J . Phys. Chem. 1984, 88, 1285-1286
1285
Orientational Ordering and Site-Selective Photochemistry of UF, Isolated in Argon Matrices Llewellyn H. Jones,* Basil I. Swanson, and Scott A. Ekberg Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545 (Received: February 6, 1984)
High-resolution studies of the v3 mode of UF6 in argon matrices show two dominant sites with a high degree (80%) of orientational ordering. Upon photolysis UF, is formed but recombines in part to form UF6 on metastable sites. One site is considerably more amenable to photolysis than the other. On high-temperature annealing (40 K) all the UF, is converted back to UF6 and the original dominant sites are re-formed to a considerable extent. Through all this photoexcitation, dissociation, recombination, and annealing the original ordering is preserved.
With high-resolution infrared spectroscopy it is possible to observe multiple site structure and site-symmetry splitting for molecules trapped in low-temperature matrices.' For several systems of spherically symmetrical molecules, namely, SF6,24 SeF6,3and cc12 isolated in rare gas solids, lowering of symmetry has been conclusively demonstrated for some sites resulting in splitting of the triply degenerate infrared-active stretching mode into a doubly degenerate and a singly degenerate mode. In most cases, particularly for depositions made in the pulsed mode, these lower symmetry sites show significant orientational ordering in that a high percentage (50-90%) of the guest molecules on these particular molecular sites have the singly degenerate mode vibrating perpendicular to the plane of the substrate surface while the doubly degenerate mode lies parallel to this plane.2,3*5This is explained as arising from registry of a C3 axis of the impurity molecule with the (1 11) growth plane of the rare gas host lattice3 The symmetry of the site is thus C3,,or possibly C3,either of which will give rise to splitting of the triply degenerate mode of the isolated Oh or Td molecule into a doubly degenerate (E) and a singly degenerate (A) mode. It is important to recognize that we are not implying significant geometrical distortion of the impurity molecule, but rather that it is the impurity plus the surrounding host atoms (primarily nearest neighbors) which make up the site and determine its symmetry. We have recently extended such studies to UF6 in rare gas solids to investigate the possibility of orientational ordering. This system has the further advantage that UV radiation decomposes UF6 isolated in an argon matrix to UFS F; the products recombine over a period of time to re-form UF6.6 A question we wish to address is whether or not UF6 on sites exhibiting orientational ordering before photolysis are scrambled following photodissociation and the subsequent recombination to form UF6. This letter is intended to report preliminary results on these studies.
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Experimental Section A matrix of argon/UF6 50001 1 was deposited at 20 K by using large (-2.5 torr L) pulses. Preconditioning of the stainless steel mix vessel and feed line with UF6 is necessary as it reacts with traces of impurity and is absorbed on the walls. Due to its absorption and desorption as well as reaction with the steel or other sources of silicon to form SiF4 it is difficult to know the concentration of UF, in the matrix. The infrared absorption due to the antisymmetric U-F stretch,
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(1) B. I. Swanson and L. H. Jones in "Vibrational Spectra and Structure" Vol. 12, J. Durig, Ed., Elsevier, Amsterdam 1983, pp 1-67, and references therein. (2) L. H. Jones, B. I. Swanson, and H. A. Fry, Chem. Phys. Lett., 87,397
(1982). (3) L. H. Jones and B. I. Swanson, J . Chem. Phys. 79, 1516 (1983). (4) B. I. Swanson and L. H. Jones, J . Chem. Phys. 74, 3205 (1981). (5) L. H. Jones and B. I . Swanson, J . Chem. Phys., in press. (6) R. T. Paine, R. S . McDowell, L. B. Asprey, and L. H. Jones, J Chem. Phys. 64, 3081 (1976).
0022-365418412088-1285$01.50/0
v3, was observed at 0.04-cm-' resolution with a Nicolet 7199 FTIR spectrometer. To assess the extent of orientational ordering the spectrum was observed with the window at 45O (about the vertical axis) to the incident beam and with a polarizer in the beam to analyze separately the horizontal and vertical absorbances of the absorption peaks.3 Photolysis studies were carried out with an AH4 mercury lamp with quartz envelope, with a KrCl excimer laser (220 nm), aqd with a KrF excimer laser (249 nm). Irradiation with a XeCl excimer laser (308 nm) did not lead to significant dissociation.
Results and Discussion The spectrum in the v3 region for a dilute matrix of UF6 in argon is shown in Figure 1. There appear four prominent peaks A, B, C, and D. The polarization shows that A and B arise from vibration in the matrix plane while C and D arise from modes perpendicular to this plane. This shows that we have two dominant sites for UF6 in argon matrices, both showing site-symmetry splitting and orientational ordering. However, we cannot tell from this spectrum which of C or D arises from the same site as A or B. Photolysis studies solve this dilemma for us because one site is dissociated to UF6 to a greater extent than the other. This is shown in Figure 2 along with postphotolysis annealing studies. It is apparent from Figure 2 that on photolysis peaks B and C diminish to a greater extent than A and D; thus, A and D arise from one site and B and C from another. An analysis of these polarization studies, as carried out previously for SF6in xenon? yields a value of 0.8 for the fraction on each site (A, D and B, C) oriented with the unique axis perpendicular to the surface. As discussed earlier,3*5we believe the unique axis is a C3 axis in registry with the ( 111 ) growth direction of the host lattice. The fact that there are two such similar, but distinct, sites, in terms of average frequency and evidence of a unique C3 axis, suggests the possibility of the usual cubic-close-packed environment for one site and the closely related hexagonal close packed for the other as implied from the results on CCll in k r y p t ~ n .This ~ can arise either from stacking faults or from a mixture of cubic and hexagonal crystallites. The spectrum after photolysis at 10 K is shown as B in Figure 2. Aside from the presence of UF, as indicated by absorption at 584 cm-' (not shown) we see the depopulation of the two dominant sites (B, C more than A, D) and the growth of new UF6 sites-especially three strong peaks from 622 to 624 cm-'. These must arise from rather unstable sites as they disappear completely and convert to a new set of sites on annealing at 20-30 K (see curve C of Figure 2). Some UF, has back-reacted to UF6 at this stage. After deposition of a xenon overcoat the matrix was annealed at 40 K and we find the original sites A, D and B, C have grown back in with about the same relative intensity as in the (7) L. H. Jones and B. I. Swanson, J . Chem. Phys., in press.
0 1984 American Chemical Society
Letters
1286 The Journal of Physical Chemistry, Vol. 88, No. 7, I984 B (D
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1/CM Figure 1. Infrared spectrum of the UF, v, mode in a matrix of Ar/UF6 > 5000/l a t 10 K on a cesium bromide window. The matrix was deposited at 18 K and annealed a t 30 K prior to recording spectrum. The window was rotated about the vertical axis 4 5 O to the incident beam. The solid line is for horizontal polarization, the broken line for vertical polarization.
initial matrix. There is also now a strong broad component at 624 cm-I. No UF, is now detected in the 584-cm-I region. A rather remarkable observation is that sites A, D and B, C in the final spectrum (top of Figure 2) are still strongly polarized and indicate orientational ordering to the same extent as in the initial matrix. The chain of events in this experiment can be described as follows. On photolysis at 10 K UF6 is first converted to UF, F; the F atom remains in the vicinity of the UF,. As the photolysis continues some of the UF5 recaptures a fluorine, perhaps due to local heating arising from photoexcitation of the UF,; the UF6 re-formed in this manner resides in one or more unstable sites. On warming above 20 K these unstable sites adjust themselves to a more stable configuration still different from that in the original matrix. Finally, on high-temperature annealing all of the UF, back-reacts to re-form UF6 and the dominant sites remaining are the original two (A, D and B, C) plus a third near 624 cm-’ which was originally present in a small amount but disappeared on photolysis. The natures of these various sites are not yet understood; after furthw studies we expect to report on them in greater detail. During this series of events the UFS and re-formed UF6 maintains registry with the (1 1 1) growth plane of the argon host. This result is surprising in view of the fact that photolysis with 249- and, especially, 220-nm radiation provides energy in excess of that needed to photodissociate UF6. It was
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618
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620
622 1/ C M
629
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Figure 2. Matrix of Ar/UF6 5000 deposited at 20 K. All spectra recorded at 10 K: (A) after annealing 30 min at 30 K; (B) after 3-h photolysis with an AH-4 mercury arc with glass envelope removed; (C) after postphotolysis 30 K anneal; (D) after postphotolysis 40 K anneal. The absorbance scale is the same for all spectra except that B, C, and D are shifted upward for clarity. The window was at 45’ to the beam and no polarizer was used.
originaly expected that this excess energy would result in local heating around the trapped UF, and scrambling*of the orientation of the re-formed UF6. The observation that the original orientational ordering is preserved suggests that the bulk of the excess energy goes to the F atom, facilitating its migration away from the UF, fragment which is left with insufficient energy to result in orientational scrambling. (8) There are four ( 11 1) directions in the cubic-close-packed structure, at tetrahedral angles to each other. Scrambling would equalize the number of molecules oriented in these four directions and destroy any spectral polarization effects.
