J. Phys. Chem. 1985,89, 2982-2983
2982
actions between this host and Chl a.1oaIn another system involving unsaturated lipids, complete mixing has been observed in mixed monolayers of all-trans-retinal and dioleoylphosphatidylcholine below the collapse pressure.Iob The upper limit obtained in these present studies for the lifetime of Chl a singlet in the DOL matrix is effectively 5.5 f 0.3 ns, in quite close agreement with that reported for dilute Chl a in phosphatidylcholine vesicles.1g This implies that the quantum yield in such a system approaches 90% of that obtained in nonpolar media.20 Comparison of our quenching curves with the earlier work of Tweet et al. in oleoyl alcohol would suggest a similar high yield in their ~ y s t e m .From ~ Figure 3d in which all the data plotted by molecules/cm2 appear to fall on a single curve, one may suggest that “squeezing” the molecules in this environment does not appear to force the creation of additional trapping sites beyond that of simply increasing concentration. In the case of a Chl a/myristylpho~phatidylcholine,~ Chl a/dipalmitylphosphatidylcholine,2’ or, indeed, where hexadecane has been used as a diluent at the nitrogen-water interface,6 more pronounced changes in fluorescence intensity have been associated with the phase transition in that system. Indeed, it may be generally suggested that the most pronounced changes in photophysical behavior for probes in monolayers are associated with phase transitions where reordering (19) Beddard, G. S.; Porter, G.; Weese, G. M. Proc. R. SOC.London, Ser. A 1975, A342, 317-25.
(20) Connolly, J. S.; Janzen, A,; Samuel, C. B. Photochem. Phofobiol.
of the constituents occurs.13~21,22 The concentration for half-quenching (4.55 f 0.1 l o L 2molecules/cm2) is not dramatically different from that reported by Costa et al. (4.3 X 10l2 molecules/cm2) in transferred phytol monolayers as well as that given by Tweet et al. in oleoyl alcohol eq 1 to their data gives Co 4.0 mentioned a b o ~ e . ~(Applying .~ X lo1, molecules/cm2.) The fit of both kinds of Chl a data to a close second-order dependence on concentration over the range of these measurements indicates the dominance of trapping involving statistical pairs of molecules, as has long been suggested.Ig However, the agreement in C, between the two types of measurements suggests the dominance of a dynamic process, either by diffusion of the excited molecule to a trap or migration of the exciton among molecules in the array. The diffusion coefficients for materials in lipid monolayers are too small to explain this behavior by diffusion, but previous studies have shown that in the concentrations range of interest here energy may readily move through the system by Forster t r a n ~ f e r .Should ~ Forster transfer be the rate-limiting step, and should traps exhibit unit quenching efficiency, one might expect another dependence on concentration. The exact nature of the trapping mechanism may be quite complex; any successful model must account for the correspondence of T and 4 data as well as Chl-Chl energy transfer dependence on concentration. The usual classical kinetic model would appear insufficient. Efforts are underway in this laboratory to further explore the characteristics of such a quenching entity.
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1982, 36, 559-63.
(21) Chauvet, J.-P.; Agrawal, W. L.; Patterson, L. K., to be submitted for publication.
(22) Vaidyanathan, S.; Patterson, L. K.; Mobius, D.; Gruniger, H.-R. J . Phys. Chem. 1985,89, 491-7.
Infrared Spectra and Structure of Thin Films of Solid Oxygen L. H. Jones,* B. I. Swanson, S. F. Agnew, and S. A. Ekberg Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545 (Received: April 4, 1985)
Using high-resoluton infrared spectroscopy,we have observed a metastable phase of oxygen formed in thin films from vapor deposition at 10 K. On heating above 12 K this phase converts irreversibly to the stable low-temperature a-phase. Isotopic studies and addition of small amounts of CO indicate that the unit cell must be large (at least 8 molecules per unit cell) and of low symmetry (with 4 crystallographically distinct 0, molecules).
Introduction Many years ago it was that thin films of solid oxygen deposited at liquid helium temperature exhibited infrared absorption in the fundamental 0-0 stretching region despite the fact that such transitions are vigorously electric-dipole forbidden for homonuclear diatomics in a unimolecular cell such as that of a-oxygen. It was further observed that this absorption disappeared irreversibly when the sample was warmed to 18 K. The authors2 suggested that the infrared absorption was due to lattice defects, since the well-annealed a-phase shows no absorption in the fundamental region. We have carried out high-resolution infrared studies of thin films of solid oxygen and conclude instead that the unexpected absorption arises from a metastable crystalline phase (1) R. V. St. Louis and B. L. Crawford, Jr., J . Chem. Phys., 37, 2156 (1962). (2) B. R. Cairns and G. C. Pimentel, J . Chem. Phys., 43, 3432 (1965).
0022-3654/85/2089-2982.$01.50/0
which irreversibly converts to the a-phase on heating above 12 K. Results
The spectrum of the fundamental 0-0 stretching region for O2deposited on a CsBr window at 10.5-1 1 K is shown in Figure 1. We note that there are five peaks in this region (A-E), the sharpest (peak D) being about 0.13 cm-I full-width at halfmaximum. Upon heating above 12 K, the peaks A, B, C, and D simultaneously disappear (in a matter of several minutes to hours depending on the temperature). Peak E remains at temperatures below 20 K but does gradually disappear on annealing in the 20-25 K range. Peaks A, B, C, and D show orientational ordering as demonstrated by the polarization observed in Figure 1. The dipoles responsible for peaks A and B are more parallel to the surface while C and D are more perpendicular. The experimental technique for polarization studies has been described3 0 1985 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2983 A further observation is that 5% of nitrogen or argon stabilizes the metastable phase up to temperatures above 20 K.
In 0
Discussion 0
E? z a
m
g? a
I
/I c
1
U
isdr
islis
islia
1/CM
1550
is+,2
is+,+
Figure 1. Infrared spectrum of 0-0 stretch for deposit of O2 (99.98% pure) at 10.5-11 K. Deposit of 44 torr L at 50-35-torr pressure in 13-cm3 pulses. Window is rotated about vertical axis at 45O to the incident beam. V is for vertical polarization for modes parallel to matrix plane only. H is for horizontal polarization with incident electric vector at 45O to matrix plane. Asterisks mark H 2 0 vapor peaks not cancelled out. Resolution = 0.04 cm-I.
earlier. We note at this point that electron diffraction studies4 and X-ray diffraction studies5 of thin films of solid oxygen, formed by condensation from the gas phase onto a low temperature substrate, showed many new diffraction lines which are different from those of the a-phase and suggest a complex, low-symmetry cell. This evidence of a new phase along with the sharpness of the infrared peaks and their orientational ordering indicates to us that these absorptions are not due to lattice defects but rather to a metastable phase of oxygen, termed m-0,. This phase irreversibly converts to a - 0 , above 12 K, further supporting its crystalline nature, since we do not expect defects to anneal out until 20 K or above for oxygen. We have studied mixtures of 1802 and 1602 and find as many as eight different peaks for each of the two primary isotopic species. were present and absorbed weakly in Small amounts of I6O I80 an intermediate region. In some experiments 0.05% CO was included. Four CO absorption peaks appeared, all of which correlated with the peaks A-D of m-02. That is, on heating above 12 K the four CO peaks diminished concurrently with the O2peaks A-D of Figure 1. An additional CO peak increased in intensity during the same warming interval and is correlated with CO in a matrix of the a-phase. Thus, the CO serves as a marker of the percentage of the oxygen in the metastable phase (as high as 75% in some deposits). (3) L. H. Jones and B. I. Swanson, J . Chem. Phys., 79, 1516 (1983). (4) E. M. Hod, Acta Crystallogr.,Sect. E , 25, 2515 (1969). (5) I. N . Krupskii, A. I. Prokhvatiov, Yu. A. Freiman, and A. I. Evenberg, Preprint ITP-79-4E, Information Department, Institute for Theoretical Physics, 252130, Kiev-130, USSR, March, 1979.
As noted above, we believe a metastable phase, which we shall call m-02, is formed on deposition of thin films of oxygen at 10-1 1 K. The infrared absorption must arise from a coupling between two or more neighboring O2molecules vibrating out of phase with each other. We note that the high-pressure t-phase of 0, exhibits extremely intense 0-0 stretch fundamentals and combination bands in the IR, which have been attributed to strong intermolecular interactiom6 Such bimolecular coupling is also well documented in the electronic spectroscopy of a- and p-O,.’ When CO is present it gives rise to four peaks associated with CO trapped in the m-phase. The pattern of these four peaks is similar to that of the four 0, stretches observed. From this we conclude that there are four types of oxygen occupying different crystallographic positions in the m-phase unit cell. From the large number of peaks mixtures we conclude that there are at observed for 1602-1802 least eight O2molecules per unit cell in the m-phase. The structure is undoubtedly of low symmetry, yet it cannot be greatly different from the a-phase structure of one molecule per unit cell because of the ease of conversion of m-phase to a-phase at 13 K At present we believe that the formation of the metastable m-phase under low-temperature vapor deposition conditions results from a unique structure established in the first few monolayers. Once formed, this surface structure locks in the growth of the m-phase bulk crystal through epitaxy. We speculate that the driving force for formation of the m-phase structure, which differs from the a-phase packing arrangement, is the absence of threedimensional spin interaction in the former. We note that several studies, neutron diffraction,8 heat ~ a p a c i t yLEED,’O ,~ and X-ray diffraction,” of a few monolayers of 0, on graphoil give evidence for a structure differing from that of a-02 with a phase transition at 12 K, exactly where we see disappearance of the m-phase. The implications of this coincidence require further study. The stabilization of the m-phase by impurities could occur because of a disruption of the three-dimensional antiferromagnetic ordering, which is a prime factor in stabilizing the a-phase. As stated above, we believe that this metastable phase gives rise to the unusual diffraction lines previously o b ~ e r v e din ~ *low ~ temperature oxygen. Further experiments are in progress to better characterize the metastable phase found in thin films of oxygen deposited from the vapor at 10 K. (6) B. I. Swanson, S.F. Agnew, L. H. Jones, R. L. Mills, and D. Schiferl, J . Phvs. Chem.. 87. 2463 (1983). (7j A. F. Prikhot’ko, Yu G. Pikus, and L. I. Shanski, Sou. J . Low Temp. Phys., 6 , 518 (1980). ( 8 ) M. Nielsen and J. P. McTague, Phys. Reu. E , 19, 3096 (1979). (9) J. Stoltenbern and 0.E. Vilches. Phvs. Rev. E , 22. 2920 (1980). (IO) M. F. T o n e i R . D. Diehl, and S.C. Fain, Jr., Phys. Reu. E , 27, 6413 (1983). (1 1) P. A. Heiney, P. W. Stephens, S.G. J. Mochrie, J. Akimitsu, R. J. Birgeneau, and P. M. Horn, Surf. Sci., 125, 539 (1983).
