Probing of the hot-band excitations in the photodissociation of OCS at 288 nm by DC slice imaging

Article abstract of DOI:10.1139/v04-072

The photodissociation dynamics of OCS at 288 nm has been investigated using the DC (direct current) slice imaging technique, which is a recently developed high-resolution slicing approach that directly measures the central slice of the photofragment distribution in imaging experiments. By analyzing a DC sliced image of S(¹D₂) photofragments we observe dissociation originating from OCS molecules excited up to v₂ = 4 in the molecular beam. The measured translational energy distribution was used to determine the branching ratio for the contribution from each initial bending state (0 v₂ 0) of OCS and relative photodissociation cross section ratios compared to v₂ = 1. Large negative anisotropy parameters determined as a function of the S(¹D₂) fragment recoil speed indicate that the photodissociation of OCS at 288 nm occurs exclusively from the 1¹A″(¹Σ^-) bending excited potential surface that can be accessed through a perpendicular transition.

Full text of DOI:10.1139/v04-072

8
80
Probing of the hot-band excitations in the
photodissociation of OCS at 288 nm by DC slice
1
imaging
Myung Hwa Kim, Wen Li, Suk Kyoung Lee, and Arthur G. Suits
Abstract: The photodissociation dynamics of OCS at 288 nm has been investigated using the DC (direct current) slice
imaging technique, which is a recently developed high-resolution “slicing” approach that directly measures the central
1
slice of the photofragment distribution in imaging experiments. By analyzing a DC sliced image of S( D )
2
photofragments we observe dissociation originating from OCS molecules excited up to v = 4 in the molecular beam.
2
The measured translational energy distribution was used to determine the branching ratio for the contribution from each
initial bending state (0 v 0) of OCS and relative photodissociation cross section ratios compared to v = 1. Large
2
2
1
negative anisotropy parameters determined as a function of the S( D ) fragment recoil speed indicate that the
photodissociation of OCS at 288 nm occurs exclusively from the 1 A′′( Σ ) bending excited potential surface that can
be accessed through a perpendicular transition.
2
1
1 –
Key words: DC slicing imaging, OCS, photodissociation, hot-band excitation.
Résumé : On a étudié la dynamique de la photodissociation du OCS à 288 nm en faisant appel à la technique de
l’imagerie de raccordement à courant direct, une approche de « raccordement » à haute résolution développée récem-
ment qui mesure directement le raccordement central de la distribution des photofragments dans des expériences
1
d’imagerie. En analysant une image coupée en courant direct de photofragments de S( D ), on observe la photodisso-
2
ciation qui tire son origine dans les molécules de OCS excitées jusqu’au niveau v = 4 dans un faisceau moléculaire.
2
On a utilisé la distribution d’énergie de translation mesurée pour déterminer le rapport de bifurcation pour la contribu-
tion à partir de chaque état initial de déformation (0 v 0) de OCS ainsi que les rapports de section droite de photodis-
2
sociation relatifs comparés à v = 1. Les valeurs très négatives des paramètres d’anisotropie qui ont été déterminées en
2
1
fonction de la vitesse de recul de la fragment S( D ) indiquent que la photodissociation de OCS à 288 nm ne se pro-
duit qu’à partir de la surface potentielle excitée de déformation 1 A′′( Σ ) à laquelle on peut accéder par une transition
2
1
1 –
perpendiculaire.
Mots clés : imagerie de couplage en courant direct, OCS, photodissociation, excitation d’une bande chaude.
[
Traduit par la Rédaction] Kim et al. 884
Introduction
excited states. Excitation of these excited states is weakly al-
lowed in the bent geometry (C ), but forbidden in the linear
s
Carbonyl sulfide (OCS) has been of considerable interest
from a spectroscopic point of view (1–3) as a prototypical
triatomic molecule, and in the earth’s atmosphere as a pri-
mary source of sulfate for the stratospheric aerosol layer (4–
equilibrium geometry of the ground state molecule. Dissoci-
ation is thus strongly favored for the molecule in bent geom-
etries on the ground electronic potential surface. The results
obtained also show the bimodal distributions (7, 9, 11) for
1
+
6
). Extensive studies have thus been performed to elucidate
both the rotational levels of CO(X Σ ) and the translational
1
the photodissociation dynamics of OCS using laser-induced
fluorescence (LIF) (7, 8), ion imaging (9–12), and photo-
electron spectroscopy (13).
energy of S( D ) because of two different dissociation path-
2
1
1
ways on the 2 A′( ∆) surface, which are ascribed to adiabatic
and non-adiabatic processes. It is generally believed that the
2 A′( ∆) excited surface, corresponding to a parallel transi-
tion, is located higher in energy than the 1 A′′( Σ ) excited
surface corresponding to a perpendicular transition (7, 9).
Katayanagi and Suzuki (11) reported that the transition
probability of this non-adiabatic process is typically 0.35 at
1
1
From these previous works (7–9), it is well known that
1
+
1 1 –
photodissociation of OCS(X Σ ) in a range between 222 and
2
48 nm produces rotationally hot, but vibrationally cold
1
+
1
3
CO(X Σ ) fragments along with S( D2) or S( P ) atoms in
the branching ratio 95:5, through the 2 A′( ∆) and 1 A′′( Σ )
j
1
1
1
1 –
Received 18 November 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 26 August 2004.
2
M.H. Kim, W. Li, S.K. Lee, and A.G. Suits. Department of Chemistry, SUNY Stony Brook, Stony Brook, NY 11794, USA.
Department of Chemistry, Wayne State University, Detroit, MI 48202, USA. Chemistry Department, Brookhaven National
Laboratory, Upton, NY 11973, USA.
1
This article is part of a Special Issue dedicated to the memory of Professor Gerhard Herzberg.
Corresponding author (e-mail: arthur.suits@sunysb.edu).
2
Can. J. Chem. 82: 880–884 (2004)
doi: 10.1139/V04-072
© 2004 NRC Canada

Kim et al.
881
Fig. 1. Experimental schematic showing the slice imaging ap-
1
+
2
30 nm from the rotational distribution of CO(X Σ ) and the
1
S( D ) image obtained by using conventional velocity map
proach.
2
ion imaging technique (14).
One particularly interesting result from the recent work
9–12) is that, owing to the fact that the transition is forbid-
(
den from the linear geometry, bent geometries, and in partic-
ular vibrational bending excitation of OCS, can play an
important role in the excitation and dissociation dynamics
even if the molecular beam is translationally and rotationally
very cold. This is also true for the isoelectronic N O mole-
2
cule. These studies have largely been performed in the vicin-
ity of the local maximum in the absorption around 230 nm,
1
1
1
1 –
where both the 2 A′( ∆) and 1 A′′( Σ ) may contribute. Both
1
surfaces cross repulsive Π states in the linear geometry, but
consisting of four electrodes. The beam is perpendicularly
these crossings are avoided (and become conical intersec-
tions) for the bent molecule. Very recently, the presence of
vibrationally excited OCS molecules in a cold molecular
beam was experimentally reported from only the rotational
intersected with a laser beam tuned to a two-photon resonant
1
excitation of S( D ). The operating pressures were -2.5 ×
2
–
5
–7
1
0
in the source chamber and -5 × 10 torr (1 torr =
1
+
133.322 4 Pa) in the main chamber.
The wavelength of laser light in the present study was
produced by frequency doubling the output of a dye laser
state selection of CO(X Σ ) at 230 nm in velocity map ion
imaging by Sugita et al. (10), Katayanagi and Suzuki (11),
and Janssen and co-workers (12), independently. Janssen and
co-workers used the hexapole state selection technique (12)
to study the state-resolved angular recoil distribution of
(
Continuum Jaguar, R590/R610 dyes) pumped by a Nd:YAG
system (Quanta Ray GCR-5). The vertical polarization of
the laser beam in the laboratory frame was achieved using a
Soleil-Babinet compensator (Special Optics) with its optical
axis set at 45° with respect to the horizontal polarization
vector of the laser beam and was then focused with a lens
1
+
CO(X Σ ; v = 0|J) from the photodissociation of OCS(v =
2
0
|JM = 10) and OCS(v = 1|JlM = 111). Here, we present a
2
study of OCS photodissociation via hot-band absorption in
the very long wavelength tail of the absorption at 288 nm.
As shown in our previous papers (15, 16), DC (direct cur-
rent) slice imaging techniques, refined from the original slic-
ing approach (17), can measure directly the center slice of
the photofragment distribution and can be effectively used to
resolve more clearly some of the structures in the kinetic en-
ergy release or velocity distribution for any molecule with-
out the need for numerical reconstruction methods. We
achieved this by employing a low voltage, multilens velocity
mapping assembly that eliminates the need for pulsed elec-
tric fields and grids, and therefore, considerably enhances
the experimental resolution attainable. Also, this slicing ap-
proach is a highly sensitive and widely applicable technique
for the study of orbital polarization (16, 17).
(
f = 30 cm) into the interaction volume with the pulsed mo-
lecular beam of OCS diluted in Ar. This linearly polarized
laser beam was used for both photolysis of OCS and ioniza-
tion of S atoms in a one-color experiment. Typical output
power for the doubled dye laser beam entering the chamber
was 2 mJ/pulse and accurate wavelength calibration was
achieved using a wavemeter (Coherent WaveMaster). The
1
S( D ) photofragments were ionized by using the following
2
(
2 + 1) resonance enhanced multiphoton ionization (REMPI)
scheme:
2hv
hv
4 1
3
1
+
S 3p ( D )   → S 3p 4p( F ) → S
2
3
at 288.18 nm
In the present study, we apply the high-resolution DC
and then accelerated through the multilens velocity mapping
assembly optimized for DC slicing of the ion cloud.
1
slice imaging technique to detection of the S( D ) photo-
2
fragments following excitation of OCS at 288 nm. Even
though this photolysis wavelength is placed far away from
the weak absorption maximum of OCS around 230 nm, and
the photoabsorption cross section is very small (4, 5), the
yield of photodissociation is still sufficient to get meaningful
information about the dissociation dynamics related to the
vibrationally excited states of the parent molecules.
After flying through the field free flight path, ions were
impacted on a position-sensitive dual MCP/phosphor screen
detector of 120 mm diameter (Burle electro-optics). The
flight path was extended up to 192.5 cm from the interaction
region to the front face of the MCP to improve the velocity
resolution. In addition, a repeller voltage was set at +500 V,
keeping the optimized velocity mapping condition to stretch
the photofragment ion cloud along the time-of-flight axis to
the order of about 400 ns. We applied a narrow time gate
Experimental
(
effectively 50 ns) at the detector to select the central section
The DC slice imaging apparatus used in this study, shown
in Fig. 1, has been described in more detail elsewhere in our
previous work (15) and only a brief description will be given
here. A pulsed supersonic molecular beam of OCS is formed
by expanding OCS molecules diluted about 30% with Ar gas
into the source chamber of our differentially pumped appara-
tus at a backing pressure of 3 bar (1 bar = 100 kPa). After
passing through a skimmer and collimator, the molecular
beam enters into a velocity mapping ion optics assembly
of the distribution. The fact that the slice is a finite fraction
of the recoil velocity distribution, and in fact this fraction is
itself speed dependent, might suggest that the accurate speed
dependence will not be faithfully reproduced by the slicing
technique. However, extensive Monte Carlo simulations have
shown that the speed dependence of the distribution is re-
markably faithful over all recoil speeds, although the veloc-
ity resolution does suffer for the poorly sliced slow
fragments. The reasons for this are subtle and nonintuitive,
©
2004 NRC Canada

8
82
Can. J. Chem. Vol. 82, 2004
1
and beyond the scope of the current paper. However, a de-
tailed treatment of these issues is in preparation. The veloc-
ity resolution of the experiment is largely determined by the
Fig. 2. DC sliced image of the S( D2) from the photodissociation
of OCS at 288 nm. The probe laser polarization is aligned verti-
cally in the plane of the image.
+
S recoil owing to excess energy in the ionization step. This
–
1
is about 18 m/s, (implying an energy resolution of ~60 cm
at the v = 2 peak) and is large enough that neighboring rota-
tional levels will not be resolved.
The image was accumulated for 5.4 × 10 laser shots. In
5
this case, it was not necessary to scan the probe laser wave-
length owing to the small Doppler width of the transition
and the relatively broad laser line width. The image magnifi-
cation factor (1.157) was obtained from Simion simulations
of the ion optics. These simulations were also found to give
accurate reproduction of the focusing conditions and ion
time-of-flight.
Results and discussion
1
Figure 2 shows a DC sliced image of the S( D )
2
photofragments from the photodissociation of OCS at
2
88 nm. The electric vector of the photolysis light is parallel
to the vertical direction in the plane of the image. As can be
seen in Fig. 2, four distinct rings are clearly visible with dif-
ferent intensities. All of the rings exhibit strong anisotropic
angular distributions corresponding to a perpendicular tran-
sition, in which the transition dipole moment is perpendicu-
lar to the recoil direction. Thus, it is immediately apparent
that the contribution from the 1 A′′( Σ ) state, which is ac-
cessed through a perpendicular transition, is predominant.
According to previous results by Suzuki and co-workers (9,
erations predict modest rotational excitation in the CO
photoproduct, discussed further in the next section.
1
1 –
Figure 3(a) gives the translational energy distribution de-
termined from the sliced image. The peaks in the distribu-
tion clearly correspond to a hot-band bending progression in
the parent molecule, showing a maximum at two quanta ini-
tial excitation. If we assume no rotational excitation in the
parent molecule, then the translational energy origins for the
hot bands should be 151, 671, 1198, 1724, and 2257 cm–1
1
1), this observation can be interpreted more clearly from
1
the result that the image of fast S( D ) photofragments
2
changes from rather isotropic distribution (β ~ 0) to the
anisotropic distribution (β < 0) corresponding to a perpen-
dicular transition with changing photoexcitation wavelength
from 223 to 248 nm. This implies that the contribution from
for v = 0–4, respectively (21). The deviations of the peak
2
1
1
the 2 A′( ∆) excited state in a mixed transition (9, 18) attrib-
maxima from these positions may be used to estimate (albeit
crudely) the level of rotational excitation in the CO product.
On this basis, we find the most probable J values for the CO
1
1
1
1 –
uted to both 2 A′( ∆) and 1 A′′( Σ ) excited states relatively
decreases for decreasing excitation energy. In addition, al-
though the bimodal distribution is still shown at 248 nm in
the previous works, the contribution of the slow component
corresponding to a non-adiabatic process through only the
product to be 8, 12, 13, and 14 for v = 1–4, respectively.
2
For the most intense peak (v = 2), this implies an exit im-
2
pact parameter of 0.43 Å. By way of comparison, Suzuki et
al.’s results at 248 nm (9), show the product CO rotational
distribution peaking at a roughly J = 32, corresponding to an
exit impact parameter of 0.52 Å, in quite reasonable agree-
ment with our determination. The increasing rotational exci-
tation we observe with increasing hot-band excitation is a
natural consequence of the increasing product orbital angu-
lar momentum (i.e., the increasing relative velocity between
the photofragments).
1
1
2
A′( ∆) excited surface is considerably reduced due to the
lowering of the excitation energy. These previous experi-
1
1
mental investigations strongly suggest that the 2 A′( ∆) ex-
cited surface cannot be reached with a 288 nm photolysis
wavelength because of insufficient energy. Our observed im-
age is also consistent with this expectation, showing a pre-
dominantly perpendicular transition related to the exclusive
1
1 –
excitation of the 1 A′′( Σ ) excited state.
The available energy for our system is 151 cm–1 from en-
These results show for the first time the contribution of
ergy conservation using the dissociation energy D (0 K) of
the hot-band excitations up to v = 4 in photodissociation
0
2
–1
2
5 311 ± 72 cm recently reported by Janssen and co-workers
originating from the different bending excited states of OCS
parent molecules in a cold molecular beam. As seen in Ta-
ble 1, the branching ratios for the contribution from the ini-
tial vibrational bending state of OCS were determined by the
integration of each contribution obtained in Gaussian fits to
the peaks, a somewhat arbitrary choice that nevertheless
gave good fits to the resolved peaks. Our results clearly
show that the absorption cross section for bend-excited OCS
(
(
(
12), (this gave better results than scaling the NIST values
19) to 0 K), the spin-orbit energy of S( D ) (9239 cm )
20) with respect to the ground state S( P ), and the photon
1
–1
2
3
2
energy at 288.18 nm. There is insufficient energy for vibra-
1
+
tional excitation of CO(X Σ ) photofragments following dis-
sociation at 288 nm. Given the low recoil energy and the
rotationally cold starting sample, angular momentum consid-
©
2004 NRC Canada

Kim et al.
883
1
Fig. 3. The solid line represents the translational energy distribution for the S( D ) photofragments derived from DC sliced image in
2
Fig. 2. This was fitted to a sum of five Gaussians to generate the data in Table 1. The dotted line shows the speed dependence of the
1
anisotropy parameter (β) for the S( D ) photofragments.
2
Table 1. Contributions and relative photodissociation cross section ratios at 288 nm for different initial
states of OCS.
Relative cross section
Relative thermal population
ratio (σ(0 ν 0)/σ(0 1 0))
2
OCS vibrational
Branching
ratio
state (0 v 0)
298 K
200 K
298 K
200 K
2
(
(
(
(
(
0 0 0)
0 1 0)
0 2 0)
0 3 0)
0 4 0)
≤0.01*
0.15
0.52
0.20
0.13
1
1
na
1
44
2.2×10
1.8×10
na
1
1.5×10
2.6×10
7.8×10
–
–
–
–
2
3
4
5
–2
–4
–5
–7
8.11×10
6.37×10
5.02×10
3.84×10
2.37×10
5.35×10
1.21×10
2.64×10
2
3
4
2
3
Note: na, not available.
*
Indicates a very small or negligible contribution to the kinetic energy release.
1
1
–
to the 1 A′′( Σ ) surface at 288 nm is greatly enhanced rela-
tive to the vibrationless molecule. If we assume a Boltzmann
distribution to calculate the relative thermal populations for
each excited vibrational bending state relative to ground
state of OCS parent molecules at 298 K and 200 K (the ac-
tual vibrational temperature of the beam is not well known),
photodissociation cross section for v = 1 is at least 7 times
2
larger than that of v = 0 at 230 nm excitation, our results in-
2
dicate that the cross sections of hot bands from bending ex-
cited states of OCS are significantly enhanced at 288 nm
excitation relative to that at 230 nm. It is likely owing to
several key factors: the transition moment increases, as
shown theoretically by Suzuki et al. (9); the energy gap be-
tween the two surfaces drops dramatically with the bend an-
gle; and because the bending excitations of OCS parent
molecules considerably increase Franck–Condon overlaps
between the ground electronic state and excited states at the
lower excitation energy region (10–12).
the relative cross section ratio compared to the v = 1 vibra-
2
tional bending state can be simply estimated as given in
Table 1. It should be noted that since the vibrational temper-
ature is possibly much lower than the values used to calcu-
late the relative thermal populations in this study, the
relative cross section ratios here are a lower limit. Since the
vibrational origin gives a negligible contribution to the
translational energy release, we only indicate an upper
bound for its relative population. In comparison, even
though Katayanagi and Suzuki (11) reported the
Also shown in Fig. 3 is the recoil speed dependence of the
1
anisotropy parameter (β) for the S( D ) photofragments ex-
2
tracted by least-squares fit of the angular distribution with
respect to recoil velocity with the following equation:
©
2004 NRC Canada

8
84
Can. J. Chem. Vol. 82, 2004


1 
4π
the National Science Foundation under award number CHE-
I(θ) =   [1 + βP (cosθ)]
2
0
102174.
where θ is the recoil angle relative to the electric vector of
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7
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We have studied the photodissociation dynamics of OCS
at 288 nm by applying the high-resolution DC slice imaging
technique. The results obtained clearly showed that there are
2
progressions up to v = 4 in dissociation starting from the
2
bend-excited vibrational states of OCS parent molecules in a
cold molecular beam. Large negative anisotropy parameters
suggest that the photodissociation of OCS at 288 nm purely
2
2
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1 –
occurs on the 1 A′′( Σ ) bending excited potential surface
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photodissociation dynamics.
Acknowledgements
The authors would like to acknowledge Dr. D. Townsend
and M.P. Minitti for their help. This work was supported by
©
2004 NRC Canada