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J. Phys. Chem. B 1997, 101, 1588-1593
SERS Excitation Profiles of Phthalazine Adsorbed on Single Colloidal Silver Aggregates as a Function of Cluster Size B. Vlcˇ kova´ * Department of Physical and Macromolecular Chemistry, Charles UniVersity, HlaVoVa 2030, 128 40 Prague 2, Czech Republic
X. J. Gu The Ontario Laser and LightwaVe Research Centre, UniVersity of Toronto, M5S 1A1, Canada
M. Moskovits* Department of Chemistry, UniVersity of Toronto, M5S 1A1, Canada ReceiVed: June 11, 1996; In Final Form: NoVember 7, 1996X
The wavelength-dependent SERS enhancement and photoreactivity of phthalazine (pht) adsorbed on single Ag colloid fractal clusters deposited out of solution onto a glass surface were determined as a function of cluster size from the excitation profiles of the SERS spectra of three individual colloid clusters with dimensions 1.9 × 1.9 µm, 3.7 × 2.8 µm and 7.5 × 4.7 µm. The spectra were correlated with a given cluster by using imaging Raman microspectroscopy. The SERS excitation profiles obtained from various bands observed in the SERS spectra could be understood in terms of two wavelength-dependent contributions: (i) the photochemical activity which grows monotonically toward the blue in the spectral range (470-650 nm) used and (ii) the SERS enhancement function which increases toward the red. The variety of shapes observed for the excitation profiles can be explained in terms of the relative contribution of the pht reagent and the photoproduct to the sometimes overlapping SERS bands. The relative contributions (or lack of overlap) suggested by the SERS excitation profiles are in good agreement with the independent assignments of those bands to pht or its photoproduct. The wavelength-dependent SERS enhancement and the photoreactivity were found to be approximately independent of cluster size for the three clusters studied in keeping with predictions made in the literature relating to the optical properties of fractal clusters.
Introduction Surface enhancement of optical processes such as Raman scattering, absorption, fluorescence, photochemistry, and second harmonic generation is observed for molecules located at, or in close proximity to, roughened surfaces or small particles of freeelectron-like metals such as Ag, Au, and Cu, provided that the frequency of the incident radiation is close to the excitation frequency of surface plasmons localized in the microscopic metal structures.1 The same illumination source can produce surface photochemistry1-3 and at the same time excite SERS which can serve as the analytical signal used to follow the photochemical kinetics.4-10 This is an advantage when photochemistry is the object of the study. On the other hand, surface photochemistry can be an unwanted complication when the SERS spectrum of the pure photoreagent is desired and the claim has been made that that several SERS spectra reported in the literature were contaminated by signals due to photoproducts.10 The vast majority of SERS studies of photochemical processes have involved cold deposited films, roughened electrodes and assemblies of colloidal (hydrosol) aggregates. However, we have shown in a previous study that, by using Raman microspectroscopy, SERS spectra could be obtained from single colloidal aggregates that are gravitationally deposited on a glass surface.11 In that study we were unable to report the SERS excitation spectrum of a single adsorbate-covered colloid cluster, as had been our original intention, due to the decomposition of X
Abstract published in AdVance ACS Abstracts, February 1, 1997.
S1089-5647(96)01703-8 CCC: $14.00
the clusters at longer wavelengths. We have since succeeded in overcoming this problem by modifying slightly the method of colloid preparation and began a series of SERS studies using phthalazine-covered single colloid clusters. In the interim it was discovered that phthalazine adsorbed on Ag hydrosol aggregates undergoes photochemistry.10 Rather than switching to an adsorbate which is not photochemically active we decided to continue with the phthalazine (pht) system and extract simultaneously the colloid-cluster-size dependence both of the SERS enhancement and the photochemical activity. This is done by determining the SERS excitation profiles of vibrational modes of an adsorbate located on the surface of single Ag colloidal aggregates recognizing that, in this case, the excitation profiles will have contributions both from the enhancement and the photochemical activity. Micron-sized colloid clusters such as those used in this study are known to be fractal objects.12,13 The optical properties of fractal clusters have been the subject of considerable literature.12-15 Stockman14 showed that in addition to the Hausdorff or fractal dimension characterizing the scaling properties of the geometry of fractal clusters, certain optical properties of excited fractal clusters will scale as a power law whose exponent was dubbed the spectral dimension. Optical scaling would only occur under certain conditions relating to the wavelength of excitation and the optical properties of the material comprising the fractal. For silver it was shown14 that the optical scaling region extends over most of the visible spectral region. Furthermore, it was shown experimentally that the localization length of surface plasmon excitations localized on fractal colloidal aggregates lies in the © 1997 American Chemical Society
SERS Excitation Profiles of Phthalazine range 60-140 nm.15 The optical response of clusters with dimensions larger than this localization length should be independent of cluster size. The use of micro-Raman enables us to measure the SERS excitation profiles of selected individual clusters differing only in size (i.e., in the number of particles constituting the cluster). Electron microscopic analysis shows that the diameters of the individual Ag colloidal particles and the interparticle distance between neighboring particles within a colloid cluster are approximately constant. These are precisely the conditions assumed in the theoretical treatments of the optics of clusters. Hence one should be in a position to test some of these predictions, in particular the expectation that the SERS enhancement and its wavelength dependence are cluster-sizeindependent in the scaling region approaching, in essence, universal curves for a given material and for a given fractal type. Experimental Section Ag colloids were prepared by reducing AgNO3 (Aldrich) with NaBH4 (Merck) using the procedure described in refs 11, 16, and 17. It was discovered that by increasing the NaBH4:AgNO3 molar ratio to 6.0 as in ref 17 but otherwise following the procedure described in ref 16, Ag colloid resistant to laserinduced decomposition could be produced. This is the colloid used in this study. Briefly, a solution consisting of 3.4 mg of AgNO3 in 20 mL of water cooled to ca 10 °C was added dropwise, with constant stirring, to a solution prepared by dissolving 4.53 mg of NaBH4 in 60 mL of water precooled to 2 °C. Stirring was continued for approximately 45 min during which time the colloid came to room temperature. Distilled deionized water was used throughout. Adsorbate was introduced into the colloid by adding 20 mL of a 10-2 M solution of phthalazine (Aldrich) to 2 mL of Ag colloid. The final concentration of pht in the system was, therefore, 10-4 M. The glass-deposited colloid clusters were prepared by first diluting the pht-covered colloid to a concentration 1/5 its original then allowing the colloidal aggregates to settle gravitationally for approximately 3 h onto a Pyrex glass microscope cover slide placed on the bottom of a covered Petri dish containing the diluted colloid/pht sample. Aggregates produced in the same manner but deposited on C-coated Cu grids were examined by TEM and found to be fractal colloidal clusters.12 The colloid-cluster-covered slides were removed from solution and allowed to dry in air. They were then mounted onto the sample microscope (Zeiss 40× objective) of a Raman microspectrometer (Spex 1877 C Triplemate spectrometer) equipped with a CCD multichannel detector (Princeton Instruments), and the sample was examined in order to identify wellisolated colloid clusters of differing dimensions. Once the choice of cluster was made its SERS spectra were excited with the 468.0, 476.2, 530.9, 568.2, and 647.1 nm lines of a Kr+ laser (Lexel Model 3500) or with the 632.8 line of a He-Ne laser. The spectra reported are the results of 10 accumulations each of 10 s duration. Samples were mounted on the microscope’s XYZ stage equipped with a micrometric adjustment which was used to position the cluster in the focused laser beam, to adjust the focus of the laser, and to reposition the cluster after changing the laser line. The image of the illuminated colloidal aggregate was recorded by reflected light using a Sanyo Color CCTV camera and displayed on a Panasonic Video monitor. The microscope imaged a selected 240 × 180 µm area of a colloid-covered cover slide. The image of the area chosen was photographed from the monitor screen using a Polaroid camera. This was essential to ensure that the same clusters were being
J. Phys. Chem. B, Vol. 101, No. 9, 1997 1589 investigated following changes of excitation wavelengths. Three clusters of varying 2-D projected dimensions were chosen for study: cluster 1 (1.9 × 1.9 µm), cluster 2 (3.7 × 2.8 µm), and cluster 3 (7.5 × 4.7 µm). Since the clusters were obtained from the same sample of adsorbate-covered colloidal aggregates, it is safe to assume13 that the clusters selected for study are characterized by the same fractal dimension and vary only in the number of colloidal particles comprising each cluster. The numbers of particles in the three clusters selected were approximately 3800 for cluster 1, 7000 for cluster 2, and 14 500 for cluster 3. The image of the laser spot appeared to the eye to be approximately 3.5 µm in diameter. However, the intensity response of the eye is greatly nonlinear. This suggests that the portion of the focused laser beam (which presumably has a Gaussian profile) containing most of the energy was considerably smaller than this. When the desired cluster was selected and the laser focused and positioned appropriately the instrument was switched to Raman mode and the SERS spectrum collected. A pressed KNO3 pellet mounted next to the colloid-decorated cover slide was used as an external intensity standard. Its Raman spectrum was recorded before and after each SERS measurement. The two control spectra were consistent. As a further check on consistency a photograph was taken of the microscope image following all adjustments. The micrometric adjustment was sufficiently precise, however, that there was never any perceptible difference between the image taken after the adjustment and the original image. In this way it was ensured that the SERS spectra of the same three clusters were measured at all excitation wavelengths and that the laser beam position with respect to each of the clusters was consistent from measurement to measurement, at least as far as was possible to discern by eye. Results and Discussion We had shown previously11 that glass-deposited colloidal Ag clusters underwent a structural change following exposure to 50 mW of 647.1 nm Kr ion irradiation. The structural change appeared to be the collapse of the two-dimensionally projected fractal cluster which retains considerable three-dimensional character to a far more two-dimensional structure. We found that pht-covered aggregates obtained from colloids prepared with a higher NaBH4:AgNO3 ratio (6.0) (i.e., a more negative surface potential) than the colloid used for the original preparation (NaBH4:AgNO3 ) 4.7) were far more resistant to laser-induced structural change. Using this colloid, well-separated aggregates of pht-covered Ag particles could be deposited routinely (Figure 1). The three clusters whose SERS spectra and excitation profiles are reported in this study are indicated in Figure 1. The SERS spectra of the three clusters are shown in Figures 2-4, respectively, for clusters 1-3. Each of these spectra has been normalized for excitation line intensity by dividing the intensity of the raw SERS spectra by the corresponding intensity of the 1068 cm-1, ν1 band of the potassium nitrate standard. The SERS spectra of the three clusters (Figures 2, 3, and 4) are mutually very similar and show nearly the same excitation wavelength dependence. With 647.1 and 632.8 nm excitations, the SERS spectra consist entirely of the a1 modes of pht: 528, 803, 1023, 1230 (1330 w), 1382, 1457 cm-1. SERS spectra excited with the 568.2 and 530.9 nm lines show additional bands at 756, 946, and 1332 cm-1. With 482.5 and 468.0 nm excitation, the additional bands gain intensity with respect to the original bands of pht and more additional bands appear at 489, 501, 545, and 1278 cm-1. The additional bands occurring in the SERS spectra of pht with green and blue excitation
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Figure 1. Micrograph of pht-covered Ag aggregates deposited on a Pyrex slide obtained using the sample microscope of a Raman microspectrometer (ca. 1000× magnification). Aggregates selected for SERS spectral measurements are indicated with circles: cluster 1 (1.9 × 1.9 µm); cluster 2 (3.7 × 2.8 µm); cluster 3 (7.5 × 4.7 µm). The dark spots in the image are permanent defects in the imaging system.
correspond to those observed in the SERS spectra of pht-covered aqueous Ag colloid10 indicating that a similar photoprocess occurs with glass-deposited pht-covered Ag colloid clusters in air as it does on “three-dimensional” colloid clusters in solution. In ref 10, the new SERS lines growing in when the pht-covered colloid is exposed to light of sufficiently short wavelengths are attributed to the formation of a photoproduct wherein the N-N bond of pht is broken to form an adsorbed species resembling an ortho-substituted benzene. SERS excitation profiles were calculated for all of the bands observed in the SERS spectra of the pht-covered clusters by plotting the band areas as a function of the excitation wavelengths. Since all of the spectra were referenced to the 1068 cm-1 Raman band of the potassium nitrate standard in order to correct for laser intensity and for other perturbations produced by switching from one excitation wavelength to another, the ν4 dependence of the Raman intensity was also approximately canceled out in the process. The resulting excitation profiles are shown in Figures 5, 6, and 7 arranged in groups showing similar trends. The profiles in Figure 5 generally increase toward the red, except for the decrease shown by the profiles of the 803 and 1023 cm-1 bands of two of the clusters with the red-most excitation. The profiles shown in Figure 6 are monotonically decreasing with increasing wavelength, while those of Figure 7 show an initial decrease followed by an increase in the band intensities as the wavelength increases. All of these behaviors can be understood in terms of varying contributions of the extent of photoreactivity as a function of wavelength, coupled with the wavelength dependence of the SERS enhancement. It had been previously shown10 that the degree of photoreactivity of phthalazine adsorbed on colloidal silver aggregates
Figure 2. SERS spectra of pht originating from cluster 1 (1.9 × 1.9 µm) excited with (A) 468.0, (B) 476.2, (C) 530.9, (D) 568.2, (E) 632.8, (F) 647.1 nm laser light.
increases toward the blue. The partial reactivity achieved with some excitation wavelengths (even after long exposure times), for example, with green light was postulated to be due to the fact that the absorption which excites the molecule to its photoreactive state is inhomogeneously broadened, and hence an excitation in the green will only cause a fraction of the adsorbed phthalazine molecules to react. If R(λ) and P(λ) are respectively the fraction of the initial reagent left on the surface and the fraction of the original reagent converted to product, then for a single-branched process R + P ) 1. A SERS band will, in general, consist of overlapping contributions from the reagent and the product. (Since the product proposed in ref 10 resulting from the photodecomposition of pht is structurally
SERS Excitation Profiles of Phthalazine
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Figure 3. As in Figure 2 but for cluster 2 (3.7 × 2.8 µm).
Figure 4. As in Figure 2 but for cluster 3 (7.5 × 4.7 µm).
related to pht, it is not surprising that some of the bands will have significant overlap.) Accordingly, the (integrated) SERS band intensity will, in general, be given by an expression of the form
to the product P(λ) G(λ). The bands at 756 cm-1 and the doublet at 483, 501 cm-1 which was integrated as a single band are attributed in ref 10 to the photoproduct; in particular the first was attributed to the substituent sensitive, (i.e., a so-called X-sensitive) ν6 mode and the doublet to the ν11 modes of an ortho-substituted benzene derivative. These bands are almost completely free of contribution from the pht reagent. This implies that the product function, PG, is a monotonic decreasing function with increasing wavelength. Contrariwise, bands whose intensities are dominated by contribution from the reagent are expected to be strongly monotonic increasing toward the red. In particular, a band belonging entirely to the reagent would have a profile which would be proportional to G-PG. Since G largely increases toward the red while PG is monotonic increasing toward the blue, the difference would be a strongly monotonic increasing function towards the red. When σP ) σR the band intensity
I ) [σR + (σP - σR)P(λ)]G(λ)
(1)
where I is the intensity of the SERS band and σR and σP are respectively the relative SERS cross sections of the reagent and product contributing to the band independent of the electromagnetic enhancement factor. That is, σP and σR will, in general, include contributions from so-called chemical enhancement factors. G(λ) is the SERS enhancement, which affects all species equally. This will consist mainly of the electromagnetic enhancement. For a band belonging entirely to the photoproduct (i.e., when σR ) 0), eq 1 suggests that the wavelength dependence of a SERS band will be proportional
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Figure 6. SERS excitation profiles of a group of bands ((a) 756 cm-1; (b) 483, 501 cm-1) whose intensities increase toward the blue. CL1, CL2, and CL3 refer to the three clusters whose spectra are Figures 2, 3, and 4, respectively.
Figure 5. SERS excitation profiles of a group of bands ((a) 528 cm-1, (b) 803 cm-1, (c) 1023 cm-1) which show a general increase in intensity toward the red. CL1, CL2, and CL3 refer to the three clusters whose spectra are Figures 2, 3, and 4, respectively.
becomes approximately proportional to G(λ), and hence the profiles exhibited by bands with equal reagent and product contributions approximately reflect the electromagnetic enhancement function. It is, therefore, difficult to determine with confidence which bands are entirely due to reagent and which have some contribution from the product but with σP < σR. The bands at 528, 803, and 1023 cm-1 bands belong to this group (the last two of which are a1 modes of pht). These bands appear to be dominated by the reagent, or at the very most have equal reagent and product contributions. The 1382 cm-1 band also shows a general trend toward the red; however, the slight reproducible initial decrease in intensity with increasing wavelength lead us to believe that it has a greater contribution from the photoproduct than was the case for the three, previously mentioned, bands. For this band σP ∼ σR, hence its profile approximately follows the function G(λ). All of the other bands appear to have significant contributions from both reagent and product to a greater or lesser extent. The modes at 946, 1230, and 1457 cm-1 have profiles which are characteristic of bands for which σP > σR (but not overwhelmingly so). The dominance of the photo-product contribution over that of the pht reagent to the band intensity appears to increase in the order 946, 1230, and 1457 cm-1.
The mixed nature of these bands is also suggested by the increase in bandwidth on moving from green to blue excitation (Figures 2-4) which is likely due to the evolution of new photoproduct bands slightly shifted from their counterparts in the SERS spectrum of the reagent. For example, the pht band at 1231 cm-1 was shown10 to shift to 1241 cm-1 after irradiation, while the pht band at 1457 cm-1 becomes a doublet at 1432 and 1465 cm-1 in the SERS spectrum of the photoproduct. In ref 10 the 1241 cm-1 band was attributed to a mode similar to the ν7a mode of o-xylene; however, we believe that this band is more likely to be an X-sensitive mode of the ortho-substituted benzene derivative. The bands at 1432 and 1465 cm-1 are assigned to modes similar to ν19a and ν19b of o-xylene, respectively.10 Furthermore, in the SERS spectra of pht-covered clusters excited at 482.5 nm (Figures 2, 3, 4; spectra b) the new band which evolves in the vicinity of the 946 cm-1 of pht corresponds to the ν10a mode of ortho-substituted benzene derivatives which suggests that the photoproduct is not rigorously perpendicular to the surface, as implied in ref 10. Finally, the SERS excitation profiles obtained from the SERS spectra of clusters 1, 2, and 3 (Figures 5-7) are all very similar. No systematic cluster-size-dependent differences are observed. This is consistent with the suggestion by Stockman et al.14 based on an extensive theoretical treatment of the optical properties of fractal clusters, and more recently by Markel et al.14b based on computation, that certain optical properties of fractal clusters and specifically the wavelength dependence of the SERS enhancement and the inhomogeneously broadened localized surface plasmon absorption spectra are independent of size within the scaling region. The results (Figures 5-7) also show that the overall SERS intensity is independent of cluster size. This is due to the fact that the majority of the laser beam intensity was confined to a region approximately the size of the smallest cluster, and hence the same number of colloidal particles were excited in each cluster. Since the enhancement is predicted14c to be independent of cluster size, one expects, and observes, the SERS intensity to be more or less constant for the three clusters. Of course, if the entire cluster were illuminated, the SERS intensity (as opposed to the SERS enhancement) would have scaled with the size of the cluster (or, more correctly, with the number of particles illuminated). The results also suggest that the photochemical activity which manifests itself in the function P(λ) is also cluster size independent.
SERS Excitation Profiles of Phthalazine
J. Phys. Chem. B, Vol. 101, No. 9, 1997 1593 deposited from solution onto a glass surface. SERS excitation profiles were obtained by exciting the spectra with one of six laser wavelengths and using the Raman spectrum of a potassium nitrate pellet as an external intensity standard. Colloid clusters resistant to photodegradation were produced by using a slightly modified colloid preparation procedure. 2. As was previously reported for pht adsorbed on Ag hydrosol aggregates10 the SERS spectra recorded with green and blue lines indicate that pht adsorbed on single Ag colloid clusters in air undergoes a similar photoreaction. The SERS bands of the photoproduct are observed in the SERS spectrum. 3. The SERS excitation profiles obtained from various bands observed in the SERS spectra could be understood in terms of two wavelength-dependent contributions: (i) the photochemical activity which grows monotonically toward the blue in the spectral range used (470-650 nm) and (ii) the SERS enhancement function which increases toward the red. The variety of shapes observed for the excitation profiles can be explained in terms of the relative contribution of the pht reagent and the photoproduct to the, sometimes overlapping, SERS bands. The relative contributions (or lack of overlap) suggested by the SERS excitation profiles are in good agreement with the independent assignments of those bands to pht or its photoproduct based on other spectroscopic considerations. 4. The wavelength-dependent SERS enhancement and the photoreactivity were found to be independent of cluster size for the three clusters studies which varied in size from 1.9 × 1.9 µm to 7.5 × 4.7 µm. Acknowledgment. M.M. is grateful to NSERC for financial support. B.V. acknowledges financial assistance through grant GAUK 219/96. References and Notes
Figure 7. SERS excitation profiles of a group of bands ((a) 1382 cm-1, (b) 946 cm-1, (c) 1230 cm-1, (d) 1457 cm-1) which show amonotonic wavelength dependence. CL1, CL2, and CL3 refer to the three clusters whose spectra are Figures 2, 3, and 4, respectively.
Conclusions 1. SERS spectra were obtained from phthalazine (pht) adsorbed on single adsorbate-covered Ag colloidal clusters
(1) Moskovits, M. J. Chem. Phys. 1978, 69, 4159. Moskovits, M. ReV. Mod. Phys. 1985, 57, 783 and references therein. (2) Nitzan, A.; Brus, L. E. J. Chem. Phys. 1981, 74, 2205. (3) Das, P.; Metiu, H. J. Phys. Chem. 1985, 89, 4680. Leung, P. T.; George, T. F. J. Chem. Phys. 1980, 85, 4729. (4) Goncher, M.; Parsons, C. A.; Harris, C. B. J. Chem. Phys. 1984, 88, 4200. (5) Wolkow, R. A.; Moskovits, M. J. Chem. Phys. 1987, 87, 5858. (6) Suh, J. S.; Moskovits, M.; Shakhesemampour, J. J. Phys. Chem. 1993, 97, 1678. (7) Shi, C.; Zhang, W.; Lombardi, J. R.; Birke, R. L. J. Phys. Chem. 1992, 96, 10093. (8) Zhang, W.; Vivoni, A.; Lombardi, J. R.; Birke, R. L. J. Phys. Chem. 1995, 99, 12846. (9) Feichenfeld, H.; Chumanov, G.; Cotton, T. J. Phys. Chem. 1996, 100, 4937. (10) Suh, J. S.; Jang, N. H.; Jeong, D. H.; Moskovits, M. J. Phys. Chem. 1996, 100, 805. (11) Vlcˇ kova´, B.; Tsai, D.; Gu, X.; Moskovits, M. J. Phys. Chem. 1996, 100, 3169. (12) Weitz, D. A.; Oliveria, M. Phys. ReV. Lett. 1984, 52, 1433. (13) Weitz, D. A.; Lin, M. Y.; Sandroff, C. J. Surf. Sci. 1985, 158, 147. (14) (a) Shalaev, V. M.; Stockman, M. I. Zh. Eksp. Teor. Fiz. 1987, 92, 509 [SoV. Phys. JETP 1987, 65, 287]; Z. Phys. D 1988, 10, 71. Markel, V. A.; Muratov, L. S.; Stockman, M. I.; George, T. F. Phys. ReV. B 1991, 43, 8183. Stockman, M. I.; George, T. F.; Shalaev, V. M. Phys. ReV. B 1991, 44, 115. (b) Markel, V. A.; Shalaev, V. M.; Stechel, E. B.; Kim, W.; Armstrong, R. Phys. ReV. B 1996, 53, 2425. (c) Stockman, M. I.; Shalaev, V. M.; Moskovits, M.; Botet, R.; George, T. F. Phys. ReV. B 1992, 46, 2821. (15) Tsai, D. P.; Kovacs, J.; Wang, Z.; Moskovits, M.; Shalaev, V.; Suh, J. S.; Botet, R. Phys. ReV. Lett. 1994, 72, 4149. (16) Vlcˇ kova´, B.; Matejka, P.; Sˇ imonova´, J.; Pancˇ osˇka, P.; C ˇ erma´kova´, K.; Baumruk, V. J. Phys. Chem. 1993, 97, 9719-29. (17) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790.
