J . Phys. Chem. 1990, 94, 6201-6208
-
-
20-valence-electron XY2 molecule, the lowest energy transition could be either the allowed B2 A, or the forbidden A2 A I . A I which include Vibronically allowed transitions such as A, a change in the asymmetric stretch quantum number should, however, be missing the 000-000 band as well as those transitions of the form 000 LMO. We are reasonably confident of our identification of both types of progressions 000 LMO and 000 L M I . Alternative assignments were not nearly as consistent or reasonable. One possibility that has been considered is that we are seeing transitions of the form 000-LMO and 100 LMO which would not involve the asymmetric stretch. This assignment could be made if the vl' frequency is coincidentally 817 cm-I. Several factors would seem to oppose this. Population of this vibration level in the excited electronic state would imply that the v i bend would also be excited; however, progressions of the form 010-LMO are rather weak. Also, no isotopic shifts in the opposite direction of those in Figure 4 are detected. We, therefore, lean toward an assignment of this transition to the forbidden A2-AI, where progressions built upon the asymmetric stretch frequency are expected. The existence of the progressions not involving the asymmetric stretch would be unusual, however, although these transitions could gain intensity through magnetic dipole selection rules or another coupling
-
-
-
-
-
-
6201
mechanism as have been observed in the AIAz RIA2 transition in thi~formaldehyde.~~ Unfortunately, the results of the electronic energy calculations on SFz summarized in Tables V and VI do not resolve the issue of whether the spectrum is due to an electric dipole allowed or vibronically allowed transition. The character of the excited state should be relatively easy to determine by an absorption spectroscopy experiment. A wide scan of moderate resolution could determine the possible activity of the three excited-state frequencies. A high-resolution scan of several bands would determine their type and hence the symmetry of the electronic excited state. Acknowledgment. R.J.G. and F.W.K. separately acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. C.D.T. thanks the Chapter 606 Student Monies Allocation Committee of TTU for the purchase of a sample of the isotopic CS,. We are grateful to H. R. Martin for the precise calibration of the monochromator. We thank D. C. Moule for pointing out the unusual appearance of the asymmetric stretch. Registry No. SF2. 13814-25-0;F2,7782-41-4; CS2, 75-15-0; OCS, 463-58-1; S,7704-34-9; "S, 13981-57-2;34S, 13965-91-4. ~~~
(19) Judge, R. H.; King, G. W. J. Mol. Spectrosc. 1979, 74, 175.
Vacuum-Ultravlolet Absorption and Fluorescence Excitation Spectra of IC1 Kenneth P. Lawley, Elinor A. Kerr,+ Robert J. Donovan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K.
Andrew Hopkirk, David Shaw, SERC Daresbury Laboratory, Daresbury. Warrington WA4 4AD, U.K.
and Andrew J . Yencha* Department of Chemistry and Department of Physics, State University of New York at Albany, Albany, New York 12222 (Received: October 9, 1989; In Final Form: March 15, 1990)
Absorption and fluorescence excitation spectra for IC1 in the vacuum-ultraviolet (1 25-195 nm) have been recorded using tunable synchrotron radiation. Fluorescence occurs mainly from the E(0') ion-pair state, and pronounced resonance structure is observed. These resonances result from an unusual three-state interaction. Both homogeneous and heterogeneous coupling between the E(0') ion-pair state and degenerate Rydberg states takes place. Pronounced broadening of the Rydberg levels occurs through heavy predissociation by a repulsive valence state or states.
Introduction The amount of information available concerning the higher excited states of diatomic halogens and interhalogens was sparse until the advent of synchrotron radiation. The majority of the earlier work was on the absorption spectra in the vacuum-ultraviolet (VUV), recorded using photographic methods, which yielded a reasonably complete picture of the Rydberg-state manifolds of ICI,l I,? and BrP3 More recently, synchrotron radiation has been used to study the absorption and fluorescence properties of CI2$-' Br2,* BrCI,IOICI," and IBrI2J3 in the VUV spectral range. The earliest report on the VUV absorption spectrum of IC1 was a photographic study in the wavelength range of 167-191 nm, in which two band systems were identified and ana1y~ed.I~ These same two (Rydberg) systems were again observed photographically much later on with the additional discovery of two new systems between 139 and 191 nm.I5 An analysis of these 'Present address: Structural Materials Branch, BP Research Centre, Chertsey Road, Sunbury-on-Thames T W 16 7LN, U.K.
OO22-3654/90/2094-620 1$02.5O/O
newly found Rydberg systems followed, and, in addition, preliminary assignments were made on numerous other Rydberg tran( I ) Venkateswarlu, P. Can. J . Phys. 1975, 53, 812. (2) Venkateswarlu, P. Can. J . Phys. 1970, 48, 1055. (3) Venkateswarlu, P. Can. J . Phys. 1969, 47, 2525. (4) Wormer, J.; Moller, T.; Stapelfeldt, J.; Zimmerer, G.; Haaks, D.; Kampf, S.; LeCalvi, J.; Castex, M. C. Z . Phys. D 1988, 7 , 383. ( 5 ) Lee, L. C.; Suto, M.; Tang, K. Y. J . Chem. Phys. 1986, 84, 5277. (6) Moller, T.; Jordan, B.; Zimmerer, G.; Haaks, D.; LeCalvE, J.; Castex, M.-C. Z . Phys. D 1986, 4 , 73. (7) Moeller, T.; Jordan, B.; Gfirth, P.; Zimmerer, G . ;Haaks, D.; LeCalvt, J.; Castex, M.-C. Chem. Phys. 1983, 76, 295. (8) Austin, D. I.; Donovan, R.J.; Hopkirk, A,; Lawley, K. P.; Shaw, D.; Yencha, A. J . Chem. Phys. 1987, 118, 91. (9) Donovan, R. J.; MacDonald, M. A.; Lawley, K. P.; Yencha, A. J.; Hopkirk, A. Chem. Phys. Lett. 1987, 138, 571.
(IO) Hopkirk, A.; Shaw, D.; Donovan, R. J.; Lawley, K. P.; Yencha, A. J. J . Phys. Chem. 1989, 93. ( I I ) Kerr, E.; MacDonald, M.; Donovan, R. J.; Wilkinson, J. P. T.; Shaw, D.; Munro, I. J . Photochem. 1985, 31, 149. (12) Yencha, A. J.; Donovan, R. J.; Hopkirk, A,; Shaw, D. J. Phys. Chem. 1988, 92, 5523.
0 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
sitions observed in a study covering the wavelength range of 130-1 60 nm.I6 A comprehensive, high-resolution photographic study was subsequently performed over the wavelength range of 122-190 nm, in which 23 Rydberg series were identified, converging to the two spin-orbit components of the ground-state ion, IC1’ (2n3/2,1/2).’ The presence of ion-pair states in the same spectral region as the Rydberg states was inferred both from the study of discharge-excited emission spectral7 and more recently from the extensive characterization of many of these states using laser optical-optical-double-resonance (OODR) techniques.’*-22 The data obtained from OODR studies reveal that the ion-pair state absorptions in the VUV are frequently obscured by the stronger Rydberg transitions, because the ion-pair states have very deeply bound wells with large equilibrium bond lengths; this leads to extended absorption systems with very dense structure that is difficult to resolve. More recently, observations in the VUV using synchrotron radiation have shown in combined absorption and fluorescence excitation studies some resolved components of the ion-pair systems in Br,8 and 12.23 Additional fluorescence excitation studies using coherent, monochromatic, tunable VUV radiation generated by four-wave sum-mixing in Mg vapor have been performed on Br24*25 and IC126in which ion-pair states have been probed in detail. Ion-pairformarion (Le., X+ + Y-) has also been detected mass spectrometrically in the halogen-containing C12,29Br2,3*33 I 2, 30-34-36 HF,27b,28 DF,2*C H 3 F,37 systems F2,27q28
( 1 3) Lawley, K. P.; Austin, D.; Tellinghuisen, J.; Donovan, R. J. Molec. Pfiys. 1987, 62, 1195. (14) Cordis, H.; Sponer, H. Z . Phys. 1932, 79, 170. (15) Donovan, R. J.; Husain, D. Trans. Faraday SOC.1968, 64, 2325. (16) Donovan, R. J.; Robertson, P. J. Spectrosc. Lett. 1972, 5, 281. ( I 7) Venkateswarlu, P.; Verma, R. D. Proc. Indian Acad. Sci. 1957, A47, 150. (18) (a) Brand, J . C. D.; Deshpande, U . D.; Hoy, A. R.; Jaywant, S. D. M.; Woods, E. J. J. Mol. Spectrosc. 1983, 99, 339. (b) Brand, J. C.; Hoy, A. R.; Risbud, A. C. J . Mol. Specfrosc. 1985, l l 3 , 4 7 . (c) Brand, J. C. D.; Deshpande, U.; Hoy, A. R.; Jaywant, S. M. J. Mol. Spectrosc. 1983, 100, 416. (19) (a) Ishiwata, R.; Tanaka, 1. Cfiem. Phys. Lett. 1984, 107, 434. (b) Ishiwata, T.; Fujiwara, 1.; Shinzawa, T.; Tanaka, 1. J. Cfiem. Phys. 1983, 79, 4779. (c) Ishiwata, T.; Takunaga, A,; Shinzawa, T.; Tanaka, I. J . Mol. Spectrosc. 1984, 108, 314. (d) Ishiwata, T.; Ohtashi, H.; Tanaka, I. J. Chem. Pfiys. 1984,81, 2300. (e) Ishiwata, T.; Ohtoshi, H.; Sakaki, M.; Tanaka, I. J. Chem. Phys. 1984, 80, 1411. (20) (a) King, G. W.; Littlewood, 1. M.; Robins, J. R. Cfiem. Phys. 1981, 62, 359. (b) King, G. W.; Littlewood, 1. M.; McFadden, R. H.; Robins, J. R. Cfiem. Pfiys. 1979, 41, 379. (c) King, G . W.; Littlewood, I. M.; Robins, J. R. Chem. Pfiys. 1981, 56, 145. (21) Rousseau, D. L.; Williams, P. F. Pfiys. Reu. Left. 1974, 33, 1368. (22) Donovan, R. J.; Holmes, A. J.; Langridge-Smith, P. R. R.; Ridley, T. J. Cfiem. Soc., Faraday Trans. 1988,84, 541. (23) Hiraya, A.; Shobatake, K.; Donovan, R. J.; Hopkirk, A. J . Chem. Phys. 1988, 88, 52. (24) Lipson, R. H.; Hoy, A. R.; Flood, M. J. Chem. Phys. Lett. 1988, 149, 155.
(25) Lipson, R. H.; Hoy, A. R. J. Mol. Spectrosc. 1989, 134, 183. (26) Lipson, R. H.; Hoy, A. R. J. Cfiem. Pfiys. 1989, 90, 6821. (27) (a) Dibeler, V. H.; Walker, J. A.; McCulloh, K. E. J . Chem. Phys. 1969,564592. (b) Dibeler. V. H.: Walker, J. A,; McCulloh, K. E. J. Chem. Pfiys. 1969, 51, 4230. (28) Berkowitz, J.; Chupka, W. A.; Guyon, P. M.; Halloway, J. H.; Spohr, R. J. Cfiem. Phys. 1971, 54, 5165. (29) Berkowitz, J.; Mayhew, C. A.; Ruscic, B. Cfiem. Pfiys. 1988, 123, 317. (30) Morrison, J. D.; Hurzeler, J.; Inghran, M. G.; Stanton, H. E. J. Cfiem. Phys. 1960, 33, 821. (31) Dibeler, V. H.; Walker, J. A.; McCulloh, K. E. J. Cfiem. Phys. 1970, 53, 4715. (32) Dibeler, V. H.; Walker, J. E.; McCulloh, K. E.; Rosenstock, H. M. Int. J. Mass Specfrom. Ion Pfiys. 1971, 7, 209. (33) Yencha, A. J.; Kela, D. K.; Donovan, R. J.; Hopkirk, A,; Kvaran, A. Cfiem. Pfiys. Letf. 1990, 165, 283. (34) Myer, J. A.; Samson, J. A. R. J. Cfiem. Pfiys. 1970. 52, 716. (35) Akopyan, M . E.; Vilesov, F. 1.; Sergeev, Y. L. Opt. Spectrosc. 1973, 35, 472. (36) Kvaran, A.; Yencha, A. J.; Kela, D. K.; Donovan, R. J.; Hopkirk, A. Cfiem. Pfiys. Submitted for publication. (37) Krauss, M.: Walker, J. A,; Dibeler, V. H. J. Res. Natl. Bur. Stand. 1968. 72A. 28 1.
Lawley et al. n 120 z
t
I
I
I
I
130
140
150
160
170
180
I90
Wavelength / nm
Figure 1. Vacuum-ultraviolet absorption spectrum of IC1 in the presence of C12 (1:2 mixture). Spectral resolution, AA = 0.1 nm. IC1 pressure = 25 mTorr, C12 pressure = 50 mTorr.
CH3C1,37-39CH2DC1,37and CH3Br,39as well as in the systems 02,4(t42 NO,42C0,42-43HCN,44 and S02.45 We have previously shown that the fluorescence spectra resulting from the excitation of ion-pair states in the VUV, using synchrotron radiation, lasers, or even a commercial spectroThis fluorimeter, can be an invaluable tool for their is a result of the fact that (1) the Rydberg states appear to be extensively predissociated (hence their fluorescence yields are very low) and (2) fluorescence from ion-pair states usually gives rise to oscillatory continuum emission in the visible and near-ultraviolet. In the past we have reported preliminary fluorescence excitation and absorption spectra for ICI” and also a study of the reaction of an ion-pair state of IC1 with Xe.SO In this paper we present a higher resolution and more detailed study of the absorption and fluorescence excitation spectra of IC1 in the VUV.
Experimental Section Full details of the experimental methods for collecting absorption and fluorescence excitation spectra have been published elsewhereI2 and only a brief resume will be given here. All the experiments were performed by using port 13.2 of the S E R C Daresbury Laboratory Synchrotron Radiation Source. All spectra were recorded in multibunch mode, in which 160 discrete bunches of electrons orbit in the synchrotron, thus providing a quasicontinuous excitation source. The synchrotron radiation was dispersed by using a normal incidence 0.5-m Seya monochromator and after focussing was passed into an aluminum sample chamber, which was connected to a gas-handling line. The absorption and fluorescence excitation spectra were recorded simultaneously over the wavelength range of 125-195 nm with a 0.1-nm excitation band-pass resolution on a mixture of IC1 + CI2 (1.2 ratio) at various total pressures by (a) monitoring the transmitted VUV light by using a sodium salicylate coated window and (b) measuring the total fluorescence signal at right-angles to the excitation beam axis through various filter/photomultiplier combinations (see figure captions for specific conditions). The (38) Dibeler, V. H.; Walker, J. A. J. Cfiem. Pfiys. 1965, 43, 1842. (39) Munakata, T.; Kasuya, T. Cfiem. Phys. Left. 1989, 154, 604. (40) Dibeler, V. H.; Walker, J. A. J. Opt. SOC.Am. 1967, 57, 1007. (41) Dehmer, P. M.; Chupka, W. A. J. Cfiem. Phys. 1975, 62, 4525. (42) Oertel, H.; Schenk, H.; Baumgartel, H. Cfiem. Pfiys. 1980, 46, 251. (43) Locht, R.; Diirer, J. M. Cfiem. Pfiys. Lett. 1975, 34, 508. (44) Berkowitz, J.; Chupka, W. A.; Walter, T. A. J. Cfiem. Pfiys. 1969, SO, 1497. (45) Dujardin, G.;Hellner, L.; Olsson, B. J.; Besnard-Ramage, M. J.; Dadouch, A. Pfiys. Rev. Lett. 1989, 62. 745. (46) Wilkinson, J. P. T.; MacDonald, M.; Donovan, R. J. J. Photochem. 1986, 35, 123. (47) O’Grady, B. V.; Donovan, R. J. Cfiem. Pfiys. Left. 1985, 122, 503. (48) Wilkinson, J. P. T.; MacDonald, M.; Donovan, R. J. Cfiem. Phys. Lett. 1983, 101, 284. (49) MacDonald, M.; Wilkinson, J. P. T.; Fotakis, C.; Martin, M.; Donovan, R. J . Chem. Pfiys. Lett. 1983, 99, 250. (50) Wilkinson, J. P. T.; Kerr, E. A.; Lawley, K. P.; Donovan, R. J.; Shaw, D.; Hopkirk, A.; Munro, I. Chem. Phys. Left. 1986, 130, 213.
IC1 Absorption and Excitation Spectra TABLE I: Band Heads for the b'6(O+) Rydberg Systems in IC1
-
X(O+) and b,(l)
AVP,b/nm u/cm-l
b'6(O+)
-
58 X(O+)
0-0
174.08 172.96 171.83 170.52 169.26 168.04 166.84 165.64 173.74 172.44 171.12 169.91
57445 57817 58197 58644 59081 59510 59938 60372 57557 57991 58438 58885
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The Journal of Physical Chemistry, Vol. 94, No. 16. 1990 6203 X(0')
Aulcm-' L
378 374 447 437 429 43 1 434
0. 6
..
,0. 4
-.
n m
x
; 0. 2 -. V
m
V
434 447 417
Main component assignment. Each of these bands has unresolved hot-band components. Estimated error in relative peak wavelength AX = 1 0 . 0 3 nm (*I0 cm-I). Absolute error of wavelength scale AA = f0.1 nm (*33 em-I). intensity of transmitted light through the empty cell was recorded immediately after the absorption spectrum in order to convert the transmitted light signal into an absorption cross section. Both types of spectra (absorption and fluorescence excitation) were corrected for the variation in synchrotron light intensity with time and for the transmission function of the Seya monochromator. The photomultiplier signals were sampled by using conventional single-photon counting techniques and digitized by using a PDP 1 1 /04 microcomputer and CAMAC interface. IC1 (BDH Chemicals Limited) of stated 98% minimum purity was used after being degassed at 0 OC and vacuum distilled, using liquid nitrogen, until an IC1 vapor pressure at ice temperature of -4.2 Torr was obtained. Excess C12 (either BOC chemically pure grade, 99.5% minimum purity, or Mathieson ultrahigh purity) was added to suppress the intense I2 fluorescence, due to the equilibrium mixture of I2 and C12that exists with IC1 under normal conditions. ( a ) Vacuum-Ultraviolet Absorption Spectrum of ICI. The absorption spectrum of IC1 is shown in Figure 1 at a resolution of 0.1 nm. It should be noted that, for the sharp absorption features, the cross section will be a function of the resolution, thus the absorption data in Figure 1 are a lower limit to the true cross section. For broad features, >0.1 nm, the true cross section is obtained. No attempt was made to subtract out the C12 contribution in the mixed-gas spectrum, because C12 is noninterfering for wavelengths >I40 nm.5 The absorption spectrum of IC1 is dominated at longer wavelengths (>165 nm) by two prominent Rydberg series assigned by Venkateswarlu' to transitions from the ground state of IC1 to the two spin-orbit split ('l'I3/2)6~~ - (21'Il/2)6suRydberg states. They were identified as a6(Q = 1) (179-190 nm) and b6(Q = 0+,1) (165-173 nm), respectively, by Venkateswar1u.I In each case, the structure corresponds mainly to a vibrational progression in the upper state, with some hot-band structure. Both of these systems appear to be predissociated on the basis of the diffuse nature of the absorption bands reported by Venkateswarlu,' although weak fluorescence from these systems back to the ground state has been observed5' in condensed and high-frequency discharges in 1C1. Upon closer examination of Figure 1 several additional weak absorption peaks, not reported by Venkateswarlu,' are observed between the b6 Rydberg bands. The position of these bands and the b6 absorption system are given in Table I. From a comparison of the spacings between the vibrational levels of the b6 Rydberg state and the new bands, it appears that the new system I S also Rydberg in origin. We assign this new band system as the 0 = 0' component of the (2111,2)6soRydberg configurations and label it b'(O+), based on correlations that exist between the fluorescence (51) Haranath, P. B. V.;Rao, P. T. Indian J . Phys. 1957, 31, 156.
C
I
0.0
+
r
130
150 160 170 Uavetength / nm
140
180
190
Figure 2. Vacuum-ultraviolet fluorescence excitation spectrum of IC1 in the presence of CI, (1:2 mixture). Excitation wavelength resolution, AA = 0.1 nm; detection, 270-480 nm. Gas pressures same as in Figure I.
excitation and absorption spectra (to be discussed below) and the similar intensities of the a6(Q = 1) and b6(Q = 1) Rydberg systems. A region of what appears to be continuous absorption is observed between 158 and 190 nm, underlying absorption to the a6 and b6 Rydberg states, and rising to a maximum at 158 nm (see Figure 1). We identify this absorption as being due to the E(O+) X(O+) ion-pair transition in ICI. To shorter wavelengths (
