J . Phys. Chem. 1986, 90, 2159-2163
-
efficient population: D* D+ inefficient population: D*
-
+ e-
D+
current production
+ e-
-
(la) (1b)
D+ + eD recombination (2) In eq l a the excited dye would inject an electron through an efficient pathway into the bulk of the semiconductor to produce photocurrent with high efficiency. In eq l b and 2 the electron would be trapped at the surface from which it returns to the parent oxidized dye, yielding little or not net photocurrent. One is inclined to seek the physical cause for the different @ of these two populations of dye in the nature of the site at which the dye is adsorbed. For example, Bressel and Gerischer” have examined the recombination reaction of (lb) and (2) at SnOa and ZnO and with thermal desorption techniques have characterized these sites as hydroxyl groups on the surface of the semiconductor. In this work an attempt has been made in the experiments of Figure 4 to distinguish these different sites by measuring the relative enthalpies of adsorption of the dye at these two locations on the surface. With the assumption that dye molecules weakly adsorbed to the surface will desorb during a wash procedure before those which (17) Bressel, B.; Gerischer, H. Eer. Bunsenges. Phys. Chem. 1983, 77,963.
2159
are more strongly adsorbed, the data of Figure 4a for rhodamine B indicate that the efficient dye population is less tighly bound to the surface than the inefficient one. This follows from the observation that CP falls rapidly as 8 declines from 0.3 to 0.1, Those dye molecules left on the surface are those most strongly adsorbed, they sensitize photocurrent least efficiently. This also appears to be the case for rose bengal a t higher 8, but to a lesser extent. However, AHadsis higher for rose bengal resulting in a rate of desorption which is much slower than rhodamine B. If the AH difference between efficient and inefficient sites is small, the resultant difference in the desorption rates may not be as significant for rose bengal as for rhodamine B over the 10-min time span of this dynamic mesurement. Such tentative interpretations must await confirmation through more thorough studies of this relation between 0 and AI&&. However, it is apparent that there is a correlation between the arrent-sensitizing ability of a molecule and strength of its adsorptive interaction with the surface.
Acknowledgment. The author thanks M. A. Ryan and E. C. Fitzgerald for their assistance in the laboratory with these experiments. He is also indebted to Prof. R. Helbig for the gift of the ZnO single crystals and to A. Nozik for his support at SERI while the author was on sabbatical leave. This work was supported by the Office of Basic Energy Sciences of the Department of Energy. Registry No. ZnO, 13 14- 13-2; rhodamine B, 8 1-88-9; rose bengal, 11121-48-5.
Surface Chemistry of Phosphine on Clean and Oxidized Iron R. I. Hegde and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: November 12, 1985)
The interaction of PH, adsorbed on clean and oxidized iron at 100 K has been studied by temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). In TPD after PH3 adsorption on clean iron, both molecular desorption and dissociative processes are evident. On oxidized iron, the dissociation of PH, is inhibited. Coadsorption of D2 and PH, leads to no detectable deuterium incorporation in the desorbing phosphine, but all the isotopic forms of molecular hydrogen desorb in relatively large amounts. Preadsorbed PH3 inhibits D2 adsorption, while postdosed PH, not only displaces D2but also changes its desorption peak shape. Compared to clean Fe, adsorption of D2is strongly inhibited on both oxidized and PH,-covered oxidized iron surfaces at 100 K.
Introduction The surface chemistry of adsorbed organophosphorus molecules has been the subject of recent investigation^.^-'^ Recently we a simple reported the surface chemistry of phosphine (PH3),192,4 phosphorus-containing molecule, and dimethyl methylphosphonate (DMMP),, a relatively large organophosphorus compound, on Rh( 100) using various surface science spectroscopic techniques. (1) Hegde, R. I.; Tobin, J.; White, J. M. J . VUC.Sci. Technol. A 1985, 3, 339. (2) Hegde, R. I.; White, J. M. Surj. Sci. 1985, 157, 17. (3) Hegde, R. I.; Greenlief, C. M.; White, J. M. J . Phys. Chem. 1985,89, 2886. (4) Greenlief, C. M.; Hegde, R. I.; White, J. M. J. Phys. Chem. 1985,89, 5681. (5) Ertl, G.; Kuppers, J.; Nitschke, F.; Weiss, M. Chem. Phys. Lett. 1977, 52, 309. (6) Nitschke, F.; Ertl, G.; Kuppers, J. J . Chem. Phys. 1981, 74, 2911. (7) Kiskinova, M.; Goodman, D. W. Surf. Sci. 1981, 108, 64. (8) Yu, M. L.; Myerson, B. S. J. Vac. Sci. Technol. A 1984, 2, 446. (9) Shanahan, K. L.; Muetterties, E. L. J . Phys. Chem. 1984, 88, 1996. (10) Garfunkel, E. L.; Maji, J. J.; Frost, J. C.; Farias, M. H.; Somorjai, G. A. J. Phys. Chem. 1983, 87, 3629. (11) Friend, C. M.; Muetterties, E. L. J. Am. Chem. SOC.1981, 103, 773. (12) Tsai, M.-C.; Muetterties, E. L. J . Am. Chem. SOC.1982, 103, 2534. (13) Tsai, M.-C.; Friend, C. M.; Muetterties, E. L. J. Am. Chem. SOC. 1982, 104, 2539.
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As part of our continuing investigation of the surface chemistry of phosphorus-containing molecules on a variety of transition metals, we report here the behavior of PH3 on clean, phosphorus-covered, deuterium-covered, and oxidized Fe surfaces. The experiments include temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and hydrogen-deuterium isotope exchange. To our knowledge, the surface chemistry of PH3 on an iron surface has not been previously reported. Ertl and co-workers5 have studied a related molecule, PF3, which dissociates on Fe.
Experimental Section The experiments were carried out in a stainless steel ultrahigh-vacuum chamber pumped by a 450 L/s turbomolecular pump which has been described elsewhere.’i3J4 The base pressures were in the low 10-l0-torr range. Briefly, the chamber was equipped with (1) a line-of-sight mass spectrometer which was computer interfaced to multiplex up to nine peaks in TPD, (2) an argon ion gun for sputtering, and (3) a single-pass cylindrical mirror analyzer with a coaxial electron gun for AES. The polycrystalline Fe was a thin rectangular disk, 1 cm X 1 cm X 0.1 cm, metallographically etched and polished. This sample (14) Hegde, R. I.; White, J. M. J . Phys. Chem. 1986, 90, 296
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was spot-welded to 0.05-mm-diameter tantalum wires which were connected to a sample holder cooled by liquid nitrogen. By resistively heating the tantalum wires, the temperature could be controlled between 100 and 1000 K. A chromel-alumel thermocouple was spot-welded to the back of the iron crystal for temperature measurements. For TPD, heating rates between 16.5 and 25.0 K/s were used and temperatures were kept below 600 K to avoid segregation of impurities to the surface. The crystal was cleaned by prolonged argon ion gun sputtering at high temperatures. Initially, the iron sample was contaminated with large amounts of sulfur, carbon, chlorine, oxygen, and nitrogen. Extensive sputtering between 600 and 900 K using a 2-kV, 1O-MAargon ion beam for 8 h reduced the oxygen, chlorine, and nitrogen impurities to acceptable levels (measured by AES). At this stage, carbon and sulfur were the main impurities. The carbon was depleted by exposing the sample to 0, at 300 K and flashing to 1000 K. Oxygen accumulation was removed by heating in H,, sputtering, and flashing to 900 K. The dissolved sulfur was reduced by heating to 900 K and sputtering for several days. This continued cleaning by argon ion sputtering reduced the sulfur contamination to less than 1% of a monolayer. The effective surface coverages were calculated by using measured AES peak-to-peak heights and standard sensitivities taken from the Physical Electronics Handb00k.l~ PH3, D,, and 0,(all obtained from Union Carbide, 99.999% min purity) were stored in lecture bottles. The purities of all the gases were verified by mass spectrometry. Pressure readings were corrected by using ion gauge sensitivity (relative to nitrogen) factorsI6 of 2.6 for PH,, 0.35 for D,, and 1.O for 0,. All the gases were adsorbed at 100 K by backfilling the chamber to p C 5 X torr. Exposures are given in langmuirs; 1 langmuir = torr s.
Results and Discussion Adsorption of PH3 on Fe. PH, was adsorbed on clean Fe at 100 K and examined by both TPD and AES. The AES spectrum of clean Fe is shown in Figure 1A. TPD after adsorption of PH3 at 100 K leads to both molecularly and dissociatively adsorbed PH,, as the following evidence indicates. Both PH, and H, desorption signals were observed in the thermal desorption spectra. Near saturation PH3coverage, the molecular PH3desorbs around 170 K ( T J . The calculated activation energy for the molecular PH, desorption is 9.6 kcal/mol, based on the experimental value of Tp= 170 K, and assuming simple first-order desorption kinetics with Y = l o t 3s-1,17 The desorption temperature of PH3 from Rh(100)' is considerably higher (325 K) than from Fe (170 K), indicating stronger binding on Rh. Another piece of evidence for dissociative adsorption is that the AES shows a significant amount of phosphorus residue on the Fe surface (Figure 1B) after TPD to 600 K and recooling. The hydrogen thermal desorption spectra (derived from the decomposition of PH,) depended on the PH3 exposure. At low PH3 exposure, the H2 desorption shows a broad peak centered around 41 5 K just as it does for low coverages of H2 on a clean surface. With increasing exposure the H2desorption spectra shift to lower temperature by as much as 100 K (between 0.1 and 0.4 langmuir of PH, exposure) suggesting second-order H-H recombination-desorption kinetics. On both Rh and Fe surfaces PH3 is characterized by significant amounts of irreversible dissociation at low temperatures (see below). The irreversibility is indicated by the lack of significant incorporation of deuterium in the desorbing molecular phosphine in coadsorption experiments; once P-H bonds begin to break, they are not readily reformed. PH3 on Phosphided Fe. In the TPD of PH3from a phosphided Fe surface (prepared by decomposition of phosphine) 10% more PH3 desorbed for a given dose than on a clean surface, but the (1 5 ) Handbook of Auger Electron Spectroscopy; Physical Electronics Industries, Inc. Minnesota, 1972. (16) Summers, R. L. NASA Technical Note TND-5285;NASA: Washington, DC, June 1969. (17) Redhead, P. A. Vacuum 1962, 12, 203.
Hegde and White I
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Figure 1. (A) Auger spectrum of clean iron after argon ion bombardment, oxidation, and reduction cycles. (b) Auger spectrum of polycrystalline iron after thermal desorption of a dose of phosphine.
PH, desorption peak position was not altered. For a given PH, dose, there was a 40% decrease in the amount of H 2 desorption on a phosphided iron surface compared to that on a clean surface. These results suggest that PH3adsorption on a clean Fe surface leads to phosphiding before PH3 desorbs. Coadsorption of D2 and PH3 on Fe. Details of the D2/Fe system will be reported separately.I8 The only points of significance here are (1) deuterium adsorption on iron is dissociative, as generally observed on group VI11 (groups 8-10)19 metals for adsorption at 100 K, and (2) a 2000-langmuir exposure is sufficient to saturate the surface. As shown in Figure 2, for saturation D(a) followed by 1.0 langmuir of PH3, there was no detectable exchange between PH3 and D2, i.e., no PH2D, PHD,, or PD3 desorbed. Thus, no desorbing molecular phosphine is produced by recombination of an intermediate PH, or PH species with surface deuterium (hydrogen). Since there is no D-for-H isotope exchange in the molecularly desorbing PH,, and since a major fraction of the adsorbed PH, does dissociate (see below), PH3dissociation on clean Fe is highly (18) Hegde, R. 1.; White, J. M.; Callings, E. W., to be published. (19) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the pblock elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., 111 3 and 13.)
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Surface Chemistry of Phosphine on Iron
The Journal of Physical Chemistry, Vol. 90, No. 10, 1986 2161
5 v)
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probable and strongly irreversible under the conditions of our experiments. The amount of PH3desorbing molecularly increased slightly in the presence of D(a) but the desorption energetics remain the same. The total amount of hydrogen desorbing due to decomposition of PH, also decreased in the presence of D(a). Thus preadsorbed deuterium decreases (by roughly 10%) the fraction of PH, that decomposes. Deuterium desorption from a surface without coadsorbed PH, is shown in Figure 3A. For a surface saturated with D(a) (2000 langmuirs) and then dosed with 2.0 langmuirs of PH,, all the isotopically labeled hydrogen molecules were observed in subsequent TPD (D2 and H D are shown in Figure 3B,C). Coadsorbing PH, changes both the D2desorption peak area and the peak shape. In the absence of PH,, the D2 TPD is broad (which may be considered as a superposition of more than one kind of desorbing state) and centered around 355 K (see Figure 3A) in agreement with previous work.'* When 2.0 langmuirs of PH, was postdosed, the high-temperature part of the desorption peak attenuated and at the same time D2 desorption around 260 K increased (Figure 3B). In the same temperature region, significant amounts of H D (Figure 3C) and Hz (not shown) also desorb. After a 2-langmuir exposure of PH,, the Dz desorbed is reduced by about 50%. In part, this is the result of reaction with H(a) to form HD, which desorbs around 260 K from the Fe surface. However, the total amount of deuterium desorbing (D2 0.5HD) is about 30%lower than without the PH3 dose. Since incorporation of D into PH, is not significant, some D(a) is displaced during PH, exposure. The adsorption of D, was strongly influenced by the presence of adsorbed phosphine and its decomposition products. The D2 desorption spectra from clean Fe and an Fe surface with 0.7 monolayer of phosphorus (produced by a 2-langmuir PH3 dose and estimated by AES) are compared in Figure 4. In both cases, the D2dose was 2000 langmuir. On the PH,-exposed iron surface, no significant D2 adsorption was observed. Thus most of the sites available for dissociative adsorption of D, are filled by PH, and its decomposition products. The coadsorption interactions observed here are similar to those found on Rh( loo)., As on Rh, PH3 displaces preadsorbed D(a) from the Fe surface. For both metals, the desorption peak shapes in D2TPD are modified by the interactions with PH, adspecies and the high-temperature D2 state is strongly attentuated and shifted to lower temperatures. This indicates repulsive interactions between the coadsorbed species. Finally, on both metals, H2(D2) adsorption was strongly inhibited by the presence of adsorbed PH,
200 300 4 0 0 5 0 0 TemperatureIK
Figure 3. (A) Thermal desorption spectrum of deuterium from clean polycrystalline iron. The dose was 2000 langmuirs at 100 K. (B and C) D, and HD thermal desorption spectra after dosing 2000 langmuirs of D2and then 2 langmuirs of PH3 on polycrystalline iron at 100 K.
+
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TemperaturelK Figure 4. Thermal desorption spectra of D, from polycrystalline iron (A) and from polycrystalline iron predosed with 2 langmuirs of PH, (B). In each case the dose of D2 was 2000 langmuirs at 100 K.
and its decomposition products. Adsorption of PH, on Oxidized Fe. A thin layer of iron oxide was prepared by exposing Fe to 5 X torr of 0,for 500 s at 300 K. This procedure gives an FeO, surface (where the value
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The Journal of Physical Chemistry, Vol. 90, No. 10, 1986
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Figure 7. Dependence of the molecular phosphine TPD area (from Figure 6 ) as a function of phosphine exposure at 100 K on polycrystalline
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Figure 6. Molecular phosphine desorption from an oxidized iron surface for several exposures of phosphine at 100 K.
Figure 8. AES spectra of oxidized polycrystalline surfaces before (A) and after (B) exposure to phosphine.
of x varied from 1.4 to 1.8) as estimated by AES. PH, was adsorbed on oxidized iron at 100 K and examined at that temperature by AES. A measure of the relative phosphorus coverage was calculated from the following ratio of peak heights: P(120 eV)/[Fe(651 eV) + O(S10 eV)] (Figure 5 ) . This intensity ratio initially increases very rapidly but approaches saturation very slowly (more than 10 langmuirs of PH3 is required). No quantitative data on the absolute initial sticking coefficient can be derived since the maximum coverage is not known. When PH3 was adsorbed a t 100 K on oxidized Fe, only molecular PH3was detected in the thermal desorption. The observed H, desorption peak tracked molecular PH, desorption and the
relative intensity was identical with that obtained when gas-phase PH, was measured directly. A series of PH, thermal desorption spectra from an oxidized Fe surface, as a function of PH3exposure, are shown in Figure 6. At low exposure, there is a single peak centered around 200 K. The peak shifts to lower temperature (160 K) and broadens with increasing PH3coverage, just like that from a clean Fe surface. We take this to imply that PH3 adsorption involves more than a single type of adsorption site. I n d d , the high-temperature peak is evident as a shoulder in the 2langmuir dose TPD curve. Part of the shift to lower temperatures may also be due to repulsive lateral interactions between the PH, molecules.
J. Phys. Chem. 1986, 90, 2163-2168 Figure 7 shows the variation of PH, desorption peak area as a function of PH, exposure. Comparison of Figure 5 with Figure 7 shows that PH, desorption approaches saturation more slowly than the AES P/(Fe 0) ratio. This suggests, not surprisingly, that PH, is sensitive to electron beam damage. While this was not investigated here, it is of interest for future work. Near saturation, PH, desorbs around 160 K from the oxidized iron surface (about 10 K lower than on the clean Fe surface). Assuming a preexponential factor" of IO', s-l, the desorption energy for the chemisorbed PH, is 11.5 kcal/mol in the limit of low coverage. For near-saturation coverage, the desorption energy drops to 9.1 kcal/mol (9.6 kcal/mol on the unoxidized surface). That PH3 adsorption on the oxidized Fe surface at 100 K is molecular is further supported by the fact that no detectable residue was left on the oxidized iron surface a t the end of the desorption as monitored by AES (Figure 8). Thus there is a marked difference between the behavior of PH, on clean and oxidized iron, the latter inhibiting dissociative adsorption. Comparison of the amount of PH3 desorbed from clean and oxidized iron indicates that at low coverages 70 f 15% of the adsorbed PH3 decomposes on a clean iron surface. The adsorption of D2 is strongly inhibited on oxidized Fe, with or without the presence of PH3. A 2000-langmuir D2 adsorption
+
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and subsequent TPD showed no D2 desorption from either.
Summary This work can be summarized as follows: 1. At 100 K, PH3 adsorption on clean Fe is both molecular and dissociative, while on oxidized Fe it is molecular. 2. Preadsorbed phosphorus and/or deuterium on clean iron inhibit PH3decomposition but do not change the PH, desorption energetics. 3. Postdosed PH3displaces some preadsorbed deuterium from the Fe surface and lowers the D2 desorption peak positions. 4. Preadsorbed PH3 inhibits D2 adsorption. 5. Coadsorption of D2and PH3 leads to no incorporation of D into the molecular PH, desorption, showing that PH3 decomposition on clean Fe is irreversible under the transient conditions of these experiments. 6. The adsorption of D2is inhibited on oxidized Fe in the presence or absence of coadsorbed PH3. Acknowledgment. This work was supported in part by the U S . Army Research Office. Registry No. Fe, 7439-89-6; PH3, 7803-51-2; HI, 1333-74-0
Surface Raman Excitation and Enhancement Profiles for Chromate, Molybdate, and Tungstate on Colloidal Silver Hannah Feilchenfeldt and Olavi Siiman* Department of Chemistry, Clarkson University, Potsdam, New York 13676 (Received: November 15. 1985)
The surface-enhanced Raman scattering (SERS) excitation profiles of chromate, molybdate, and tungstate ions on colloidal silver were obtained by using excitation wavelengths between 457.9 and 676.4 nm. The intensity of the strongest SERS band between 800 and 900 cm-I, assignable to v,(M-0), was in each case referenced to the 1020-cm-' band of the internal standard, methanol, in silver hydrosols to which methanol had been added. These relative SERS intensities were compared against similar intensity ratios for each oxo anion in solution and then appropriately scaled for the concentration of Mod2in solution and on the surface of the silver particles in the sols. Pedk SERS enhancements occurred at 560 nm for Cr042and at 600 nm for both MOO:- and WOZ-. The latter peak positions matched the secondary absorption band maxima that originate from aggregates of silver particles in the sols. The numerical values of the SERS enhancements were 5 X 104-1 X IO5 for CrO:-, 9 X IO' for MOO:-, and about 2 X lo5 for W04*-on colloidal silver. The somewhat lower SERS enhancements for the chromophoric adsorbate, CrO:-, which adsorbs maximally at 370 nm in aqueous solution, are attributed to a lower degree of aggregation for the sols with added Cr0:- rather than to partial quenching of its resonant excited state on the heavy metal (silver) surface. Also, the position and intensity of SERS bands of these oxo anions on the silver suface and their excitation profiles are in agreement with the existence of chemisorbed Mod2-ions on silver rather than an outer dielectric layer of a silver(1) complex, Ag2M04.
Introduction Enhanced Raman scattering from chromophoric adsorbates on silver1-" and gold12 surfaces has recently received considerable attention. The two resonance processes, surface-enhanced Raman scattering (SERS) and surface resonance Raman scattering (SRRS), may occur separately or as a hybrid, surface-enhanced resonance Raman scattering (SERRS). It is conceivable that under the appropriate conditions the individual 105-1 O6 enhancements of SERS and SRRS will combine to give total enhancements as large as 10'o-10'2. Some experimental measures of the coupling between SERS and SRRS are excitation profiles, the type of Raman spectra as a function of exciting wavelength (Le., relative intensities should change by excitation into one or the other resonance since selection rules are different), SERS enhancements, and lifetimes of resonant excited states for the surface species relative to the one in solution (connected also to On leave from the Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel.
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possible homogeneous broadening of surface Raman bands). The above experimental results can then be related to the following: (1) the overlap of SERS and SRRS excitation profiles, Le., the energy gap between their peaks and their intensity and bandwidths; (2) polarization effects on the adsorbate through lateral interaction^'^,'^ (excited-state dipole-dipole (1) Campbell, J. R.; Creighton, J. A. J. Elecfroanul.Chem. 1983, 143, 353. (2) Stacy, A. M.; Van Duyne, R. P. Chem. Phys. Lett. 1983, 102, 365. (3) Weitz, D. A,; Garoff, S.; Gersten, J. I.; Nitzan, A. J . Chem. Phys. 1983, 78, 5324. (4) Hildebrandt, P.; Stockburger, M. J . Phys. Chem. 1984, 88, 5935. (5) Bachackashvilli, A.; Katz, B.; Priel, Z.; Efrima, S. J. Phys. Chem. 1984, 88, 6185. (6) Chambers, J. A.; Buck, R. P. J. Electronanal. Chem. 1984, 163, 297. (7) Pettinger, B. Chem. Phys. Lett. 1984, 110, 576. (8) Siiman, 0.;Lepp, A,; Kerker, M. J. Phys. Chem. 1983, 87, 5319. (9) Siiman, 0.; Lepp, A. J. Phys. Chem. 1984, 88, 2641. (10) Siiman, 0.; Smith, R.; Blatchford, C.; Kerker, M. Langmuir 1985, I , 90. (11) Lepp, A,; Siiman, 0. J . Phys. Chem. 1985,89, 3494. (12) Siiman, 0.; Hsu,W. P. J. Chem. Soc., Faraday Trans. I , in press. (13) Murray, C. A,; Bodoff, S. Phys. Rev. Let?. 1984, 52, 2273.
0 1986 American Chemical Societv
