The use of externally applied magnetic fields in the study of chemisorption on metal films

Thin Solid Films, 50(1978) 371-382 ~) Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands

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T H E USE OF E X T E R N A L L Y APPLIED M A G N E T I C FIELDS IN T H E STUDY OF C H E M I S O R P T I O N ON M E T A L FILMS G. WEDLER lnstitut fiir Physikalische und Theoretische Chemie der Universitiit Erlangen-Niirnberg, Egerlandstra~e 3, D-8520 Erlangen (F.R.G.) W. GOPEL lnstitut fiir Physikal,'sche Chemie und Elektrochemie der Technischen UniversitiJt Hannover, Callinstrafle 3-3A, D-3000 Hannover ( F.R.G)

A survey is given of the current situation of thin film research work using magnetometry, ferromagnetic resonance and galvanomagnetic effects, i.e. the Hall effect and the transverse magnetoresistivity. The unsolved problems and future perspectives are discussed. It is shown that the application of these techniques to adsorption studies should offer advantages to both an understanding of adsorption phenomena and the study of catalytic processes.

I. INTRODUCTION

In spite of the extreme importance of heterogeneous catalysis in the chemical industry the mechanisms of many even very simple catalytic processes are not yet elucidated. In order to fill this gap of knowledge, which would be of great relevance to the improvement of catalysts, much effort is being made to investigate the adsorption of gases on catalysts, because adsorption is the precursor of catalysis. Without doubt the most significant progress in the understanding of the chemisorptive bond has been achieved during the last few years by the application of electron spectroscopy methods to adsorption studies on single crystals 1. There are also disadvantages, however, to these investigations: they can only be carried out under high vacuum conditions and most of them have been performed on single crystals, whereas catalysis occurs at high pressures on polycrystalline material. There might be a possibility of overcoming this gap between basic research work and technical catalysis by using evaporated polycrystalline films as adsorbents or catalysts and by applying investigation methods which can be used both under ultrahigh vacuum conditions and under high pressure. There are several methods by which changes in the physical properties of evaporated films that are characteristic of specific adsorption processes 2 can be measured. In this article we shall restrict ourselves to methods which make use of an externally applied magnetic field. It is the purpose of this article to show the present status of the subject and to point out problems that have to be solved before further progress can be achieved. Before adsorption processes can be investigated and can be reasonably discussed the properties of the evaporated films themselves must be known. Therefore this article consists of two parts, the first dealing with the properties of the clean films and the second describing the adsorption phenomena.

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Within the scope of the paper films of catalytically active metals (in general transition metals, in particular metals of Group VIII including the ferromagnetic metals iron, cobalt and nickel and metals of Group IB, especially copper) are of special interest. 2.

INVESTIGATIONS ON CLEAN METAL FILMS

Externally applied magnetic fields have mostly been used to study either ferromagnetic properties (using the techniques discussed in Sections 2.1 and 2.2) or the influence of magnetic fields on the electron transport properties of thin films that are not necessarily ferromagnetic (to be discussed in Sections 2.3 and 2.4). Some additional experimental techniques will be mentioned in the comparative discussion (Section 2.5). Research on thin ferromagnetic films has proved rewarding from two standpoints: first, the study of this specialized branch of magnetism has deepened our knowledge of the nature of magnetism in general and, second, magnetic properties are found to be highly structure sensitive. The overwhelming proportion of research in this field has been carried out on metallic ferromagnetic films. For chemisorption studies high surface-to-volume atom ratios are needed, and if these films (d ~< 50 nm) are coherent then for energy reasons they exhibit magnetically a single domain structure 3' 4. Investigation of the galvanomagnetic properties, i.e. of the Hall effect and of the magnetoresistivity, is important for an understanding of the electronic structure of the metals. 2.1. Magnetometry A variety of magnetometers have been used to study the magnetism of thin films 5. The main properties determined magnetometrically are the magnetic moments (and magnetizations) as well as the magnetic anisotropy energies. The main contribution of the magnetic anisotropy energies is of uniaxial type with the film normal as the symmetry axis s°. Experimental results indicate a strong influence of geometrical structure on magnetic properties and even superparamagnetism may occur if films consist of magnetically decoupled islands. Careful film preparation is necessary to obtain reproducible results that can be discussed in terms of convenient theoretical models. (a) Size effects due to the mere existence of the surface without changes of single-atom parameters have been estimated in various theoretical papers. For a review see, for example, ref. 6. The expected decrease of the saturation magnetization with film thickness and the critical exponents describing the temperaturedependent magnetization near the Curie temperature have been found ~ on, for example, epitaxially grown (oligatomic) Ni-Fe(l 11) films coated with Cu(111). All experimental results on carefully prepared films with well-defined geometric structure are in agreement with theory, at least qualitatively 8, and deviations apparent in earlier work may be interpreted by structural or chemical inhomogeneities. (b) Magnetically "dead" layers at fiim-substrate interfaces have been detected

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on, for example, Mn- or C-coated Ni-Fe films 9. In contrast, Cu-coated films show no reduction of ~° magnetic moment due to surface effects. Experimental results obtained under less defined vacuum conditions on nickel films could not be reproduced under ultrahigh vacuum conditions. Magnetic proximity effects resulting from interactions between magnetically differently ordered films were also observed in some cases 1~. The theoretical literature on this subject is controversial 1~-~6 and the experiments on free surfaces that are necessary for further chemisorption studies are lacking. (c) Magnetic surface anisotropies due to anisotropic magnetic interactions of surface atoms were first described by N6el ~7. For a critical review see refs. ! 1, 18 and 19. Few experimental results have been obtained magnetometrically 7.2°. Differences in the N6el surface anisotropy for various coatings of oligatomic N i - F e films are more pronounced than differences in the number of effective dead layers 2~. This indicates that surface anisotropies are more sensitively influenced by changes in the chemical state of the surface than magnetic moments are. Again corresponding measurements of anisotropies on free surfaces are lacking. Furthermore, theoretical as well as experimental work is needed to estimate the influence on the anisotropy of the mechanical stresses in real films. This effect is expected to be pronounced in materials with high magnetostrictive constants 4' 22. 2.2. F e r r o m a g n e t i c resonance

Ferromagnetic resonance ( F M R ) results from the resonant coupling of a microwave field with the Larmor precession of electron spins responsible for magnetization in an angular-dependent effective internal magnetic field. In contrast with magnetometric measurements, an influence of paramagnetic impurities can be estimated quantitatively by F M R and additional information on the film structure can be obtained from lineshapes and microwave absorption 23' 24, so. Often F M R experiments are made with an external magnetic field and magnetization in the film plane. If the magnetomechanical ratio is taken from independent measurements, magnetic anisotropies can be determined by F M R ; essentially the same results are obtained as with magnetometric measurements/°. With an external magnetic field and magnetization perpendicular to the film plane, spin wave resonance is possible for films with d > 100 nm provided that magnetic surface anisotropies differ from the bulk values 25-a2. Systematic studies of this kind of surface effect have not been made. The applicability of F M R to films in the monolayer range was demonstrated by Jannsen 32 and Bagdonat et al. aa" 34 Magnetic moments can be obtained from the integral F M R absorption after calibration provided that relaxation processes do not change 23' 2,,. Magnetizations of polycrystalline nickel films determined in this way have been used to characterize typical structures at certain film thicknesses 33-35. Magnetically incoherent island structures for d < 3 nm exhibit superparamagnetism and Gaussian F M R lineshapes owing to statistically ordered local fields. Thicker films show no significant deviations from the theoretical values of the magnetization and from the Lorentzian lines a°. Since F M R lineshapes are strongly dependent on magnetic inhomogeneities, diffusion and sintering processes can be studied qualitatively 34' 36.8 o. ~20. Many theoretical models have been proposed to explain line broadening and changes in F M R

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absorption23,24.37. For quantitative evaluations of experimental results detailed information on the geometrical structure of the sample is necessary. Since experiments of this kind have not been done, the corresponding interpretations of thin film F M R lineshapes have always been qualitative. Surface contributions to magnetic anisotropies can be determined by taking magnetic moment values from F M R absorption measurements or from independent measurements 2°' 33. Detailed studies on polycrystalline nickel films show deviations from the ideal Nbel anisotropies which indicate a strong influence of internal stresses on magnetic anisotropies 33' ,20. At present only few quantitative estimations of internal stresses can be given s°. The thermally induced stresses in nickel films that lead to magnetoelastic internal fields because of the high magnetostrictive constant of nickel are exceptional. This effect has been estimated quantitatively for F M R results on nickel films evaporated at different temperatures 33. A variety of models on the origin of stresses in films have been proposed 4' 22 The main problem for separating different magnetic anisotropies in thin films is to determine stresses and/or surface free energies. For a detailed discussion see ref. 80. 2.3. Hall effect

There is extensive literature dealing with the Hall effect in metals, especially in G r o u p IB metals and in magnetic metals 38. There are also many papers dealing with the Hall effect in evaporated films of various metals 39-52. These investigations show that the Hall effect in thin films strongly depends on : (i) the state of ordering 48-5° as influenced by the vacuum conditions, the substrate temperature during the evaporation, the rate of evaporation and the annealing temperature; (ii) the temperature 4°'45''.6"49; (iii) the film thickness 39' 4o, 43-46.49. s 1.52 Unfortunately some of the earlier work was performed under poor vacuum and/or indefinite evaporation conditions so that the results obtained often seem to be contradictory. Only a few attempts, e.g. with Cu films 49, have been made to carry out a systematic experimental investigation of the influences mentioned above. The situation becomes even more complicated when the Hall effect is studied in metals which show magnetic ordering. The Hall voltage UH, which in non-magnetic metals depends linearly on the current densityj and the magnetic field strength H UH = R j H

(where R is the Hall coefficient), shows two nearly linear branches 38" 53 : UH = R o j H + RE.jM

The first term on the right-hand side of the equation accounts for the branch above the saturation magnetization and is characterized by the ordinary Hall coefficient Ro; the second term dominates below the saturation magnetization and is characterized by the extraordinary Hall coefficient R E. There is some uncertainty45, 51, however, regarding the determination of R E since the Hall voltage of magnetic films which are not too thin (more than 7 nm in the case of nickel films) is not a linear function of the applied magnetic field strength because of hysteresis

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effects (see also Section 2.5). This also applies in the evaluation of the saturation magnetization from the intersection of the two linear branches of the curve of Hall voltage against magnetic field strength 4s' 51,53 Theory predicts two kinds of size effects--the classical "galvanomagnetomorphic" effect which results in "Sondheimer oscillations "as' 54 and the "quantum size effect" which should only be observed with extremely thin films. Recently, oscillations of Ro and R E with increasing thickness have been reported 52 for nickel films and these qualitatively agree with the Sondheimer oscillations calculated by Zebouni et al. 54 for a free electron gas. Further experiments are necessary to prove whether the oscillations are indeed Sondheimer oscillations. 2.4. Transverse magnetoresistivity

There is also extensive literature dealing with the theory of the magnetoresistivity in metals a8'55, which is defined as the change in resistivity due to the influence of a magnetic field relative to the resistivity measured in the absence of the field: Apmag _ p(H) - p(O)

p(0) In addition, the size effect in the magnetoresistivity has been studied theoretically3 a, 56-58. Non-magnetic metals exhibit a positive transverse magnetoresistivity, ferromagnetic metals a negative one. The size effect results in "Sondheimer oscillations" depending on the field strength and the film thickness. Experiments have shown that like the Hall coefficient the transverse magnetoresistivity is strongly dependent on the state of ordering TM 59, the temperature 6° and the thickness 51'52'61-64 The thickness dependence is masked by thicknessdependent elastoresistances 51. Sondheimer oscillations have been found experimentally at 4.3 K with aluminium films 61. Also in the case of nickel films an oscillatory behaviour of the magnetoresistivity with increasing thickness has been observed 52. In the region above the saturation magnetization these oscillations are in phase with the oscillations of the Hall coefficient (see also Section 2.3). Films produced under identical defined conditions give reproducible results. Much work, theoretical and experimental, however, must be done before all the observed phenomena can be explained in a satisfactory manner. 2.5. Comparative discussion

At first glance the methods discussed above seem to be unrelated. This is not the case, however, and they are in fact to some extent complementary since magnetometry and ferromagnetic resonance are only applicable to ferromagnetic materials whilst the Hall effect and magnetic resistance changes are generally applicable. Indeed all four techniques can provide a value for the saturation magnetization and in this respect can be seen even to overlap. 2.5.1. General problems

(a) A variety of measurements have been carried out under undefined conditions and thus cannot be used to test any theory on thin films. (b) All measurements discussed are very sensitive to structural changes, and

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careful film preparation is necessary with respect to vacuum conditions, substrate temperature and rate of evaporation as well as annealing temperature. (c) The best available results of the magnetic properties have been obtained with epitaxially grown (oligatomic) films embedded in a matrix ~'79"2°. Unfortunately they cannot be used for chemisorption studies on free surfaces. (d) Interpretation of the experimental results leads to basic questions concerning, for example, the band structure of clusters, amorphous structures and real films as well as their connection with magnetic and galvanomagnetic properties, which have been treated theoretically in many cases but only for rather simple systems which cannot be realized experimentally. (e) Further experiments under well-defined conditions are much needed.

2.5.2. Special problems (a) The ferromagnetic properties of thin films can be characterized by size effects and surface effects. (i) Size effects are predicted by many theories in which the dependence of thin film magnetization on the distance from the surface, on temperature and on the Curie temperature as a function of thickness is treated s'6s 7o. Magnetometric measurements only yield information integrated over the film, and for this agreement with theory has been found 8' so In recent low energy electron diffraction experiments on NiO using the extra diffraction spots caused by antiferromagnetic order, estimations were given for the magnetization in the topmost layers and for the corresponding critical indices for surface magnetization 7~ ;3. This critical behaviour was also observed in nickel films using the magneto-optic Kerr effect 74 and using a singularity in the temperature coefficient of resistivity vS. (ii) The existence of "'dead" layers also predicted in some theoretical approaches~l 13 can be detected or excluded magnetometrically 9'2°'33, by M6ssbauer spectroscopy 76' vv and, in principle, by neutron diffraction 7s. Experiments on free surfaces are highly desired. The general problem of magnetically dead layers can be solved only by an understanding of chemisorptive or interfacial bondings. (iii) Magnetic surface anisotropies have been calculated theoretically from various aspects ~ ' ~ ~9. Anisotropies in real films can be treated by different physical models more or less phenomenologically a, 23,24.33 but the separation of various contributions in real systems is not possible yet in a unique way. Some attempts have been made to determine these different effects33"35"79. From a theoretical point of view it is questionable whether an exact separation is possible at all s° (this is the same problem as treating free energies in real macroscopic ferromagnets as a summation over different parts). Anisotropies have been determined experimentally using magnetometers 7' 20.2 ~ FM R2°' 33, 80 the field dependence of the anomalous Hall effect in perpendicularly magnetized films s~ and from emission of spin-polarized electrons 82-s4. The great advantage, with respect to chemisorption studies, in studying anisotropies in thin films is that they are very sensitive to different interfaces. (b) Discrepancies still exist in the thickness dependence of the saturation magnetization of ultrahigh-vacuum-evaporated nickel films determined either by magnetometry and F M R or by galvanomagnetic effects Iv. It is hoped that the detailed information gained by the magnetic methods will lead to a better

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understanding of the shape of the curve of Hall voltage against magnetic field strength, thereby clarifying these inconsistencies. 3.

INVESTIGATIONS OF ADSORPTION EFFECTS

As already mentioned in Section 1, measurements of the magnetic and galvanomagnetic properties of metal films can be performed both under ultrahigh vacuum conditions and under catalytic conditions. It is therefore possible to compare the results obtained at higher temperatures and under higher pressures with those found under the conditions usual in basic research work and under which other modern methods can also be applied. Despite these advantages that the investigations of thin film properties offer, only a few experiments have been carried out under reproducible conditions. 3.1. Magnetometry Chemisorption may change the coupling of spins at ferromagnetic surfaces, which should be detectable magnetometrically because of changes in magnetic moments or in anisotropies, as discussed fn Sections 2.1 and 2.2. A variety of results have been obtained on polycrystalline nickel catalysts under moderate vacuum conditions 85-95. The decrease of the magnetic moment per hydrogen atom adsorbed was found to be independent of surface coverage and metal particle size over a wide range 93, whereas for CO the experimental results were contradictory 94' 95 The chemisorption results support the general view that nickel atoms on which molecules are adsorbed cease to participate in the collective magnetism of the particle, and attempts have been made to correlate magnetic data with results from other techniques applied to the catalyst. On thin films (d < 10 nm) significant changes in the magnetization could not be seen in Neugebauer's measurements under ultrahigh vacuum conditions 96'97, but Bauer found partially irreproducible decreasing magnetic moments with increasing hydrogen pressure 9a, confirming the earlier work of Freedman on superpara-

magnetic films99. Quantitative investigations of chemisorption effects on the magnetic moment and on the various magnetic anisotropies of thin films under different thermodynamic conditions are necessary for more detailed discussions. 3.2. Ferromagnetic resonance Indications for chemisorption-induced changes in magnetic surface anisotropies have been detected by spin wave resonance on Permalloy 26 and nickel films 2~ (d > 50 nm) and have been interpreted qualitatively by "spin pinning effects". Although F M R spectroscopy has been shown to give detailed information on magnetic structures in thin films, only few results have been published on reproducible chemisorption effects since F M R experiments on thin films are difficult to carry out under ultrahigh vacuum conditions. For d < 10 nm films Jannsen developed a "stress model" to explain chemisorption effects on the anisotropic field measured by F M R 32. Since the maximum change in this field due to chemisorption was the same for various gases, the effect

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was discussed in terms of complete stress release in the film. Several earlier F M R studies on the chemisorption of 0 2, H z O and CO confirm these results at least qualitatively99,100- lO2. Recent F M R measurements on polycrystalline nickel and iron films indicate 35'79 reversible changes of the spectra with H 2 and CO pressure at temperatures between 300 and 360 K. Since shifts in the resonance field of thin films ( d = 1.3 nm) are proportional to the coverage of hydrogen, chemisorption isotherms that have been obtained from FM R data are in good agreement with wellknown thermodynamic data 79. The magnetic moment of the samples determined from microwave absorption is reduced by the contribution of one nickel atom per hydrogen atom adsorbed and of about two nickel atoms per CO molecule (0 < 0.3). The effect may be interpreted as a chemisorption-induced " d e a d " layer (see Section 2.1) and in this model smaller fields of demagnetization are expected for thin films (d < 4 nm). Reduced magpetic anisotropies in the film are indicated by a chemisorptioninduced reduction of F M R linewidths 35. The first influence is a changed N6el magnetic surface anisotropy. Each hydrogen atom adsorbed in the [32 state was calculated to remove the magnetic surface anisotropy of one nickel atom at the surface, and for CO even more than one 36' 1o3. Oxygen increases the linewidth due to the occurrence of antiferromagnetic NiO at the surface 1°3. No theoretical approach has been made yet to interpret these effects within the frame of convenient models on metal chemisorption. The second effect on changes in magnetic anisotropies arises from variations in interatomic distances in the film due to the reduced surfacefree-energy-induced compressive stress after chemisorption. For metals with high magnetostrictive constants this effect results in anisotropic magnetic fields in the film. (Compare, for example, the simple Bethe-Slater theory, by which the physical origin of this effect can be illustrated.) The main problem to be solved before more detailed quantitative chemisorption studies are possible is to estimate free energies and stresses both experimentally and by adequate theories. This problem has been discussed for clean films in Section 2.2, but in addition more realistic models should be developed to calculate chemisorption effects on mechanical stresses without neglecting real geometrical structures and atomic arrangements at the surface 8°' ~20 F M R is a useful experimental tool for chemisorption studies although the information, e.g. from lineshapes, can be used for more or less phenomenological considerations only. The theoretical treatment of thin film F M R in realistic systems has to be improved by estimating the F M R spectra of well-defined films for which additional information resulting from independent measurements exists.

3.3. Hall effect Investigations of the effect of gas adsorption on the Hall coefficient were originally made in ordec to decide whether the changes in the electrical resistivity which occur as a consequence of gas adsorption 2' 1o4 can be explained by (i) a change in the number of conduction electrons 1°5, (ii) a change in the electron mobility 1°4' 106 or (iii) demetallization l°v which results in a decrease in thickness. The earlier experiments 1°8, 109 seemed to support the idea of demetallization. It must be remembered, however, that some of these results could not be reproduced 49

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and furthermore that in some cases oxygen, which is known to oxidize metal films rather than to be chemisorbed, was used as the adsorbate. It has also been found 49' 110,111 that in addition to the above-mentioned effects a change in the mechanism of scattering of conduction electrons at the surface can influence the transport properties. Bast1112 reaches similar conclusions. It now seems to be clear that it is not possible to separate the various effects, that the observed influence is a superposition of these effects and that they may differ in importance in their influence on the various transport phenomena. It is therefore essential to combine as many methods of investigation as possible 2. In order to obtain reliable results regarding the influence of gas adsorption on the Hall effect it is necessary to carry out systematic investigations including the dependence of the change in the Hall coefficient on coverage, on film thickness and on temperature. First attempts in this direction have recently been made 113,114 for the adsorption system Ni/CO. In addition to an overall increase in the ordinary Hall coefficient, which is in agreement with earlier observations l°a' 109,115, an oscillation of R o with increasing coverage was found. These oscillations resemble the Shubnikov-de Haas effectas, which indicates a change in the band structure of the nickel due to chemisorption of CO. The coverage dependence ARE(P ) of the change in the extraordinary Hall coefficient shows a connection with the coverage dependence Ap(0) of the change in resistivity (without the presence of a magnetic field) of the type AR~(0) = aAp2(O) + bAp(O) which can be understood on the basis of the theory of Karplus and Luttinger 116 when it is assumed that the adsorbed CO molecules act as impurity centres. Further investigations including other adsorption systems are necessary to elucidate the real physical background of these new observations.

3.4. Transverse magnetoresistivity Only a few papers 113-115,11 '7 describe measurements regarding the influence of adsorption on the transverse magnetoresistivity, in spite of the fact that it can easily be measured with the Hall voltage. An overall decrease of the magnetoresistivity is observed both for the adsorption of 0 2 on nickel 117 and for the adsorption of CO on nickel 11a-ll 5. The curve of ARmag against the applied field strength exhibits a maximum. The position of this maximum is shifted on adsorption, which might indicate a change in the magnetoelastic properties. An influence on the magnetoelastic properties of nickel due to adsorption has been reported 11s' 119. Below the saturation magnetization the shape (no extrema, maximum, minimum and maximum 45) of the curve of ARmag against the field strength depends strongly on thickness. The coverage dependence of the change in the transverse magnetoresistivity has therefore only been investigated for the region above the saturation magnetization. It exhibits an oscillating behaviour similar to that of the Hall coefficient R 0 (see Section 3.3) 113,114. The reproducibility, however, is worse than in the case of the change in R o. It is not yet possible to give a satisfactory explanation of the observed phenomena.

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3.5. Comparatit, e discussion Because of the great advantage of magnetic and galvanomagnetic measurements in catalytic work, a number of experiments have been carried ou! under moderate vacuum conditions on highly dispersed catalysts s5 05. These results, as well as results on thin films not obtained under ultrahigh vacuum conditions ~8' 99 only yield qualitative information but do show strong chemisorption effects. Theoretical interpretations of the few experimental results obtained under clean conditions are difficult, firstly because of problems in understanding the magnetic and galvanomagnetic behaviour of clean films alone (see Section 2.5). Basic theoretical problems remain;concerning the influence of chemisorption on magnetization, magnetic anisotropies and electron transport properties, whereby the involvement of spins in localized or non-localized bonding models must be taken into account. However, phenomenological approaches are available in which chemisorbed species are treated as scattering centres, film thicknesses and bond numbers are estimated, and changes in the number of free electrons are related to changes in band structure. An established theoretical framework has been developed for simple metal systems, principally for bulk material. It is desirable in the future to strengthen this framework for thin film material and to combine it with theories now being developed for chemisorption. In the experimental sphere there is a need for reliable data with which to test the theoretical predictions and to discover correlations. REFERENCES

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