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J . Phys. Chem. 1987, 91, 4776-4779
found to be unreactive toward the alkynes, and only molecular species were observed on this surface. The lack of reactivity of the 0-polar surface is due to the absence of accessible acid-base site pairs on this surface. The formation of propargyl intermediates following the dissociative adsorption of methylacetylene on Zn0(0001), and the decrease in internal polarization of ionic intermediates on the ZnO(0001) surface relative to similar ionic intermediates on the Ag( 110) surface, demonstrate the importance of surfaceadsorbate
interactions in determining both the stability of surface species and the selectivity of surface acid-base reactions.
Acknowledgment. We gratefully acknowledge the support of the National Science Foundation (Grant CBT 831 1912) and of E. I. du Pont de Nemours and Co., Inc. Registry No. C2Hz, 74-86-2; CH,CCH, 74-99-7; C,HSCCH, 53674-3; ZnO, 13 14-13-2.
ESR Study of Reduced Mixed Llgand Complexes: 2,2'-Blpyrlmldlne Complexes wlth Ruthenium(I I ) Jean-Noel Gex, William Brewer, Kathy Bergmann, C. Drew Tait, M. Keith DeArmond,* Kenneth W. Hanck, and Dennis W. Wertz Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 (Received: February 2, 1987)
The ESR spectra of reduced RuL3"+complexes [L = 2,2'-bipyrimidine (bpm)] containing one and two redox electrons give an S = spectrum that produces temperature-dependent line broadening, while the three-electron product, [Ru(bpm),]-, does not give line broadening. Reduced mixed ligand complexes such as [R~(bpy)~(bpm-)]+ produced hyperfine structure (hfs) associated with the bpm ligand. The hfs of the one-electron-reduced dimer [R~~(bpy),bpm-]~+ species as well as the hfs of reduced free ligand bpm produces spin densities in the bpm that are consistent with intuition. A comparison of the charge localization and electron-transfer process in these and other mixed valent metal complexes is presented.
Introduction In prior publication^,'-'^ the unique magnetic and electronic properties of reduced Ru(I1) complexes of ?r electron diimine ligands have been elaborated by use of electron spin resonance as well as other spectroscopic methods. The ESR technique has characterized the charge distribution and dynamics for these ligand localized paramagnetic species. The ESR, resonance Raman, and electronic spectroscopy for the one- and two-electron-reduction products of R u L T structure indicates that these species can each species with the redox electron localized in be treated as S = orbitals on the individual chelate rings. Moreover, the temper(1) DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem. Rev. 1985, 64, 65.
(2) Ohsawa, Y . ;DeArmond, M. K.; Hanck, K. W.; Morris, D. E.; Whitten, D. G.; Neveux, P. E., Jr. J . A m . Chem. SOC.1983, 105, 6522. (3) Ohsawa, Y.; Hanck, K. W.; DeArmond, M. K. J . Electroanal. Chem. Interfacial Electrochem. 1984, 175, 229. (4) Morris, D. E.; Ohsawa, Y.; Segers, D. P.; DeArmond, M. K.; Hanck, K. W. Inorg. Chem. 1984, 23, 3010. ( 5 ) Ohsawa, Y.; Whangbo, M. H.; Hanck, K. W.; DeArmond, M. K. I n o m Chem. 1984. 23. 3426. (g) Donohoe, R.' J.; Tait, C. D.; DeArmond, M. K.; Wertz, D. W. Spectrochim. Acta. Part A 1986. 42A. 233. (7) Angel, S . M.; DeArmond,' M. K.; Donohoe, R. J.; Hanck, K. W.; Wertz, D. W. J . A m . Chem. SOC.1984, 106, 3688. (8) Morris, D. E.; Hanck, K. W.; DeArmond, M. K. J . Electroanal. Chem. Interfacial Electrochem. 1983, 149, 115. (9) Tait, C. D.; MacQueen, D. B.; Donohoe, R. J.; DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. J . Phys. Chem. 1986, 90, 1766. (10) Motten, A.; Hanck, K. W.; DeArmond, M. K. Chem. Phys. Left. 1981, 79, 541. (11) Morris, D.; Hanck, K. W.; DeArmond, M. K. J . A m . Chem. Soc. 1983, 105, 3032. (12) Ohsawa, Y . ;DeArmond, M. K.; Hanck, K. W.; Moreland, C. G. J . A m . Chem. SOC.1985, 107, 5383. (13) Gex, J. N.; DeArmond, M. K.; Hanck, K. W. J . Phys. Chem. 1987, 91, 251-254. (14) Gex, J. N.; Hanck, K. W.; DeArmond, M. K. Inorg. Chem., in press. (15) Gex, J. N.; Cooper, J. B.; Hanck, K. W.; DeArmond, M. K. J . Phys. Chem., in press.
0022-3654/87/2091-4776$01.50/0
ature-dependent ESR line b r ~ a d e n i n g ' has ~ ' ~ been postulated to result from the electron movement between equivalent ligands. Finally, minimizing this electron hopping rate by increasing the barrier does permit observation of hyperfine structureI3 for species such as [Ru(bp~-)(bpy)~]+ (bpz = 2,2'-bipyrazine), thus verifying the line-broadening rationale and allowing further characterization of the wave function for the reduced species. The absence of room temperature hfs for the three-electron-reduction products, for which the barrier to hopping is large, was recently rationalized by the observation at high temperature of hfs for [Ru(bpy-)J-.I4 Such ESR data with the complementary spectroscopic results also verify the spatially isolated (localized) orbital model proposed for these materia1s.l This treatment of the one-, two-, and threeelectron-reduced species as a radical, diradical, and triradical species, respectively, does not yet fully explain the absence of exchange and dipole-dipole coupling (S = 1 and S = 3/z) for these species. Whatever the rationale of the minimal magnetic coupling, an intramolecular electron transfer does occur for these metal-containing heterocycles. Such a result is reminiscent of that phenomenon occurring in mixed oxidation metal dimer complexes such as the Creutz-Taube complexI6 where Ru" and Ru"' rather than bpy- and bpy are the species involved in the electron transfer. More relevant to our data are the temperature-dependent ESR studies of Pope's group'8-20 and Launay and co-workers2' on reduced polyanions in solution containing various transition metal ions linked together. Further, the ESR, Mossbauer, and IR results (16) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1. (17) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (18) Prados, R. A.; Pope, M. T. Inorg. Chem. 1976, 15, 2547. (19) Varga, G. M.; Constantinow, E.; Pope, M. T. Inorg. Chem. 1970, 9, 662. (20) Altenau, J. J.; Pope, M. T.; Prados, R. A,; So, H. Inorg. Chem. 1975, 14, 417. (21) Sanchez, C.;Livage, J.; Launay, J. P.; Fournier, M.; Jeannin, Y . J . Am. Chem. Soc. 1982, 104, 3194.
0 1987 American Chemical Society
ESR Study of Ru(II)-2,2’-Bipyrimidine Complexes
The Journal of Physical Chemistry, Vol. 91, No. 18, 1987 4717 TABLE I: ESR Parameters of the Free Radical Anion bpm- and the Complexes [Ru(bpm-)(bpy)$ (I) and [(bpy)2Ru(bpm-)Ru(bpy),]3+ (11)“ bPm
I
I1
a
nx
n
0.137 0.47 0.27 0.07 0.15 0.39 0.274 0.137 0.35
4N 2H 2N 2N 2H 1 Ru 4N
l , l ‘ , 3,3‘
2H
5,s
1,l‘ 3,3’
5,s’ l , l ‘ , 3,3’
5,s’
1 Ru
Coupling constants are given in milliteslas.
0
-5
-10
-15
E ( V o l t vs SCE)
Figure 1. Cyclic voltammogram of (a) R ~ ( b p m ) , ~ +(b) , R~(bpm)~(bpy)2+, (c) R u ( b p ” w ) , 2 + , and ( 4 [(bpy)2Ru(bpm)Ru(bpy)2I4+. (a), (b), and (c) are measured in 0.1 M TEAP/DMF, and (d) is measured in 0.1 M TEAP/ACN.
of Hendrickson and co-workers22-26for mixed oxidation state sandwich compounds, although primarily concerned with solidstate species, afford an interesting comparison with these metal diimine systems. Elschenbroi~h*~*~~ has reported sandwich-complex ESR spectra for chromium that are phenomenologically similar to that of the bpy species. In this study, ESR data, including both hfs and temperaturedependent line broadening, are reported for the reduction products of complexes of Ru(1I) with the ligand 2,2’-bipyrimidine (bpm). From these results further perspective is gained for the electron-transfer phenomenon in these systems especially by comparison with that reported for other metal-containing complexes. At a minimum, this effort should permit definition of new experiments for these reduced species.
Figure 2. (a) ESR spectrum and (b) second-derivative spectrum of [R~(bpm-)(bpy)~]+ (solvent 0.1 M TEAP/DMF; T = 298 K).
The compounds were purified by passing a concentrated acetone solution through a Sephadex LH-20 column, collecting the apExperimental Section propriate fraction, and recrystallizing it from acetone/isopropyl Materials. The purifications of the supporting electrolyte alcohol or acetone/diethyl ether. Punty was checked by comparing tetraethylammonium perchlorate (TEAP) and the solvents N,the luminescence spectrum of the compound at different excitation N’-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) frequencies. were performed as reported elsewhere.29 Acetonitrile (ACN) Cyclic voltammetric measurements and total electrolysis were (Aldrich, Gold Label) was prepared by passing it 3 times through carried out by using a three-compartment H-cell under a nitrogen an activated-alumina column in the inert atmosphere of the atmosphere with -0.1 mM solutions. Electrochemical apparatus glovebox. The ligands were purchased commercially, with bpy and procedure have been reported previously.8 The number of from Aldrich and bpm from Alfa. electrons added to the parent species is designated by n. The following ruthenium compounds were made from literature ESR spectra (X-band) with -0.1 mM solutions were measured preparations: [ R ~ ( b p m ) , l ( P F ~ )[Ru(bpm)(b~y)21(PF6)2,3~,~~ ~,~,~~ with an IBM-Bruker Series 200 spectrometer equipped with [R~(bpm)2(bPY)l(PF6)2?3 and [(Ru(bPY)2)2(bPm)l(PF6)4.30*32’33’34 temperature controller Model 5500 and cryostat LTR-3 from Air Products. Data acquisition was done with an AT&T 6300 computer with EPRDAS.,’ Simulation of spectra were done by using (22) Morrison, W. H.; Hendrickson, D. N. Inorg. Chem. 1975,14,2331. SIMESR.36 (23) Oh, S. M.; Hendrickson, D. N.; Hassett, K. L.; Davis, R. E. J . Am. Chem. SOC.1984, 106, 7984. (24) Dong, T. Y . ;Hendrickson, D. N.; Iwai, K.; Cohn, M. J.; Geib, S. J.; Rheingold, A. L.; Sano, H.; Motoyama, I.; Nakashima, S. J. Am. Chem. SOC. 1985, 107, 7996. (25) Dong, T. Y . ;Cohn, M. J.; Hendrickson, D. N.; Pierpont, C. G.J. Am. Chem. SOC.1985, 107, 4777. (26) Dong, T. Y . ;Kambera, T.; Hendrickson, D. N. J . Am. Chem. SOC. 1986, 108, 5857. (27) Elschenbroich, C.; Heck, J. J . Am. Chem. SOC.1979, 101, 6773. (28) Elschenbroich, C.; Gerson, F.; Stohler, F. J. Am. Chem. SOC.1973, 95, 6956. (29) Ohsawa, Y . ;Hanck, K. W.; DeArmond, M. K. J . Electroanal. Chem. Interfacial Electrochem. 1984, 175, 229. (30) Hunziker, M.; Ludi, A. J. J . Am. Chem. Soc. 1977, 99, 7370. (31) Lin, C.-T.; Bottcher, W.; Chou, M.; Creutz, C.; Sutin, N. J . A m . Chem. SOC.1976, 98, 6536. (32) Dose, E. V.; Wilson, L. J. Inorg. Chem. 1978, 17, 2660.
Results Some part of the voltammetry for these materials has previously been reported by Saji and co-workers3’ and Rillema and coworkers.)) The voltammograms for the mono-,bis-, and tris(bpm) complexes as well as the dimer are given in Figure 1 and do illustrate the redox patterns characteristic of these systems possessing orbitals localized on single chelate rings.38 The patterns (33) (34) (35) (36) (37) 327.
Rillema, D. P.; Mack, K. B. Inorg. Chem. 1982, 21, 3849. Goldsby, K. A.; Meyer, T. J. Inorg. Chem. 1984, 23, 3002. Adaptable Laboratory Software, Inc., Rochester, NY. Daul, C. University of Fribourg (Switzerland). Watanabe, J.; Saji, T.; Aoyagui, S. Bull. Chem. SOC.Jpn. 1982, 55,
4778 The Journal of Physical Chemistry, Vol. 91, No. 18, 1987
Figure 3. (a) ESR spectrum and (b) second-derivative spectrum of [(bpy),Ru(bpm-)Ru(bpy)J3+ (solvent 0.1 M TEAP/ACN; T = 298 K).
permit the individual waves to be associated with either the bpm
or bpy chelate rings. The ESR spectra of the n = 1 mono(bpm) complex exhibits hfs (Figure 2; Table I). Unfortunately, the n = 2 and n = 3 species are insoluble in both D M F and CH3CN; consequently no spectra are available. Only the n = 1 [Ru(bpm-)(bpm)bpy]+ species gave an ESR spectrum with a temperature-dependent S = I / , spectrum; the [Ru(bpm'),(bpy)] complex did not show temperature-dependent line broadening or hfs at elevated temperatures (120 "C in DMSO). The tris complex [Ru(bpm)J2-" behaved normally, with temperature-dependent line broadening occurring (in DMF) for the n = 1 and n = 2 species and no line broadening occurring for the n = 3 species. High-temperature spectra in D M F (60 "C) produced no hfs for the [Ru(bpm-),]species. The dimer complex [Ru2(bpy),bpmI4+ gave hfs structure for the one-electron product (Figure 3; Table I ) , but the twoelectron product is ESR silent, suggesting that this material is diamagnetic.
Discussion The voltammogram (Figure 1) for the tris(bpm) complex is a three-wave pattern characteristic of a tris(diimine) and typical of spatially isolated localized redox orbitals. Furthermore, the resonance Raman spectra39of the reduced complexes clearly show the presence of both bpm and bpm- vibrational peaks on the oneand twc-electron-reduction products, necessitating a localized redox orbital description for the [ R ~ ( b p m ) ~ ] ~redox - " series. The voltammetry for the mono(bpm) complex indicates, as expected, that the first redox process involves the bpm and that it is considerably displaced from those reductions associated with the Ru-bpy moiety (second and third waves). The voltammogram of the dimer bpm complex shows a relatively positive redox couple, implying a strong electrostatic attraction for a redox electron on a bpm between two Ru2+ centers, and further shows a larger separation between the first and second reduction processes than that for the mono complex. Indeed, the waves likely are both associated with the Ru-bpm unit such that the two-electron product should be diamagnetic, consistent with the ESR result. The n = 1 and n = 2 tris(bpm) complexes do show temperature-dependent line broadening in DMF as do other tris complexes. The magnitude of the activation energy for the n = 1 species is somewhat smaller (700 cm-') than that for other tris complexes (38) Vlcek, A. A. Coord. Chem. Rev. 1982, 43, 39. (39) Tait, C D.; Wertz, D. W.; DeArmond, M. K. Inorg. Chem., in press.
Gex et al. (normally4J0~"E 1000 cm-'), while that of the n = 2 is of normal magnitude ( E = 500 cm-I). Consequently, the n = 1 and n = 2 activation energies are nearly equal. Since the role of the solvent in the electron hopping process cannot yet be understood, this Occurrence cannot be rationalized. The n = 3 species, as other tris n = 3 species, exhibits no temperature-dependent line broadening due to the high barrier to hopping predicted for this species. As indicated (Results), high-temperature (60 "C) spectra in DMF did not resolve the hfs as has been done for [Ru(bpy-),]in DMS0.14 For the bis complex where the DMSO solvent was required, no unusual line shape was noted nor was hfs observed at temperatures up to 120 OC. The activation energy for the n = 1 bis complex is of comparable magnitude to that observed for the n = 1 and n = 2 tris complexes. As predicted by the model, [ R ~ ( b p m - ) ~ ( b p yand ) ] ~ [Ru(bpm-),(bpy-)]- do not exhibit temperature-dependent line widths due to the high barriers to hopping in these complexes. The ESR for the reduction products of [ R u ( b ~ m ) ( b p y ) ~is] ~ + limited to that of the n = 1 species because of the solubility problems in ACN and DMF. Moreover, the spectrum of the n = 1 species is complex due to the presence of two pairs of equivalent nitrogens in these complexes. Consequently, at least 75 lines (5 X 5 X 3) are expected from the hyperfine interactions of the sites expected to have the largest spin density. Comparison of the coordinating nitrogen coupling to those of the free ligand (Table I) does indicate that the ruthenium ion polarizes the nitrogen spin density as expected. While the spectrum is too complicated to extract a precise Ru(1I) hfs, the general shape implies that the splitting due to Ru(I1) is much less than the 4-5-G magnitude found for the m ~ n o ( b p z )and ' ~ mono(4,4'-C0,et-bpy) c ~ m p l e x e s .The ~ ~ n = 1 [Ru2(bpm-)(bpy),lS+ species produces a well-resolved pattern since now the four nitrogens are again equivalent. The magnitude of the Ru(I1) hfs is less than that of the mono (bpz) and mono ester complexes as is that of the nitrogen splitting (Table I). Indeed the overall line width is here less than that of the other complexes exhibiting hfs, suggesting that substantial spin density resides on the bridging carbons which have no attached hydrogen. Such a distribution of the spin density would be reasonable for this symmetric dimer structure. This series of papers detailing dynamic effects in diimine metal complexes necessitates that appropriate comparison to other paramagnetic metal containing complexes be done to provide perspective. The occurrence of intramolecular electron hopping in metal complexes has been reported for other metal complexes including iron and chromium sandwich compounds22-2sas well as reduced heteropolyanions.18-21 The results for the sandwich compounds include Cr(0) systems with the unpaired electron associated at least in part with the a ligand portion of the molecule. A spatially isolated orbital permitting localization of the charge on each of the separate Cr-arene units is apparently possible for bis(7-arene)chromium complexes, since a paramagnetic triplet (S = 1) species can be observed for the d i c a t i ~ n . ~The ' ~ ~rigid ~ side-on arrangement of the two chromium units results in a parallel orientation of the 7 orbitals for the monomeric units whereas in our trimer species the a orbitals of the units are perpendicular to one another. Furthermore, Weissman and c o - w ~ r k e r s have ~~~' reported S = 1 states for two-electron bis(diimine) complexes generated by chemical reduction. In these materials the two diimine ligands are tetrahedrally coordinated, therefore giving orthogonal 7 electron systems. The hopping and delocalization for the reduced heteropolyanions involves d electrons of the metal atom.'8-2' Moreover, these heteropoly systems are capable of the addition, in some cases, of up to six electrons. However, similar to the sandwich compounds above, the addition of more than one electron into the polymer unit produces predictable magnetic behavior: here diamagnetism for the even electron species. The reduced heteropolyanion material does exhibit hfs in frozen solution, which permits the (40) Brown, I. M.; Weissman, S. I. J . Am. Chem. SOC.1963, 85, 2528. (41) Brown, I . M.; Weissman, S. I.; Snyder, L. C. J . Chem. Phys. 1965, 42, 1105.
4779
J . Phys. Chem. 1987, 91, 4779-4788 evaluation of the degree of delocalization even for those species for which localization is occurring and an energy of activation can be measured.21 This class I1 mixed valence behavior is as expected. These data for the heteropolyanions suggest that the barrier to electron hopping is primarily an intramolecular one rather than an extrinsic one since in some cases hopping occurs in rigid matrices. Indeed, Launay and co-workers2lhave rationalized in detail the temperature-dependent behavior of the line broadening for the heteropolyanions extracting activation energies as well as preexponential (tunneling) factors for various molybdates. At this stage, k’s are not available directly from our data; however, efforts are planned to extract rate constants from the ESR line width. An alternative of calculating k from the intervalence transfer band has been criticized by Hendrickson and co-workers.22 Moreover, for our systems, the tail of an intense r - x * absorption often obscures this band. Extending the work of Launay?’ Hendrickson22-26proposes that anion and solvent molecules play a role in the electron transfer for his systems with evidence derived from X-ray structures of crystalline materials. The mechanisms proposed by Hendrickson emphasize the role of anions and solvate molecule in modulating the intramolecular electron transfer rate associated with the sandwich species. Our inability to obtain crystalline samples free Qf supporting electrolytes precludes many of the experimental techniques used by these workers. However, the mechanism proposed is conceivably appropriate to explain the electron hopping occurring in the high dielectric constant solvents (ACN, DMF, DMSO) used to date. That is, the absence of ion pairing and the ability of the solvent to move freely reduce the barrier to hopping and permit electron hopping. Testing of such a mechanism is not easy but might be possible in low dielectric constant solvents where ion pairing with the metal ion complex occurs.
Conclusions Symmetric ML3n+complexes give localized n = 1, 2, and 3
reduction products whose ESR spectra in fluid solution exhibit an Arrhenius temperature-dependent line broadening that results from intramolecular electron hopping for the first and second reduction products. Hyperfine structure can be observed for these predominantly ligand-localized radical species when the barrier to hopping is increased by using a mixed ligand complex as [RuL2L’I2+where L’ is a ligand reduced at a potential -0.5 V more positive than L. The observation of hfs verifies, consistent with g shifts, that the Ru character in the redox orbital is small. For the mono(bpm) complex reported here, the poor resolution of the hyperfine is a result of the spectral density, a second factor important in the observation of resolved hfs. Nevertheless, comparison of the hfs for the free ligand, mono(bpm), complex and a dimeric bpm complex indicates that the spin densities are consistent with intuition for these systems. Comparison of the charge localization and intramolecular electron transfer processes in the reduced metal diimines with that known for reduced mixed valent polyanions containing Mo, V, and W as well as some sandwich compounds indicates general similarities in the phenomena for the different systems and suggests new experiments. A primary difference in the results from those systems and ours is the large amount of metal character in the redox orbital of the polyanions and the sandwich compounds, in contrast to the minimal metal character in our system. As a consequence, spin-orbit coupling plays a significant role in those systems and a lesser role here. The orthogonality of the *-electron systems for the trimer diimine complexes may be a factor in the minimal magnetic spin-spin coupling for these systems; however, the res ~ l tfor ~ the ~ ,chemically ~ ~ produced alkaline earth complexes of diimines is not consistent with such a rationale. Registry No. [Ru(bpm) 3](PF6)2, 85 3 3 5-55-3; [Ru(bpm) ( b ~ y )- ~ ] (PF6)2, 65013-23-2; [Ru(bpm),(bpy)](PF,),, 85335-57-5; [(Ru(bpyM,(bpm)l (PF6)4, 65013-25-4.
Ion-Beam-Induced Chemical Changes in the Oxyanions (MO,,”-) and Oxides (MO,) Where M = Cr, Mo, W, V, Nb, and Ta S. F. Ho, S. Contarini, and J. W. Rabalais* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: February 19, 1987)
The chemical changes induced by 4-keV Ar+ bombardment of the transition-metal oxyanion compounds N2W04(N = Li, Na, Ag, NH4), Na,MO, (M = V, Cr, Nb, Mo, Ta), and the corresponding transition-metal oxides have been investigated by X-ray photoelectron spectroscopy (XPS). Both the atomic concentrations and the binding energy shifts in the altered layer show that a steady-state composition is attained at a total irradiation dose of 1017ions/cm2. Bombardment of all of the oxyanions resulted in reduction of the transition-metal atoms to lower oxidation states; reduction to the metallic state is observed in all cases except V03- and Cr042-. In contrast, bombardment of the metal oxides produced lower oxidation states of the metal in some, but not all, cases. The mechanism of ion-induced decomposition is discussed in terms of the thermal spike model and the behavior of specific compounds is shown to correlate with decomposition pressure at elevated temperatures.
-
I. Introduction Rare gas ion bombardment of solid surfaces is widely used in surface science to remove surface layers in sample preparation,’ for obtaining depth concentration and as an excitation source in secondary ion mass spectrometry (SIMS)4,5and ion ( I ) Walls, J. M.; Christie, A. B. In Surface Analysis and Pretreatment of Plastics and Metals; Brewis, D. M., Ed.; Applied Science: London 1982. (2) Chang, C. C. Surf. Sci. 1971, 25, 53. (3) Smith, R.; Walls, J. M. Philos. Mag. 1980, A42, 235. (4) Benninghoven, A. Surf. Sci. 1973, 35, 427. (5) Werner, H. W. Surf. Interface. Anal. 1980, 2, 56.
0022-3654/87/2091-4779$01.50/0
scattering spectrometry (ISS).6 In many however, ion bombardment induces damage and results in an altered surface layer as a result of the large quantities of energy deposited in localized regions near the collision sites. This energy can induce ~
~~
(6) Ball, D. J.; Buck, T. M.; McNair, D.; Wheatly, G. H. Surf. Sci. 1972, 30, 69. (7) Betz, G.; Opitz, M.; Braun, P. Nucl. Instrum. Methods 1981, 182, 63. (8) Holloway, P. H.; Nelson, G. S.J . Vac. Sci. Technol. 1979, 16, 793. (9) Christie, A. B.; Sutherland, I.; Walls, J. M. Vacuum 1984, 34, 659. (10) Strop, S.; Holm, R. J. Electron Spectrosc. Relat. Phenom. 1979, 16,
183.
0 1987 American Chemical Society
