J. Phys. Chem. 1995,99, 6763-6766
6763
Evidence for a Four-Center Mechanism in the Photoreaction of HI Clusters Karen L. Randall and D. J. Donaldson” Department of Chemistry and Scarborough College, University of Toronto, 80 St George St., Toronto, Ontario Canada M5S IAl Received: February 13, 1995; In Final Form: March 22, 1995@
Emission from ion-pair states of 12 is observed when HI clusters are photolyzed at 248 nm. The emission depends quadratically on the laser intensity and quartically on the stagnation pressure of neat HI. The results H2 12* takes place in HI clusters, strongly suggest that the four-center photoinduced reaction (H1)2 followed by excitation of the I2* to the ion-pair levels.
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Introduction
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In the past decade, new experimental techniques have allowed the study of chemical reactions between reagents which are, to some extent, initially aligned. These techniques rely on the intermolecular forces operative in molecular clusters1or between coadsorbates on a solid surface2to restrict the initial geometries of the reactants. In these approaches the intermolecular forces act to align the reagents somewhat; the reaction dynamics are often quite sensitive to this effect. In most of this work, however, the actual chemical reaction being studied will also take place in the absence of such alignment; it is the dynamics, rather than the chemistry, which is affected. We have recently described a variant in which the chemical reaction takes place only in a dimer;3 that is, a cooperative effect gives rise to the observed reaction. Alkyl iodide dimers, (RI)z, absorb one photon of UV radiation (in the wavelength region in which the monomer dissociates directly into R I) and produce I2 with very little internal energy. We showed that this is not the result of I (or I*) atom reactions within the complex but must be due to some cooperative effect present in the electronically excited alkyl iodide cluster. Very similar results hold for photolysis of HI clusters as well.3c However, in this case we observe a high-pressure threshold for the production of I2 (ca. 1 atm as opposed to a few Torr) and a very different dependence of the 12 signal on stagnation pressure (linear as opposed to quadratic). These observations suggest that the expansion conditions which are conducive to 12 formation give rise to highly clustered beams (thus the linear pressure dependence), suggesting as well that clusters larger than the HI dimer are involved. We developed a model for these cooperative dynamics: “frustrated dissociative a t t a ~ h m e n t ” . In ~ ~summary, the photochemistry is initiated when an electron is excited into the R-I antibonding orbital of one of the partners in the dimer (where R = C or H). If the antibonding orbitals of the two monomer units enjoy sufficient physical overlap, the excited electron may be shared between them. If such sharing occurs, it will induce partial anionic character in the second partner. Since RI anions are directly dissociative, this process gives rise to a simultaneous weakening of both R-I bonds. At the same time, the excited electron, residing in an orbital which is shared to some extent between the partners, gives rise to an incipient bond between the two iodine atoms. Under favorable conditions then, a concerted reaction may occur:
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@Abstractpublished in Advance ACS Abstracts, May 1, 1995.
as is ~ b s e r v e d .This ~ process is not expected to deposit a lot of energy into the I2 internal degrees of freedom if, at the transition state, the dimer geometry places the two iodine atoms near the I2 equilibrium bond length. A different sort of cooperative effect has also been reported in (HX), complexes. Polanyi and co-workers2 observed equal amounts of H2 and Xz (X = C1 and Br) upon irradiation of HX adsorbed on LiF (001) surfaces. These products had quite different translational energy release than those seen from surface reaction of H atoms or from thermal or photochemical desorption. The surface reaction between two HX monomers was proposed as the source: 2HX(ad) H2 X2. This process is forbidden on orbital symmetry grounds4 in the electronic ground state but is allowed for reagents in an excited state, in which case electronically excited X2 should be produced. A similar mechanism was suggested by Buntine et a1.j to explain their observation of translationally very cold H2 product when neat expansions of HI were excited near 218 nm. Those authors speculated that the four-center reaction (HI)2 H2 12 might take place, producing slow H2 and electronically excited
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More recently, Young and co-worker& have reported I2+ as a product in the 240.3 nm photoexcitation of seeded beams of HI. The Iz+ signal varies nonlinearly with pressure and follows a 3.6-photon laser intensity dependence. It was suggested there, in contrast to our conclusions,3c that two photons are used to break two H-I bonds; I2 recombines in the cluster and is ionized by absorption of a further two photons. In the following, we present evidence for the formation of electronically excited molecular iodine, I2*, as a product of HI cluster photolysis. This product is formed under different conditions than the ground state product we identified previously. Taken together with the Polanyi2 and Buntine et a1.j results, this provides strong support for the idea of a four-center, excited state mechanism operative in (HX), photochemistry. The joint formation of 12 and I2*, from two different mechanisms, might help bring the results of Young6 and our earlier work3c into agreement.
Experimental Section The experiments were performed in a modification of the apparatus described p r e v i o ~ s l y .A ~ ~complete ~ description will be published elsewhere.* Neutral clusters of HI were prepared in a supersonic jet expansion of the neat gas. The expansion was formed using a commercial pulsed molecular beam valve
0022-3654/95/2Q99-6763$09.00/0 0 1995 American Chemical Society
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6764 J. Phys. Chem., Vol. 99, No. 18, 1995
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Emission wavelength (nm) Figure 1. Emission spectrum from 12 ion-pair states observed following 248 nm excitation of HI clusters. The prominent features are labeled with their assigned transition^.^,^^ The spectrum was obtained at an HI stagnation pressure of 1300 Torr and a 248 nm laser pulse energy of approximately 2 dlpulse.
of 0.5 mm orifice size. In this apparatus configuration, at about 10 nozzle diameters downstream from the throat of the nozzle, the expansion is intersected by the mildly focused, 248 nm output of a JSrF excimer laser. Excimer laser pulse energies of 0.15-5 mJ per 15 ns pulse were used, focused with a 0.5 m lens to a point approximately 10 cm beyond the jet. In the laser-jet interaction region, the laser spot size was about 0.3 cm2. Emission from electronically excited species is collected by a 3 mm diameter UV-transmitting fiber bundle and imaged onto the entrance slit of a l/8 m scanning monochromator. Light passed by the monochromator is detected by a photomultiplier whose output is directed either to a boxcar integrator or to a digital storage oscilloscope. The output from these devices is sent, in turn, to a laboratory computer for processing.
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Figure 2. (a) Laser power dependence of the D X emission feature at 324 nm. The laser pulse energies range from 0.15 to 3.2 mJ/pulse. The solid line shows a quadratic fit to the low-intensity data. Similar results are seen for the D’ A‘ feature at 340 nm. (b) A log-log plot of the data in (a). The solid line is a fit to the data and has a slope of 2.3 f 0.3, indicating that a two-photon process is responsible for
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Results Figure 1 shows a fluorescence spectrum recorded at 2 nm resolution. The emission bands are all assignable to transitions originating from ion-pair states9of iodine, I2@. The fluorescence lifetimes we measure’with the digital oscilloscope correspond to those we observe for I2 excited directly to the ion-pair manifolde8 The emission appears promptly, within the 5 ns rise time of our detection electronics. Apart from the total fluorescence intensity, the appearance of the spectrum is independent of the stagnation pressure of HI behind the nozzle. Careful scans to the red of 500 nm showed no clear features within our currently achievable signal-to-noise ratio. In separate experiments, we attempted to use 248 nm radiation to excite 12 expanded with He directly. In those experiments, no emission from 12 ion-pair states is observed. Figure 2a displays the dependence of the intensity of the emission band seen at 324 nm on the laser pulse energy, in the range 0.15-3.2 &/pulse. Similar results hold for the feature at 340 nm. A two-photon dependence is inferred from the slope of 2.3 & 0.3 in the lower pulse energy regime of the log-log plot shown in Figure 2b. At higher laser pulse energies, the falling off in the slope provides evidence of saturation. Since all the relative peak intensities observed here are independent of the experimental conditions, we conclude that two 248 nm photons are required to produce the I*@whose emission spectrum we observe.
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HI stagnation pressure (Torr) Figure 3. Stagnation pressure dependence of the intensity of the emission feature observed at 324 nm. The solid line at pressures less than 700 Torr shows a quartic fit to the low-pressure data. A squareroot dependence of signal on pressure is observed at higher pressures; this is indicated by the dashed line. The dependence of the signal intensity observed at 324 nm on the stagnation pressure is plotted in Figure 3. The intensity is strongly nonlinear; in fact, at lower stagnation pressures, up to 1 atm, it varies quartically with pressure. Since at least one “third body” species is required to carry away excess energy in
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J. Phys. Chem., Vol. 99, No. 18, 1995 6765
cluster condensation, this implies that HI dimers and trimers are involved in the observed photochemistry. At pressures higher than about 1 a m the dependence is much less than quartic: the signal varies roughly as the square root of the pressure. In our previous work,3cwe reported the formation of very cold ground state 12 from the one-photon photochemistry of HI clusters. Ion-pair state emission was not observed in that work: the optical filters used in the laser-induced fluorescence measurement of ground state 12 do not pass light of ;1 < 590 nm. The pressure dependence of the ground state 12 signal is considerably different than that given here: it is linear with HI backing pressure (slope of ln(intensity) vs ln(pressure) plot = 1.12 f 0.05), with an appearance threshold of approximately 1 atm. This very different stagnation pressure dependence is strong evidence for different mechanisms of formation for I2 and IziP.
Discussion The first candidate for a new mechanism is the 248 nm photodissociation of an HI unit in a cluster, followed by reaction of the product I (or I*) with a remaining HI unit. The 12 formed in this process would then absorb a second 248 nm photon to access the ion-pair states, which we observe. This mechanism, though intuitive, is not energetically possible. The reaction
I*
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is endoergic by about 52 kT/mol, much more than the energy available to the I* fragment upon HI photodissociation. Reaction with ground state I atoms is even less favored energetically. A consecutive two-photon mechanism, in which the fixst photon creates one I atom and the second creates another, which recombine inside the remaining HI cluster,6 is also not sufficiently energetic to populate the ion-pair states of the I2 product. An energetically possible reaction is the sequence (HI),
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+ H + :1 + (H1)n-2 1 ;
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Here, the fxst photon induces a reaction in the cluster to produce ro-vibrationally excited ground state iodine, I2#, and the second excites this species to the ion pair states. Though energetically possible, we consider this highly unlikely because 248 nm excitation does not excite low-lying v levels (those populated at room temperature and below) of 12 to the ion-pair states. There is a large geometry difference between the X and the ion pair giving rise to very small Franck-Condon factors for such transitions. Presumably, the I2# formed in the first step could contain sufficient vibrational excitation to give rise to favorable FCF's. However, in our earlier work,3cwe saw only vibrationally and rotationally cold ground electronic state 12 from (HI), photolysis. As well, the very different stagnation pressure dependence observed for ground state 12 argues for a substantially different formation mechanism. A final mechanism is suggested by the work of Buntine et aL5 Following the ca. 218 nm photolysis of neat expansions of HI, those workers observed H2(v=1) with very little translational energy. They suggested that this product could arise from a four-center reaction: (HI)2 H2 I2* (hv). The Woodward-Hoffman rules demand that one of the products of this four-center excited state reaction be electronically excited." Buntine et al. suggested that if a four-center mechanism was operative in this case, the 12 would be born in an electronically
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excited state.5 A four-center mechanism was also suggested by Polanyi and co-workers2in discussing surface photochemistry of HBr and HC1. This process is much more favorable energetically than the preceding ones. (HIh H2 I2 (hv) has about 480 kJ/mol of available energy, which can go into internal states of H2 (as observed) and/or 12. Since we did not see evidence for highly excited ground electronic state 12 in our earlier work,3c any energy appearing in the I2 product will be at least partially electronic. This is as demanded by the Woodward-Hoffman rules. The amount of available energy is, in fact, sufficient to populate the ion-pair states of I2 directly. We consider this unlikely for two reasons. First, our measured two-photon dependence of the I2 ion-pair state fluorescence intensity is inconsistent with those states being populated directly. Laser power dependence studies are always to be viewed with caution but seldom, if ever, overestimate the number of photons involved in a multiphoton process. Symmetry considerations also suggest a two-step population of the ion-pair levels. The initial excitation of (HI), corresponds to excitation of a IT state of the HI monomer." Both the ground state of H2 and the 12 ion-pair states are of 2 symmetry. Although these (AS) coupling scheme labels are not strictly valid for the (Q,o) coupling important in HI and 12, they suggest that there is not a propensity to populate the ion-pair levels of 12. By contrast, the A, A', and B states of 1 2 are all of IT symmetry, and no such arguments hold. We conclude that our results strongly indicate a fourcenter mechanism:
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12'P
That we do not observe direct emission from these excited states of I2 is not a strong argument against the proposed mechanism. The A and A' states have lifetimes of hundreds to thousands of microseconds;12we would not see these very weak emissions in our present apparatus. Since we expect little translational energy release in the products? the I2* probably contains considerable vibrational and rotational excitation, and could well be born over a large range of (v,J) levels. A broad internal population distribution would make observation of fluorescence from the brighter B state difficult as well. The quartic dependence of the emission signal on the stagnation pressure is suggestive of the involvement of HI dimers and trimers in the photochemistry. The geometries of these species are not experimentally known, though that of the dimer has been ca1c~lated.l~ However, if trimers are involved, it might be that one HI molecule acts as a "template", aligning the reacting pair in the optimum geometry for the four-center reaction. In this way, the third molecule might act like a solid surface.2 In our apparatus, production of 12iP is favored at lower stagnation pressures, and ground electronic state 12 formation takes place at higher pressures. This could be because the IziP signal is attenuated at higher stagnation pressures by an efficient quenching of the I2* intermediate by HI. We have observed8 such efficient quenching of I2(B) emission by clusters of Ar, Kr, and Xe. Figure 4 shows the pressure dependence of each product, separately normalized. It is clear that under a range of expansion conditions both products may be formed. It might be that the experiment of Young and co-worker& was actually probing both species: one formed via a one-photon production of 12, as we concluded p r e v i o u ~ l yfollowed ,~~ by a three-photon ionization and one via a two-photon formation of 12'P, as we
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attachment mechanism,3c and one which forms electronically excited 12, via a four-center reaction me~hanism.~
Acknowledgment. This work was supported financially by NSERC and CEMAID, a Canadian federal Center of Excellence. D.J.D. thanks NSERC for the award of a University Research Fellowship. We are grateful to Prof. R. N. Zare and Dr. M. A. Buntine for bringing this problem to our attention.
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References and Notes
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HI stagnation pressure (Torr) Figure 4. Comparison of the pressure dependence of the excited state (points) and ground state (crosses) I2 signal seen in HI cluster photolysis. An absolute intensity comparison is not possible with our present techniques; therefore, each dependence is normalized to its own maximum.
report here, followed by a one-photon ionization. A combination of these processes could give the 3.6-photon dependence observed by Young.6
Conclusions Taken together, our present and earlier3cobservations indicate that there are two photochemical processes which take place in HI clusters excited at 248 nm: one which produces ground state 12 with very low internal energy, via the frustrated dissociative
(1) (a) Soep, B.; Whitham, C. J.; Kelle, A.; Visticot, J. P. Faraday Discuss. Chem. SOC.1991, 91, 191. (b) Shin, S. K.; Chen, Y.; Nickolaisen, S.; Shape, S. W.; Beaudet, R. A.; Wittig, C. Adv. Photochem. 1991, 16, 249. (2) (a) Cho, C.-C.; Polanyi, J. C.; Stanners, C. D. J. Chem. Phys. 1989, 90, 598. Bourdon, E. B. D.; Cho, C.-C.; Das, P.; Polanyi, J. C.; Stanners, C. D.; Xu, G.-Q. Zbid. 1991, 95, 1361. (3) (a) Fan, Y. B.; Donaldson, D. J. J. Phys. Chem. 1992, 96, 19. (b) Fan, Y. B.; Donaldson, D. J. J. Chem. Phys. 1992,97, 189. (c) Fan, Y. B.; Randall, K. L.; Donaldson, D. J. lbid. 1992, 98, 4700. (4) Hoffmann, R. J. Chem. Phys. 1968, 49, 3739. (5) Buntine, M. A.; Baldwin, D. P.; Zare, R. N.; Chandler, D. W. J. Chem. Phys. 1991, 94, 4672. (6) (a) Young, M. A. J. Phys. Chem. 1994, 98, 7790. (b) Burnett, J. W.; Young, M. A. Chem. Phys. Lett. 1994, 228, 403. (7) Bishenden, E.; Fan, Y. B.; Andraos, N.; Haddock, J.; Sapers, S. P.; Donaldson, D. J. Can. J. Appl. Spectrosc. 1992, 37, 89. (8) Randall, K. L.; Donaldson, D. J. Manuscript in preparation. (9) For a recent review of 12 ion-pair states, see: Tellinghuisen, J.; Phillips, L. F. J. Phys. Chem. 1986, 90, 5108. (10) Lawley, K. P.; Donovan, R. J. J. Chem. Soc., Faraday Trans. 1993, 89, 1885. (11) Huber, K. P.; Herzberg, G . Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules; van Nostrand Reinhold: New York, 1979. (12) Bohling, R.; Langen, J.; Schurath, U. Chem. Phys. 1989,130,419. (13) Hannachi, Y.; Silvi, B. J. Mol. Struct. 1989, 200, 483. (14) Tellinghuisen, J. Chem. Phys. Lett 1983, 99, 373. JP950411F
