J . Phys. Chem. 1988, 92, 5875-5879
5875
Electronic Spectrum of OCS at 62 000-72 000 cm-' M. I. McCarthy and V. Vaida* Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-021 5 (Received: September 9, 1987; In Final Form: April 6. 1988)
We report the absorption spectrum of jet-cooled OCS in the region 62000-72000 cm-'. This spectrum is attributed to two ;z 157 nm. The effect of cluster electronic states: the linear 'Z' state originating at 154.5 nm and the bent 'n state with ,A, formation on the absorption spectrum is investigated by increasing the OCS stagnation pressure. The spectroscopic effects caused by intermolecular interactions are discussed for both electronic states.
Introduction The photodissociation of triatomic molecules is an area of current research.' Recently, particular emphasis has been given to properly characterizing the reactive potential energy surfaces and the energy disposal in the resulting photofragments. The most straightforward application of theoretical dynamics for understanding experimental product-state distributions is the case of a collinear dissociation of a triatomic molecule.2d Such a system can be modeled as a dissociation along the reaction coordinate from a linear geometry in the excited electronic state. Experimentally, it has been difficult to locate examples of linear dissociative excited states in triatomic molecules. The lZ+linear excited state of OCS, reported to appear around 150 nm, is one of the few examples under study.7-" Following excitation to the I F state, dissociation to form CO S is expected to proceed along a collinear coordinate. As a result, the 140-160-nm region in OCS has been the focus of many theoretical2" and experimental photofragment s t ~ d i e s . ' ~ *In' ~light of the interest in the photochemical dynamics of this molecule, a thorough understanding of the spectroscopy of the reactive excited state is necessary. This paper reports the absorption spectrum at 62 000-72 000 cm-' (1 40-1 60 nm) of OCS cooled in a supersonic expansion. In this molecule, the dissociation that occurs within 1 ps produces broad homogeneous vibronic line widths.I0 These large bandwidths make quantitative structural information difficult to obtain but provide insight into the excited-state dynamics. Reactive molecules generally undergo a large change in geometry on electronic ex~ i t a t i o n . ~The technique of direct absorption spectroscopy of jet-cooled molecules is ideally suited for studying systems that undergo significant structural changes and rapid diss~ciation.'~ Rotational and vibrational cooling of the molecule reduces inhomogeneous effects and directly probes the short-time dissociation dynamics. The large Franck-Condon envelope which results from
TABLE I: Absorption Spectrum Parameters Measured for Room-Temperature '*OCS and OC% band position u, cm-'
A, 8,
(12) Black, G.; Sharpless, R. L.; Slanger, T. G.; Lorents, D. C. J . Chem. Phys. 1975, 62, 4214. (13) Ondrey, G . S.; Kanfer, S.; Bersohn, R. J . Chem. Phys. 1983, 79, 179. (14) Vaida, V. Ace. Chem. Res. 1986, 19, 114.
0022-3654/88/2092-5875$01.50/0
Au, cm-I
assignt
180CS
+
(1) Leone, S. R. Adu. Chem. Phys. 1982,50, 255. Simons, J. P. J . Phys. Chem. 1984,88, 1287. Bersohn, R. J . Phys. Chem. 1984, 88, 5145. (2) Shapiro, M. Isr. J . Chem. 1973, ZZ, 691; J . Chem. Phys. 1972, 56, 2582. (3) Band, Y. B., Freed, K. F. J . Chem. Phys. 1975, 63, 3382. (4) Holdy, K. E.; Klotz, L. C.; Wilson, K. R. J . Chem. Phys. 1970, 52, 4588. (5) Atabek, 0.; Beswick, J. A.; Lefevre, R.; Mukamel, S.; Jortner, J. J . Chem. Phys. 1976, 65, 4035. (6) Heller, E. J. J. Chem. Phys. 1978, 68, 2066. (7) Herzberg, G.Electronic Structure of Polyatomic Molecules; Van Nostrand: New York, 1966. ( 8 ) Lochte-Holtgreven, W.; Braun, C. E. H. Trans. Faraday Soc. 1932, 28, 698. (9) Forbes. G.S.: Cline. J. E. J . Am. Chem. Soc. 1939. 61. 151.
isotopic shift,' cm-I
1578.6 1567.6 1562.5 1557.2 1549.2 1545.9 1542.3 1532.2 1526.4 1522.0 1508.7 1501.0 1491.3 1482.1 1474.0 1465.1 1457.8 1448.6 1443.6 1432.7
63346 63793 63999 64217 64549 64687 64837 65264 65513 65710 66280 66620 67057 67472 67842 68254 68594 69034 69271 69798
-37 -34 12 -36 -8 5 14 -20 87 32 -34 37 123 38 86 35 115 35 127 41 138
1590.5 1585.1 1579.1 1568.7 1564.1 1558.1 1549.5 1545.8 1542.7 1530.4 1526.0 1523.1 1508.4 1490.7 1473.1 1458.3 1442.8
62872 63089 63325 63748 63934 64181 64538 64692 64821 65341 65532 65656 66294 67084 67883 68572 69312
-18
447 424
nu2
( n + l)uz ( n + 2)u2 0-0
826 767 1933 777 852 785 782 752 780 677 764
VI
2ul ~3
3u,
+ ~1 +2~1 ~3 + 3Ul 6~1 ~3 + 4Ul
~3
4ul ~j
0~34s
(I
-16 11 77 0 -74 9 -4 10 13 11 23 11 -6 57 0
nu2
453 423
( n + 1)u2 ( n + 2)u2
433
( n + 3)u2 0-0
840
~1
762 790 799 689 740
2vl 3Ul
4u, 5Ul 6~1
Isotopic shifts correspond to (OCS- isotope).
significant geometry changes during excitation gives qualitative information about the structural changes involved in the transition. The jet-cooled, direct absorption technique employed here is ideally suited for the reexamination of the spectroscopy of OCS. In addition, the current work uses clusters to investigate the effects of intermolecular interactions on the spectrum of OCS in this energy region. These results are discussed in light of previous studies on the dissociation dynamics of the m ~ l e c u l e . ' * ~ ' ~ ~ ' ~ (15) van Veen, N.; Brewer, P.; Das, P.; Bersohn, R. J . Chem. Phys. 1983, 79, 4295.
0 1988 American Chemical Society
McCarthy and Vaida
5876 The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
WAVELENGTH
8,
(
WAVELENGTH (
) 1400
15pO
1450
14pO
15PO
8,
) 15pO
I5,OO
14,SO
16?0
16,OO
>
a
FYz W I-
E
W
zt 4W U
I 70
,
I 68
I 66
1
t
64
62
FREQUENCY ( x 1 0 3 cm-‘ ) Figure 2. Jet-cooled absorption spectrum of OCS from 140 to 160 nm. Scans were run with 15 Torr of OCS seeded in N2 to 100-Torr total stagnation pressure. TABLE 11: Absorption Spectrum Parameters Measured for Jet-Cooled OCS‘,b
band position P , cm-I
A, 8,
I
72
I
I
I
I
70
I
68
FREQUENCY
I
I
88
I
64
1
62
( x103 cm-i
1
Figure 1. Room-temperature vapor absorption spectrum of OCS and isotopically labeled samples: (a) OCS, (b) I8OCS,(c) OC34S. All traces were run with 15-20 Torr in the cell.
1606.6 1593.7 1561.1 1581.1 1568.4
62 243 62 746 64 058 63 247 63 757
1545.5 1549.5 1541.5 1530.6 1525.3 1521.1 1507.2 1498.5 1490.2 1480.1 1472.7 1461.5 1457.0
64 704 64 537 64 870 65 332 65 561 65 142 66 347 66 733 67 106 67 563 67 906 68 421 68 633
Au, cm-’
assignt
503
(n + l b 2
501 510
(n + 2 b 2 (n + 312.
v2
0-0
857 786 2029 759 830 800 858 727
VI 211
v3
3u1 u3 +
PI
4u1 u2
+ 2P‘
5U‘
“The,,A of the broad bands from 62 243 to 63 757 cm-I (Figure 3) are accurate to *50 cm-’. bThe positions of the bands from 64704 to 68 633 cm-’ are accurate to *5 cm-I.
2 represents an average of 33 scans. A series of data were taken Experimental Section with different stagnation pressures behind the nozzle, and these The technique of UV/vacuum-UV direct absorption specare shown in Figure 4. Figure 4a is the average of 12 scans taken troscopy of supersonic jets and its applications to the study of with 90 Torr of OCS seeded in N2 at a stagnation pressure of 180 photodissociating molecules have been described p r e v i ~ u s l y . ~ ~ ~ ” ,Torr. ~ ~ Figure 4b is the average of 14 scans of 200 Torr of neat In the present experiment, the source of vacuum-UV radiation OCS. Figure 4c is the average of eight scans of 300 Torr of neat is an Opthos krypton-filled electrodeless lamp powered by an OCS. All the reported data were taken with an instrumental Opthos microwave generator. The vacuum-UV radiation is resolution of 16 cm-I. dispersed by a 1-m Acton monochromator and crosses the sample beam 0.5 cm below the 1-mm nozzle. Details of the signal colResults lection and processing have been reported earlier.’* Carbonyl The room-temperature absorption spectra of OCS, I8OCS,and sulfide, 16012C32S, was obtained from Matheson. The labeled OC34Svapor in the region 140-160 nm are presented in Figure isotopes l80CS and OC34Swere obtained from ICON Services 1 and Table I. A structured progression appears atop a continuum Inc. All samples were used without further purification. 18012C32S, and that peaks at e157 nm. The spectra of 16012C32S, The room-temperature cell spectra of OCS, l80CS,and OC34S 16012C34S show qualitatively the same features. Figure 2 and were taken in static cells with 15-20-Torr pressure (Figure 1). Table I1 show the jet-cooled absorption spectrum of the natural Fifteen to twenty scans of each were averaged. The jet-cooled OCS isotope in this region. These data were obtained with 15 spectrum (Figure 2) was taken at a stagnation pressure of 100 Torr of OCS seeded in N 2 at 100-Torr stagnation pressure. Torr, with OCS seeded in N2 at a 6:l (N2/OCS) dilution. Figure Cooling the OCS sample in this manner reduces the background continuum by about 25%. The low rotational temperature ob(16) Sivakurnar, N.; Burak, I.; Cheung, W.-Y.; Houston, P. L.; Hepburn, tained in the jet reduces the inhomogeneous line broadening effects J. W. J . Phys. Chem. 1985.89, 3609. and results in narrower observed vibronic bands. The 155-162-nm (17) Leopold, D. G.; Vaida, V.; Gra3ville. M. J. J . Chem. Phys. 1984, 81, region is enlarged and presented in Figure 3. These data were 421. taken in the jet with 170 Torr of neat OCS. (18) McCarthy, M. I.; Vaida, V. J . Phys. Chem. 1986, 90, 6759
Electronic Spectrum of OCS at 62 000-72 000 cm-I
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 5877
WAVELENGTH 1400
1460
(
A
)
1160
1600
1600
F R E O U E N C Y ( x i 0 3 cm-1)
Figure 3. Jet-cooled absorption spectrum of OCS: (-)
assiant VI
u2
u3
b
I-
TABLE 111: Normal Modes of OCS
o=c=s o=c=s o=c=s
m
a
peak width (fwhm).
normal mode"
I
170 Torr of neat
OCS;(---) Guassian model of a 5 0 0 - ~ m -progression ~ with 300-cm-'
ground-state frea, cm-I 859 527 2062
'Normal modes are similar to those seen in ClCN (ref 23). *Ground-state frequencies from ref 9.
The structured progression in the spectrum of jet-cooled OCS originates at 64 700 cm-' and has an average spacing of 786 cm-'. The normal modes in the ground state of OCS are shown in Table 111. The v 1 mode has been described as a "CS stretch" or "symmetric stretch" and has a frequency of 859 cm-' in the ground state.'JO The bending v2 mode has a ground-state frequency of 527 cm-'. The u3 mode is described as a "CO stretch" or an "asymmetric stretch" and is 2062 cm-' in the ground state. The main band in this excited state is assigned to a progression in the v 1 mode. The splittings observed in the first two bands in the jet-cooled spectra probably result from the rotational profile. Three to five quanta in a second progression appear at the blue end of the spectra. This second progression is assigned to u3 + nu1. The shift of the v3 frequency in the '*OCS spectra (Figure lb) supports this assignment. The high-energy end of the OC34S spectrum shows diffuse features which appear as shoulders on the v 1 progression. These bands have not yet been unambiguously assigned. A jet-cooled spectrum of the low-energy region (62 000-64 000 cm-') is presented in Figure 3. Four bands in a broad, diffuse progression with an average spacing of 500 f 50 cm-' are evident. This progression has A, = 157 nm. All of the room-temperature data also show at least two members of this diffuse, but identifiable, progression. The 500-cm-I frequency corresponds to the v2 bending mode. Figure 4 illustrates the effect on the absorption spectrum of increasing the stagnation pressure and consequently increasing the degree of cluster formation. A significant change in the observed features occurs as the back-pressure of OCS is increased. The most pronounced effect is evident at low energies. The diffuse bands centered around 157 nm appear to gain enough intensity in this region to overwhelm the highly structured v I progression.
Discussion Detailed information about the spectral region 62 000-72 000 cm-' (140-160 nm) in OCS is difficult to obtain due to the rapid dissociation occurring at these energies. In light of recent experimental' ',I3 and theoretica12-6interest in the photodissociation dynamics of OCS at 157 nm, a reexamination of the spectroscopic data is particularly important. The following discussion examines the new spectroscopic data presented here in the context of the electronic structure and photodissociation dynamics of this molecule. This discussion contains two parts. The first part
z w
$ d K c
q
I
72
I TO
I
08
FREQUENCY
I
66
I 04
I
62
( x103 cm-')
Figure 4. Jet absorption spectrum of OCS with increased OCS stagnation pressure: (a) 90 Torr of OCS in 180 Torr total with N2; (b) 200 Torr of neat OCS; (c) 300 Torr of neat OCS.
considers the assignment of the spectrum in this energy region. The second part examines the effect of cluster formation on the spectrum. Electronic Structure. The spectrum at 62 000-72 000 cm-' is reassigned in light of the data obtained by jet-cooling the molecule. The intense structured band originating at 64700 cm-' in the jet-cooled spectrum is assigned to a progression in v 1 in the linear-linear 'E' transition. At energies below 64 000 cm-l all of the spectra measured here show a broad, diffuse band with ,,A = 157 nm. This feature has previously been assigned as part of the IE' 'E' transition.I0 The present work suggests that this broad band is due to a transition to a 'n state. An assignment of an additional 'n electronic state in this region is consistent with the observed diffuse progression at low energies. Our arguments for this assignment are as follows: 1. The features in the vicinity of 157 nm are not consistent with the Franck-Condon envelope of a single transition ('Z' IZ+).The correlation diagram (Figure 5) and the electronic structure calculation^^^ on an analogous system, C 0 2 ,indicate that the only other electronic state expected at an energy near
-
-
-
(19) England, W. B.; Ermler, W. C.;Wahl, A. C.J . Chem. Phys. 1977, 66, 2336.
McCarthy and Vaida
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
5878
+
-
COOV
t 0 (1s)
+ s
+ O 0- i
0
t s
cs ('E+) + s
cs
+
co
OCS
0
+
s
CS
4 -% 1 12
10
cs
+
co
ocs
0
+
s
Figure 5. Correlation diagram for the dissociation of OCS to form CS + 0 (left) and CO + S (right): (a) in C,, symmetry; (b) in C, symmetry.
the '2' state is of II character. 2. The progression evident in Figure 3 has a frequency of 500 f 50 cm-I and is assigned to the 2 bending mode. This progression is extremely diffuse; therefore, an accurate determination of the frequency is not possible. However, the values obtained are inconsistent with the 786 f 10 cm-l progression in v 1 evident in the '2' IZ+transition (Table 11). In addition, the v2 bending mode is expected to be active in the 'II excited state, but not in the linear lZ+state. Previous work showed that excitation of the bending Z ' (Arnx mode dominates the spectrum of the lower energy 'II I i= 167 nm) transition.'* The calculated spectrum (Figure 3b) is modeled by using four Gaussians with a 500-cm-' spacing between the centers and a fwhm = 300 cm-' for each band. This model accurately reproduces the diffuse structure present in the data. 3. The line widths observed in the proposed 'II state are much broader than the vibronic bands seen in the I Z ' state. This suggests different dynamical channels are available for dissociation from the two excited electronic states. It seems unlikely that a single electronic state would show vastly different dynamic effects over this energy region. 4. There is also supporting evidence for the presence of this 'II state from previous studies.20,21 The molecular orbital configuration of OCS in the ground state is
-
-
OCS(XI Z+) : (6a)2(7a)2(8u)2(9a)2(2a)4(3a)4(4a)o(lOu)O( 11u)'
-
-
-
-
The I Z ' I Z ' results from a 3.ir 4.ir excitation. The I Z ' I I I transition is a 3a 110 excitation.' Matsunaga and Watanabe20 report the presence of a broad shoulder on the red end Z ' IZ+transition in their photoelectron ionization of the I potential studies. LeClerc et aL2' report in electron loss spec-
-
(20) Matsunaga, F. M.; Watanabe, K. J. Chem. Phys. 1967, 46, 4457. (21) LeClerc, B.; Poulin, A,; Roy, D.; Hubin-Franskin, M.-J.; Delwiche, J . J . Chem. Phys. 1981, 75, 5329.
troscopy the presence of a progression in the 311 state from the 11a excitation. This state falls to the red of the state. 3a Those authors claim that the asymmetry of the low-energy end Z ' 'II (37 of the '2' state is due to the presence of the I 11a) transition. Ab initio studies on C 0 2 predict the 'II state to be energetically in the region of the IZ+state.I9 The presence of an additional electronic state in the 157-nm region poses serious problems to the interpretation of product distributions. The current work does support the claim that the 'Z' state is linear. However, the observation of the v2 in the 'II state is indicative of a bent geometry. Many of the photoproduct Z' state have been done using the 157-nm line studies on the I of a F2 l a ~ e r . ' ~The , ~ ~current work indicates that excitation at 157 nm will probe predominantly the bent I I I excited state. The quantum yield studies of S(IS) from OCS over the 140160-nm region show a sharp onset of S('S) production at 140 nm1.I2 The S(lS) tails off more gradually in the 160-nm region. The collinear dissociation (Cmusymmetry) of OCS from the '2' state correlates to production of S(lS) (Figure 5). If the OCS molecule is bent, the symmetry is reduced to C, and the 'II state will correlate to CO S('S). The shape of the S(lS) production curve Z ' state at high in the Black et a1.I2 study could result from the I energies and in the low-energy region from the 'II state. Houston reports from his studies on the recoil of the C O fragment that the C O is formed with a rotational temperature of 700-1400 K from dissociation at 157 nm.22 This distribution is consistent with some excitation in the v2 bending mode in this region. However, preliminary results22 show that excitation at 157 nm involves a parallel transition, but the calculated /3 value of
