemental sulfur or by the oxidation of sulfides.
However, it is the unintentional production of sulfur oxides as
the byproduct of common combustion of hydrocarbon fuels
containing sulfidic sulfur, and its subsequent release into the
atmosphere, that is of concern to the scientific community. It
has been determined that at least 90% of sulfur present in fossil
fuel is released in the gas phase as SO2 during combustion.2 It
is also present in the atmosphere due to volcanic emissions and
through the oxidation of various sulfur-containing gases liberated
through biochemical cycles. Because of this fundamental
importance to atmospheric science, sulfur oxides have been
increasingly researched and their chemistry discussed in the
literature.3-6
Previous work has been conducted in our laboratory on the
chemistry of SO2 with other species.4,6-10 Some years ago,
studies were undertaken to identify reactions with neutral
hydroxyl radicals as being a significant mechanism of its initial
chemical conversion in the atmosphere. Subsequently, we
initially investigated the reaction of SO2 with OH-(H2O)n)0-2
and found the presence of HSO3-(H2O)m in the mass spectral
distribution of reaction products.8 From this, the following
reaction was postulated:
The photodissociation of SO2 has been investigated for several
electronically excited states. At 193 nm, its photodissociation
yields SO + O exclusively.11,12 Kawasaki and Sato investigated
SO2 at 193 nm and, from the angular distribution of the
dissociative O atom, concluded that the dissociation into SO +
O was predissociative. In particular, they found the distribution
to be isotropic, indicating that the lifetime of the dissociation
is much longer than a rotational period. In other studies, Katagiri
et al. experimentally and theoretically explored the C˜ state
dissociation of SO2. They invoked rotational line broadening
to claim that the predissociative lifetime of the SO2 was on the
order of 10-100 ps, though the rate was found to increase
exponentially as the energy was increased.13
Investigations of SO2 at higher energies showed that along
with SO + O, S + O2 could become a significant dissociation
channel.14,15 Photolysis at 123.6 nm was performed on SO2 (in
the presence of H2 to minimize experimental difficulties that
did not interfere with photodissociation) to yield both dissocia-
tion channels in a plethora of excited-state fragments. Lalo and
Vermeil suggested that the dissociation process at 123.6 nm to
yield S + O2 proceeded by a direct mechanism. Other studies
by Venkitachalam and Bersohn at 285-311 nm (using two-
photon excitation) produced S, O2, SO, and O as photodisso-
ciation products.16 Effenhauser et al. investigated the two-photon
photodissociation of SO2 at 248 and 308 nm.17 They reported
that excitation with photons at 248 nm produced a myriad of
photoproducts that yield mostly SO + O, but also fragmentation
pathways leading to S + O2. Furthermore, Effenhauser et al.
reported that two-photon excitation at 308 nm resulted in
SO + O, in various states. Two-photon photodissociation at
308 nm also produced S photofragments as a minor pathway.
Sato reported two-photon (286-309 nm) photodissociation
studies of SO2 that focused on the production of S photofrag-
ments, by detection of S by resonance-enhanced ionization.18
OH-(H2O)n + SO2 f HSO3 (H2O) + (n - m)(H2O) (1)
-
Further studies by Yang and Castleman of SO2 with various
hydrated anion clusters yielded clues to the role of SO2 in
atmospheric processes.9 Hydration of anionic species will enable
some reactions to become more endothermic due to the
stabilization of the reactant ions. More recently, Zhong et al.
investigated the dissociative pathways of sulfur dioxide clusters
and mixed water-sulfur dioxide clusters.4 Zhong found that
Sato et al. also reported that a “three-body” dissociation of
SO2 into S + O + O would require three photons of 308 nm
light.18 Two-photon excitation is possible due to the long-lived
B˜ state of SO2. The three-photon process has not been studied
and discussed in the literature. This is most likely due to two-
photon dissociation that occurs before a third photon is absorbed
in nanosecond excitation sources. The Sato “three-body” dis-
+
sulfur dioxide cation clusters have an SO2 ion core solvated
† Part of the special issue “R. Stephen Berry Festschrift”.
* Corresponding author.
‡ Department of Chemistry.
§ Departnment of Physics.
10.1021/jp020535+ CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/08/2002

10844 J. Phys. Chem. A, Vol. 106, No. 45, 2002
Wisniewski and Castleman
sociation has been simulated with three different models: the
simultaneous fragmentation, the sequential fragmentation, and
the classical impulsive model.18 For simultaneous fragmentation,
S (3P) and S (1D) obtain 0.27 and 0.17 eV of translation energy,
respectively. For the classical impulsive model, S (3P) and S
(1D) obtain 0.21 and 0.13 eV, respectively. Sato invoked the
data obtained from the simulations to conclude that the S atoms
produced by two-photon photodissociation (0.21 eV for S (3P)
and 0.09 eV for S (1D)) are in reasonable agreement with a
proposed simultaneous photodissociation pathway, though Sato
claims the two-photon process is most likely. The sequential
photodissociation simulation predicts much larger values for the
S photofragment; one scenario yielded a value of 0.65 eV for
the S (3P) while others yielded values too low to be compared
to the experimentally determined energy.
group velocity dispersion. The amplified laser pulse is <125 fs
in duration (sech2) determined by autocorrelation, has 1.0-1.5
mJ of energy, and has a wavelength centered at 624 nm.
To acquire 312 nm pulses required for pumping the molecules
in the two-color pump-probe experiments reported herein, the
fundamental laser beam was focused through a â-barium borate
(BBO) crystal to generate second harmonic light. The remaining
light that is not converted to ultraviolet (UV) is harvested and
used as the probe pulse. The UV light is sent through a delay
stage so that the optical path lengths of the pump and probe
lasers can be adjusted for temporal overlap. A 40 cm optical
lens, placed between the focal points of the pump and probe
laser pulses, is used to loosely focus the laser pulses between
the acceleration grids of the mass spectrometer. It is important
to note that only minimal ion signal is present in the mass
spectral distribution when either the pump or probe laser pulses
are blocked. Hence, neutral excited-state dynamics are inves-
tigated and molecular ion states are not probed in the present
experiment.
Herein we report the femtosecond pump-probe dynamics
of SO2 monomer on an excited-state surface and the temporal
dynamics deduced from the resulting photodissociation products
SO, S, and O. Excitation of SO2 was achieved by a femtosecond
laser system with pump photon bandwidth centered at 312 nm.
Results and Discussion
Excited-state SO2 was prepared by multiphoton absorption
of the second harmonic output (312 nm) of the CPM laser
system. Photodissociation products of SO, S, and O are exhibited
in a typical mass spectrum seen in Figure 1. Due to the mass
overlap of S and O2, mass 32 was determined to be sulfur by
analysis of the 34S peak that is evident at approximately 4% of
mass 32. Further isotopic analysis reveals the presence of a mass
66 amu corresponding to 34SO2 and 50 amu corresponding to
34SO.
Experimental Section
The production of SO2 was indirectly achieved while first
attempting to study a unique inorganic species, N-thionylaniline.
SO2 monomer was prepared by the hydration of N-thionyl-
aniline. Molecular reaction and subsequent rearrangement of
the N-thionylaniline resulted in the production of aniline and
SO2 (reaction 2):
C6H5NSO + H2O f C6H7N + SO2
(2)
Two-body dissociation of the SO2 following excitation to an
excited-state yields two major pathways, with a variety of
excited and ground-state products:
The reaction products were entrained in helium with a backing
pressure of 20 psig. The helium, along with the entrained vapor
and other gaseous species, was expanded into the mass
spectrometer through a pulsed nozzle. The molecular beam
produced in this fashion was collimated with a skimmer prior
to ionization with femtosecond laser pulses, which were directed
to intercept clusters in a region between the time-of-flight grids.
The time-of-flight mass spectrometer is of the Wiley-McLaren
design.19
SO2 2hυ8 SO + O
SO2 2hυ8 S + O2
(3)
(4)
If both eqs 3 and 4 were operative within the time window of
the experiments discussed here, SO2 would decay via two or
more distinct rates. Furthermore, if SO2 absorbed three pump
photons to undergo either a concerted or a stepwise photodis-
sociation mechanism into S + O + O, the kinetics corresponding
to this process also would be expected to be revealed in the
dynamics of the SO2 optical transient. Figure 2 displays the
pump-probe transient for SO2. The transient was well fit
mathematically to a single-exponential decay, yielding a time
of 271 ( 8 fs. The fitting procedure is outlined elsewhere.20
Attempts were made to mathematically fit the data to single,
bi-, and triexponential decay functions; however, the fitting for
the bi- and triexponential decay failed to converge. This
evidence for a single decay rate for SO2 is suggestive that only
a single decay channel is contributing to the products in these
studies.
Figure 3 illustrates the pump-probe transient for SO. Clearly,
the production of SO involves a mechanism corresponding to
eq 3 but this does not explain the appearance of S, nor the
apparent dynamics exhibited by this species which is displayed
by the data plotted in Figure 4. If the production of S is due to
eq 4, while the dynamics in Figure 3 are explained by eq 3,
then the dynamics exhibited in Figure 2 would be expected to
be best fit to a biexponential decay since two separate processes
would be occurring that decay into two distinct rates. Since this
Under typical operating conditions, the potential applied to
the first time-of-flight grid is between 4 and 5 kV while the
potential applied to the second grid is roughly 3 kV. With the
reflecting potential applied to the front of the reflectron, the
total flight path for the detected ions is 2 m. The detection
scheme employs a pair of microchannel plates coupled to a
digital oscilloscope (Agilent Technologies 54820A).
Ionization was achieved with an amplified colliding pulse
mode-locked ring dye laser (CPM). In this arrangement, a gain
jet containing rhodamine 590 tetrafluoroborate is pumped by a
continuous wave Nd:YVO4 laser (Coherent Verdi V5). Continu-
ous wave lasing from the gain jet is interrupted by a second jet
containing the saturable absorber DODCI. Laser pulses on the
order of 100 fs are generated at 90 MHz with pulse energies of
only ∼200 pJ. Amplification of the laser pulses is achieved in
four stages using a six-pass bowtie amplifier and three succes-
sive Bethune cells, where the beam is progressively expanded
from a 2 mm initial diameter to a final beam diameter of 12
mm. All amplification is achieved by transverse pumping of
sulforhodamine 640 by the second harmonic of a 10 Hz Nd:
YAG laser (Spectra Physics GCR-4). Recompression is per-
formed using a grating pair (1200 lines/mm) to compensate for

Femtosecond Photodissociation of Excited-State SO2
J. Phys. Chem. A, Vol. 106, No. 45, 2002 10845
Figure 1. Mass spectrum of SO2 taken at zero delay between pump and probe laser beams.
Figure 2. Pump-probe transient for the sulfur dioxide. The data was fit to a single-exponential decay, with deconvolution of the laser pulse width,
to yield a time constant of 271 fs.
is not the case, the production of S appears to be due to a
secondary fragmentation of SO. The kinetics of the various
processes for SO are outlined below:
First, consider the possibility that sulfur dioxide absorbs two
photons to generate the excited E˜ state of SO2. Following this
excitation, photodissociation results to form SO + O. In this
mechanism, only those SO molecules with appreciable internal
energy (electronically excited) will absorb enough probe photons
to become ionized and be imaged; ground-state SO (IP ) 10.360
eV) would require six probe photons for ionization. However,
the decay dynamics for SO in this mechanism are unaccounted
for. Furthermore, the apparent dynamics on the sulfur photo-
fragment also remain unexplained.
k1
SO2
9
8 f(SO + O)
(5)
(6)
(1 - f )SO f S + O
The initial excitation of SO2 leads to the formation of SO. After
the initial production of SO by the decay of SO2, the ensuing
dynamics on the sulfur monoxide are unclear. Several processes
may be occurring as discussed in what follows.
A second possible scenario is that the sulfur dioxide may
absorb three pump photons and decay into S + O + O. If this

10846 J. Phys. Chem. A, Vol. 106, No. 45, 2002
Wisniewski and Castleman
Figure 3. Pump-probe transient for the sulfur monoxide photofragment. The data was fit to kinetic rate equations, with deconvolution of the laser
pulse width, to yield a formation time of 271 fs and a decay time of 156 fs. The inset figure is the original SO data, unshifted along the time axis.
Figure 4. Potential energy surface of SO2 illustrating the intermediate B state that is accessed in a two-photon absorption scheme.
mechanism was operative, the dissociation must occur in a
stepwise fashion so sulfur monoxide is first formed and then
decayed. This mechanism fails to account for the SO ion signal
that remains at longer delay times. Further experiments to
attenuate the pump photon density failed to yield dissociative
information suggesting that only two-photon excitation is
occurring, and three-photon pumping is negligent. On this basis,
excitation via a two-photon absorption scheme through the E˜
state is surmised.
Finally, SO2 may be absorbing two photons to reach the E˜
state, see Figure 5. Dissociation may then result in SO + O as
before, but the sulfur optical transient arises due to the post-
ionization fragmentation of SO+. This explains the dynamics
attributed to the sulfur photofragment but fails to fully explain
the decay dynamics on the SO fragment. Effenhauser noted that
1
the formation of SO (b Σ+) is possible by analysis of the
photodissociation energetics and noted that a minor contribution
of this excited-state cannot be excluded from the data despite
the inability to discern whether this state is produced.17 If SO
Figure 5. Three individual components comprise the fit displayed in
Figure 3. A Gaussian distribution models the experimental pulse width.
Rise and decay dynamics of the SO fragment as well as an additional
curve corresponding to rise dynamics completes the fitting.
(b Σ+), (a ∆), and (X Σ-) were simultaneously produced in
the two-photon dissociation of SO2, it is conceivable that the
ionization of each species would behave differently in the mass
spectrometer. The ground-state SO may not absorb enough
photons for ionization while the SO (a 1∆), already with ∼0.75
eV internal energy, would require one less photon for ioniza-
1
1
3
dynamics observed in these experiments. This mechanism may
also explain why Effenhauser was unable to discern this excited
state.
Considering the third proposed mechanism, the pump-probe
transient for SO was fit using the kinetics equations convoluted
1
tion.21 However, SO (b Σ+) may quickly radiatively decay to
ground-state sulfur monoxide which would account for the

Femtosecond Photodissociation of Excited-State SO2
J. Phys. Chem. A, Vol. 106, No. 45, 2002 10847
Figure 6. Pump-probe transient for the sulfur fragment. The inset contains the transient of the oxygen cation.
TABLE 1: Ionization Potentials of the Various Fragments
of SO2
along with the pulse width of the femtosecond laser to yield
the production and decay times. The data for the SO fragment
was best fit using the following equations to determine kinetic
rates:
cation
parent
IP (eV)
+
SO2
SO2
SO
S
12.349
10.294
10.360
13.618
15.930
16.334
18.690
SO+
S+
[A]0k1
O+
O
1
2
[B] )
[e-k t - e-k t
]
(7)
(8)
SO+
S+
SO2
SO2
O2
k2 - k1
O+
[B′] ) [A]0 - [A]0e-k t
2
potentials (IP) of each of the species; O requires two additional
probe photons for ionization. The IPs of various sulfur-
containing species and products are given in Table 1. Since the
IP of oxygen atom is considerably higher than the others, it is
reasonable that fewer of those atoms will arise from the
fragmentation of SO+ and retain the charge to be detected in
the mass spectrum. If fragmentation of SO results in S just after
photoionization, the sulfur atom would more likely garner the
charge from the parent molecule rather than from the oxygen
atom. Also, the generation of atomic cation species from SO2
is low considering the high fluences required for ionization and
is only appreciable around the zero overlap.
1
In eq 7, A is SO2, while B is SO (b Σ+), and C is the
undetected SO (X 3Σ-). In eq 8, A is SO2 while B′ models SO
(a ∆). The value for k1 was fit to 3.49 ps-1 (287 fs) which
1
agrees nicely, within error, to the value calculated from the SO2
pump-probe transient single-exponential decay. The value for
k2 was determined to be 6.43 ps-1 or 156 fs. Figure 6 displays
the SO data along with the underlying kinetics that compose
the fitting displayed in Figure 3. The data for SO was
mathematically fit using a least-squares method with the aid of
Microsoft Excel.
The pump-probe transient data for S atomic ion is illustrated
in Figure 4; the inset of the figure is the transient for the O
atomic ion. Due to the noise level of the data, it is not possible
to determine if the data are best fit to kinetics that describe a
formation and decay of S, or if the data simply reflect a
formation rate with a Gaussian pulse overlap. Both SO and S
can radiatively decay from highly excited states if pumped by
two additional UV photons. However, this is impossible in these
experiments due to the ultrashort nature of the light, the UV
pulse is no longer irradiating the molecules after the initial
photodissociation and hence, further excitation is not allowed.
Furthermore, considering the resolution of the TOF-MS in the
present experiments, it is not possible to discern the translational
energy obtained by the S photofragments to infer initial
excitation energy in these studies as Sato was able to accomplish.
As a result of the third proposed mechanism above, the S
photofragment is interpreted as a post-ionization fragment of
the SO cation.
Conclusions
The decay rate of the SO2 molecule was calculated to be
271 ( 8 fs. The subsequent formation of SO was produced with
the same kinetics and reaffirmed by the fitting procedures
utilized. Several independent processes may be occurring in the
photodissociation of SO2; however, the likelihood that they
simultaneously occur with the same kinetics is doubtful
considering the single-exponential behavior on the SO2 mol-
ecule. Two-photon photodissociation of SO2 may proceed after
excitation to the E˜ state. The production of SO in a higher lying
excited state must radiatively decay from the excited state to
yield dynamics exhibited in the pump-probe optical transient.
A more extensive theoretical approach to SO2 is necessary to
discern the processes studied here. Much theoretical and
experimental work has been performed on the lower excited
state of SO2; however, the energetics of two-photon dissociation
needs to be explored more completely. Further studies on SO2
in other excited states are underway to determine the effect of
The SO+ may fragment after ionization to form either S+ +
O or S + O+. The relative intensities of O and S do not scale
to the ratios present in SO. This is due to the relative ionization

10848 J. Phys. Chem. A, Vol. 106, No. 45, 2002
Wisniewski and Castleman
(8) Yang, X.; Castleman, A. W., Jr. Chem. Phys. Lett. 1991, 179,
361.
solvation upon the excited-state dynamics and the role they may
play in the formation of acid rain.
(9) Yang, X.; Castleman, A. W., Jr. J. Phys. Chem. 1991, 95, 6182.
(10) Keesee, R. G.; Kilgore, K.; Breen, J. J.; Castleman, A. W., Jr.
Aerosol Sci. Technol. 1987, 6, 71.
(11) Freedman, A.; Yang, S. C.; Bersohn, R. J. Chem. Phys. 1979, 70,
Acknowledgment. The authors gratefully acknowledge the
Department of Energy, Grant No. DE-FG02-92ER14258, for
funding this research.
5313.
(12) Kawasaki, M.; Sato, H. Chem. Phys. Lett. 1987, 139, 585.
(13) Katagiri, H.; Sako, T.; Hishikawa, A.; Yazaki, T.; Onda, K.;
Yamanouchi, K.; Yoshino, K. J. Mol. Struct. 1997, 413/414, 589.
(14) Lalo, C.; Vermeil, C. J. Photochem. 1973, 1, 321.
(15) Lalo, C.; Vermeil, C. J. Photochem. 1975, 3, 441.
(16) Venkitachalam, T.; Bersohn, R. J. Photochem. 1984, 26, 65.
(17) Effenhauser, C. S.; Felder, P.; Huber, J. R. Chem. Phys. 1990,
142, 311.
References and Notes
(1) Sander, U. H. F.; Fischer, H.; Rothe, U.; Kola, R. Sulphur, Sulphur
Dioxide and Sulphuric Acid; Verlag Chemie International Inc.: Deerfield
Beach, Fl 1984.
(2) Spiro, P. A.; Jacob, D. J.; Logan, J. A. J. Phys. Res. 1992, 97,
6023.
(3) Vincent, M. A.; Palmer, I. J.; Miller, I. H.; Akhmatskaya, E. J.
Am. Chem. Soc. 1998, 120, 3431.
(18) Sato, T.; Kinugawa, T.; Arikawa, T.; Kawasaki, M. Chem. Phys.
1992, 165, 173.
(19) Wiley, W. C.; McLaren, I. H. ReV. Sci. Instrum. 1956, 26, 1150.
(20) Fiebig, T.; Chachisvilis, M.; Manger, M.; Zewail, A. H.; Douhal,
A.; Garcia-Ochoa, I.; de La Hoz Ayuso, A. J. Phys. Chem. A 1999, 103,
7419.
(21) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular
Structure: Constants of Diatomic Molecules; Van Nostrand Reinhold
Company: New York, 1979.
(4) Zhong, Q.; Hurley, S.; Castleman, A. W., Jr. Int. J. Mass Spectrom.
Ion Process. 1999, 185/186/187, 905.
(5) Hewitt, C. N. Atmos. EnViron. 2001, 35, 1155.
(6) .Tang, I. N.; Castleman, A. W., Jr. J. Photochem. (1976/77), 6,
349.
(7) .Holland, P. M.; Castleman, A. W., Jr. J. Photochem. 1981, 16,
347.