J . Phys. Chem. 1988,92, 5523-5529 If Ill, is accessed initially, then coupling to another surface must occur. Here, the position of 3Zl is critical. If this follows Figure 4, then coupling between Ill, and 3Z1can Occur in the exit channel. As the HBr bond distance increases, the n,u character increases. As a result, the two states can couple and Br* can be produced via a perpendicular transition. If indeed 321is positioned as indicated, one might question why cannot be accessed directly via excitation from the ground state. Although this transition may occur, there is no evidence to suggest that it should be favored I&+, where parallel excitation yields over, for example, QO+ Br*. Of course, the actual relative strength of the two transitions at 193 nm is unknown. If we do not accept that 3Z1is much lower in energy than was previously thought, the alternative explanation involves coupling between QO+ and I l l l . It is then apparent that this coupling is not particularly weak, despite the different !J and electron spins. In conclusion, we suggest that the following mechanism occurs in HBr photodissociation a t 193 nm. Initial excitation (perpendicular) is primarily to I l l l . As dissociation proceeds, sufficient and as a result, Br* (14%) coupling takes place to 3Z1and/or 3Q,+,
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is produced which originated from a perpendicular transition. Selection rules tend to favor the 3Z1channel, but the actual position of the state is in question. Consequently, a definitive assignment awaits additional experimental data and theoretical calculations. Possibly, formulas such as the Landau-Zener equation% will prove helpful in determining crossing probabilities. Acknowledgment. This research was supported by the US. Department of Energy, Division of Basic Energy Sciences. We are grateful to J. de Juan for stimulating discussions and a reviewer for constructive comments. C.W. is deeply indebted to the late Prof. E. K. C. Lee, who was one of the people whose inspiration in Cambridge, England, during 197111972 caused him to enter the field of physical chemistry, a move that he has never regretted in the least. Registry No. HBr, 10035-10-6; HI, 10034-85-2; H, 12385-13-6; I, 14362-44-8; Br, 10097-32-2. (26) Nikitin, E. E. Theory of Elementary Atomic and Molecular Processes in Gases; Clarendon: Oxford, 1974.
Vacuum-Ultraviolet Absorption, Fluorescence Excitation, and Dispersed Fluorescence Spectra of IBrt Andrew J. Yencha,* Department of Chemistry and Department of Physics, State University of New York at Albany, Albany, New York 12222
Robert J. Donovan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ. U.K.
Andrew Hopkirk, and David Shaw SERC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, U.K. (Received: December 18, 1987; In Final Form: March 1, 1988)
The vacuum-ultraviolet absorption and fluorescence excitation spectra of IBr have been recorded with tunable synchrotron radiation. Fluorescence occurs mainly from ion-pair states, and pronounced resonance. structure is observed due to interactions between Rydberg and ion-pair states. Two oscillatory continuum systems are observed in the dispersed fluorescence spectra for excitation wavelengths between 169.9 and 191 nm. Electronically excited (Rydberg) iodine atoms are formed by predissociation below X = 144 nm.
Introduction The vacuum-ultraviolet absorption spectra of the halogens and interhalogens are dominated by strong Rydberg transition^.'-^ However, underlying these strong transitions are weaker but extensive ion-pair absorption systems consisting of long progressions, often extending across several Rydberg system^.^" The large increase in bond length in the ion-pair states produces a dense and extended rotational contour leading to extensive overlap of vibrational structure, and very high resolution is needed to observe individual rovibronic transitions. The high density of such transitions and the washing in of spectral features due to isotope effects makes the ion-pair absorption systems appear quasi-continuous, even at medium resolution. To the best of our knowledge, no detailed analysis has yet been made of an ion-pair absorption system in the vacuum ultraviolet by optical means, although one study on ion-pair formation using mass spectrometry has been This paper is dedicated to the memory of Professor E. K. C. Lee, who through his wisdom, knowledge, and enthusiasm was a source of great inspiration to us.
reported on the systems 02,NO, and CO.’ On the other hand, ion-pair states have been quite extensively studied by using laser optical-double-resonance Venkateswarlu, P. Can. J. Phys. 1970, 48, 1055. Venkateswarlu, P. Can. J . Phys. 1969, 47, 2525. Venkateswarlu, P. Can. J. Phys. 1975, 53, 812. Mulliken, R. S. J . Chem. Phys. 1971, 55, 288. (5) Donovan, R. J.; OGrady, B. V.; Shobatake, K.; Hiraya, A. Chem. Phys. Lett. 1985, 122, 612. (6) Kerr, E.; MacDonald, M.; Donovan, R. J.; Wilkinson, J. P. T.; Shaw, D.; Munro, I. J . Photochem. 1985, 31, 149. (7) Oertel, H.; Schenk, H.; Baumgartel, H. Chem. Phys. 1980, 46, 251. (8) Brand, J. C. D.; Deshpande, U. D.; Hoy, A. R.; Jaywant, S . D. M.; Woods, E. J. J. Mol. Spectrosc. 1983, 99, 339. Brand, J. C.; Hoy, A. R.; Risbud, A. C. J. Mol. Spectrosc. 1985, 113.47. Brand, J. C. D.; Deshpandc, U.; Hoy, A. R.; Jaywant, S . M. J. Mol. Spectrosc. 1983, 100, 416. (9) Ishiwata, T.; Tanaka, I. Chem. Phys. Lett. 1984, 107,434. Ishiwata, T.; Fujiwara, I.; Shinzawa, T.; Tanaka, I. J . Chem. Phys. 1983, 79, 4779. Ishiwata, T.; Takunaga, A,; Shinzawa, T.; Tanaka, I. J. Mol. Spectrosc. 1984, 108,314. Ishiwata, T.; Ohtashi, H.; Tanaka, I. J . Chem. Phys. 1984,8J, 2300. Ishiwata, T.; Ohtoshi, H.; Sakaki, M.; Tanaka, I. J. Chem. Phys. 1984, 80, 1411. (1) (2) (3) (4)
0022-365418812092-5523$01.50/0 0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No, 19, 1988
In previous work it has been shown that many of the halogen and interhalogen ion-pair states give rise to oscillatory continuum emission in the visible and near ultraviolet, following excitation in the vacuum ~ l t r a v i o l e t . ~ ~ ~Thus ~ - ' ~ by monitoring the fluorescence excitation spectrum in the near ultraviolet the ion-pair states can be studied without interference from the Rydberg systems that dominate the absorption spectrum in the vacuumultraviolet region. Fluorescence excitation spectra also yield valuable information on predissociations and other interactions between the higher excited states.
Experimental Section The spectroscopic studies described herein were carried out using port 13.2 of the synchrotron radiation source (SRS) at the S E R C Daresbury Laboratory. Under standard conditions the storage ring contained 160 bunches of electrons (multibunch mode) which provided light pulses of repetition frequency 0.5 GHz and 200-ps duration. The S R S was normally operated at 2.0 GeV with a current of about 200 mA. The radiation from the S R S was dispersed by a 0.5-m normal incidence Seya monochromator (Bird and Tole). The radiation from the monochromator was focused by a CaF, lens (d = 25 mm, f = 250 mm) to yield a beam of cross section 2.5 X 10.0 mm at the center of the sample chamber. The highest effective resolution obtainable with this monochromator, consistent with a usable light flux, was about 0.1 nm. Absorption and fluorescence excitation (with broad band detection) spectra were recorded with this resolution, while dispersed fluorescence spectra were obtained with a resolution of incident light of 2.2 nm. Fluorescence excitation spectra with narrow band (dispersed) detection used a resolution of excitation light of 4.5 nm. The sample chamber was constructed from aluminum and was fitted with lithium fluoride or Spectrosil quartz windows, as appropriate. Three mutually perpendicular axes were used for excitation and observation, with entrance and exit windows being separated by 100 mm. The observation axes intersected the synchrotron beam axis about 20 mm from the entrance window. Two monochromators (one for the vacuum ultraviolet and one for the ultraviolet and visible regions) were used to disperse fluorescence. Both monochromators were mounted on their side such that the entrance slits lay parallel to the direction of the excitation beam. A 0.2-m Acton (Model VM-502) vacuum monochromator, utilizing a 1200 groove mm-', aberration-corrected concave holographic grating (linear dispersion 4 nm mm-', in first order), was used for observations between 130 and 250 nm. Dispersed fluorescence spectra were obtained by using an EM1 G26K3 14LF photomultiplier (sensitivity range 120-600 nm) with entrance and exit slits of the monochromator set a t either 1.0 or 1.5 mm yielding resolutions of 4.0 and 6.0 nm, respectively. For observations above 250 nm a 0.25-m Spex (Minimate) monochromator, employing a 2400 groove mm-I aberrationcorrected plane holographic grating (linear dispersion 2 nm mm-', in first order), was used. Dispersed fluorescence spectra were obtained by using an EM1 98834 photomultiplier (sensitivity range 180-600 nm) with entrance and exit slits of the monochromator at 2.5 and 1.5 mm, respectively, giving an effective resolution of about 3.5 nm. The transmitted synchrotron light was monitored with a sodium salicylate coated window. This window was mounted on an (10) King, G. W.; Littlewood, I. M.; Robins, J. R. Chem. Phys. 1981, 62, 359. King, G. W.; Littlewood, I. M.; McFadden, R. G.; Robins, J. R. Chem. Phys. 1979, 41, 379. King, G. W.; Littlewood, I. M.; Robins, J. R. Chem.
Phys. 1981, 56, 145. (11) Rousseau, D. L.; Williams, P . F. Phys. Reu. Lett. 1974, 33, 1368. (12) MacDonald, M.; Wilkinson, J. P. T.; Fotakis, C.; Martin, M.; Donovan, R.J. Chem. Phys. Lett. 1983, 99, 250. (13) MacDonald, M.; Donovan, R. J.; Gower, M. C . Chem. Phys. Lett. 1983. 12. - - --, 97. (14) Lawiey, K. P.; MacDonald, M.; Donovan, R. J. Chem. Phys. Lett. 1982, 92, 322. (15) Donovan, R. J.; MacDonald, M.; Lawley, K. P.; Yencha, A. J.; Hopkirk, A. Chem. Phys. Lett. 1987, 138, 571. (16). Lawley, K. P.; Austin, D.; Tellinghuisen, J.; Donovan, R. J. Mol. Phys., in press.
Yencha et al. "6
t +
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