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Surface segregation in Cu70Zr30 metglass: An auger electron spectroscopy study
Surface and Coatings Technology, 34 (1988) 133
-
139
133
SURFACE SEGREGATION IN Cu,0Zr30 METGLASS: AN AUGER ELECTRON SPECTROSCOPY STUDY S. BADRINARAYANAN, A. B. MANDALE and S. SINHA Special Instruments Laboratory, Physical Chemistry Division, National Chemical Laboratory, Pune-411 008 (India) (Received January 27, 1987)
Summary The surface enrichment of metglass Cu70Zr~ was studied by Auger electron spectroscopy at various temperatures in high vacuum. Segregation of zirconium to the surface was analysed on the basis of bond breaking and elastic strain theories. The heat of segregation was estimated. 1. Introduction The rapid development of surface analytical techniques such as X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) has made it possible to determine the nature of surface atoms, their oxidation states and to a large extent their relative concentration in the first few layers. In particular, AES is a useful technique for elucidating the surface composition and its depth distribution in alloys. In this technique, the escape depth of Auger electrons is one of the important parameters in estimating the surface composition. According to the literature [1], the escape depth of Auger electrons is nearly 15 20 A in the energy range up to 1000 eV. Hence this technique is ideally suited for surface segregation studies. In the past few years, a growing amount of evidence has been produced concerning surface segregation from multi-component systems [2 8]. This phenomenon, which manifests itself as a difference in composition between the surface and the bulk or between an interface and the adjoining phase, plays an important role in several branches of materials science such as mechanical behaviour, kinetics of phase transformation, corrosion and the catalytic properties of alloys [9 11]. In this communication, we report our results of the AES studies on the surface composition of the Cu70Zr30 metglass system as a function of annealing temperature. A suitable calibration procedure was adopted to determine the surface composition. The results are analysed in the light of the existing theories on segregation. Metallic glasses are a class of materials which are produced by rapidly cooling the liquid metals. The cooling rate is often of the order of 106 K s~. -
-
-
0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
134
Under such rapid cooling, the material has no time to arrange itself in the crystalline configuration commonly found in solid-state metallic materials. The resulting structure is amorphous and possesses some unusual properties. The high rates of cooling are achieved by spreading a thin layer of liquid in good contact with a highly conductive substrate, e.g. metals and sapphire. The development of metglass alloys (or amorphous alloys as they are sometimes called) is displacing some of the conventional materials used in transformers, brazing foils, high-resolution printers, power supplies etc.
2. Experimental procedure The metglass was in the form of a thin ribbon. The sample was mounted on a suitable holder along with its pure metal components. The present AES study was performed with a V.G. Scientific ESCA-3 MKII electron spectrometer provided with a preparation and an analyser chamber separated by a gate valve. The base pressure in the analyser chamber was about 3 X iD_b Torr. The energy of the primary excitation beam was 5 kV and the modulation voltage used was 2 V (peak-to-peak). The peak-to-peak amplitudes of the Auger signals in the dN(E)/dE vs. E spectra were taken as a measure of the amount of the elements present on the surface of the sample. This proposition has been generally accepted, as long as the peaks retain their shape in the course of treatments. All the spectra were recorded at the same target current and with identical spectrometer parameters. The line positions of the Auger peaks were obtained from the maximum in the high energy lobe of the dN(E)/dE vs. E curve following the method of Bishop and Revière [12]. Spectra from the as-prepared sample showed the presence of small amounts of carbon and oxygen on the surface. Hence some surface cleaning was essential to remove these contaminations. The sample was etched by argon ion bombardment and the AES spectra were recorded between etchings. After obtaining a clean surface, the sample was annealed in vacuum at various temperatures for 1 h and the AES spectra were recorded after the completion of annealing at various temperatures. However, all the AES spectra were recorded at room temperature. To interpret the peak height of the Auger features quantitatively, a suitable calibration procedure is essential. There are several methods of calibration documented in the literature [13] for surface segregation studies in binary alloys. The most widely used calibration procedure in studying segregation in alloys is to fracture a piece of homogenized stable intermetallic compound in an ultrahigh vacuum and analyse the spectra obtained from the fractured surface. The fractured surface is presumed to contain elements of the compound in the proportions given by the stoichiometric formula. Auger spectra obtained in this way give better information about the intensities of the characteristic elemental peaks within the probing depth. This type of calibration was not possible in the present study, as the
135
samples were in the form of thin ribbon. Hence the sample surface was cleaned by argon ion sputtering and this cleaned surface was assumed to represent the initial composition of the sample. Generally, sputtering of an alloy surface by argon ion bombardment leads to preferential enrichment in a particular element of the alloy. We found that there was a surface enrichment of zirconium after prolonged sputtering. Hence we carried out the sputtering just to remove the carbon and oxygen and the surface composition obtained is taken to represent the initial composition of the metglass. The most widely used normalization technique is to record the Auger spectra of pure components together with that of the sample under study and then normalize the Auger signals from the sample by dividing by the intensities of the pure components. This will give the surface composition (within the probing depth) directly, provided that the cross-section and line shape are the same as those for as the pure components. The calculation procedures are as follows. The Auger peak intensity (peak-to-peak height) is proportional to the number of emitting atoms n. Hence IzrRiflzr
(1)
IçjuR
(2)
2fl~u
where ‘Zr and ‘Cu are the Auger peak-to-peak heights of zirconium and copper respectively from the metglass, and R1 and R2 are the Auger signal intensities obtained under identical conditions for pure zirconium and copper respectively. If X~ris the surface composition of zirconium, then ~Zr
—
Xzr
— -
fl7~+ fl~
______________ ________
Izr/Ri + IZrIR2
‘Zr
=
=
(1? 1/R 2)ICu R1/1?2.
‘Zr +
where a
=
Xzr (at.%)
=
r IZr+aICu
3
‘Zr ‘Zr +
X 100
aIcu
( (4)
Table 1 gives the surface composition of zirconium at various temperatures of annealing calculated using eqn. (3). We used Zr148 and Cu920 Auger peaks for the peak-height calculations (Figs. 1(a) and (b)).
3. Results and discussion The cleaning procedure prior to annealing reduces the carbon and oxygen contaminants to below the detectable limit for AES analysis. The disadvantage with argon ion bombardment is that it results in selective sputtering of a particular component. To check this, the sample was subjected to
136 TABLE 1 Relative intensities of zirconium and copper Treatment
Temperature (°C)
Argon ion cleaning
~Zr]/[Cü]
30
Heating in vacuum for 1 h
30 70 110 150 200 250 300 350 400 500
Surface composition (at.% Zr)
1.2 1.5
0.40 O.4 6a
1.9
050b
1.2 3.3 5.6 6.5 9.0 11.8 14.9 18.0 21.3 27.2
0.40 0.65 0.76 0.79 0.84 0.87 0.89 0.91 0.94 0.95
aAfter 1 h sputtering. bAfter 3 h sputtering.
1 a
I
HI~~j~r 200°
A
~
200
1/.0
80
950
KINETIC ENERGY (eV)
*—
890
830
KINETIC ENERGY (eV)
Fig. 1. AES spectra of Cu 7oZry~metglass for (a) zirconium region and (b) copper region at various temperatures.
argon ion etching for nearly 3 h and the spectra were recorded at intervals of 1 h etching. The surface concentration of zirconium was found to increase with the length of sputtering (Table 1), indicating that copper was preferentially sputtered. It is well known that the sputter yield is different for various metals at the same ion energy. It was also observed that the metal with the lowest heat of sublimation has the highest sputtering rate. The heats
137
of sublimation of copper and zirconium are 80.9 kcal mo11 and 145.8 kcal mol~ respectively [14] and hence the sputtering rate of copper will be higher than that of zirconium. The experimental samples were subjected to a minimum argon ion etching to remove the surface contaminant, but not to cause segregation of zirconium on the surface. All the vacuum-heated samples showed an increase in the ratio [Zr]/[Cu] which implies preferential segregation of zirconium on the surface. Figure 2 shows the plot of surface concentration of zirconium vs. the annealing temperature and shows a linear relationship between them. 28-
o
-
0
j20cc C)
0
-
0
~
-
I
I
100 200 300 400 500 TEMPERATURE C Fig. 2. Plot of concentration ratio [Zr]/[Cu]
us. temp~rature.
The surface enrichment of a constituent element can be explained on the basis of well-known thermodynamic parameters, the primary driving force being the reduction of surface free energy. The experimental results from several binary alloy systems have led to the formulation of some generally applicable rules [15 18]. Surface energy is proportional to the heat of sublimation and hence the element with the lower heat of sublimation of a binary system will enrich the surface. On interaction with active gases, the surface will become enriched in the element with the higher heat of formation. The lattice strain theory explains the surface segregation characteristics, taking into account the differences in atomic radii; the strain produced in the surface lattice on account of such differences is assumed to control the surface enrichment. Differences in the size and compressibility of the atoms in the lattice are the important parameters. We now analyse the preceeding data for CuZr in terms of bond breaking and elastic strain theories. As pointed out earlier, the heat of sublimation for copper is low compared with that of zirconium and hence, according to bond breaking theory, copper is expected to enrich the surface. The atomic radii for zirconium and copper are 1.6 A and 1.28 A respectively. Therefore, the lattice strain theory predicts the segregation of zirconium to the surface, and in fact seems to explain the present experimental results. The surface segregation in dilute alloys may be related to bulk concentrations through [19, 20] -
XBS X~~’ ~~—~exp~—
/
Z~Hseg\
RT)
(5)
where XAS and XBS are the atom fractions of component A and B in the surface, XAb and X 5” are the corresponding quantities in the bulk and L~Hseg in
138
the heat of segregation. Equation (5) is strictly valid for very dilute alloys, and shows that, for positive values of ~H, the element B will segregate to the surface. Though strictly valid for dilute alloys, eqn. (5) has been used to explain the segregation behaviour in alloys with higher concentrations of the constituent elements [19, 21]. The ratio XzrS/X~uScan be considered proportional to ‘Zr/’Cu when ‘Zr and ‘Cu are the Auger peak-to-peak heights obtained from the experimental sample. Figure 3 shows a plot of log ‘Zr/’CU vs. lIT. From the slope of the resulting straight line, the heat of segregation was calculated to be about 3 kcal moF’. We have not attempted to elaborate our results any further, since our system is not a dilute alloy.
1/T 2~~o_3 Fig. 3. Plot of log [Zrl/[Cu]
us.
l/T.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
M. P. Seah and W. A. Dench, Surf. Interface Anal., 1 (1979) 2. P. Braun and W. Farber, Surf. Sci., 47 (1975) 57. S. H. Overbury and G. A. Somorjai, Surf. Sci., 55 (1976) 209. K. Watanabe, M. Hashiva and T. Yamashina, Surf. Sci., 61(1976) 483. A. Jablonski, S. H. Overbury and G. A. Simorjai, Surf. Sci., 65 (1977) 578. F. J. Kuijers and P. Ponec, Surf. Sci., 68 (1977) 294. G. D. Parks, Appl. Surf. Sci., 5 (1980) 92. V. I. Nefedov, Y. V. Salyn, V. A. Makcev and V. I. Zelenn, J. Electron Spectrosc. Relat. Phenom., 24 (1981) 11. V. Ponec and W. M. H. Sachtler, J. Catal., 24 (1972) 250. J. J. Burton and P. L. Ganten (eds.), Advanced Materials in Catalysis, Academic Press, New York, 1977. M. W. Delgass, G. L. Haller, R. Kellerman and J. H. Lowsford, Spectroscopy in Heterogeneous Catalysis, Academic Press, New York, 1979. H. E. Bishop and J. C. Revière, Surf. Sci., 17 (1969) 462. F. F. Abraham and C. R. Brundle, J. Vac. Sci. Technol., 18 (1981) 506. G. V. Samsonov (ed.), Handbook of Physicochemical Properties of the Elements, Plenum, New York, 1968. R. A. Vansanten and M. A. M. Boersma, J. Catal., 34 (1974) 13. E. L. Willims and D. Nason, Surf. Sci., 45 (1974) 377.
139 17 R. Bouwman, L. H. Toneman, M. A. M. Boersma and R. A. Van Janten, Surf. Sci., 59 (1976) 72. 18 M. P. Seah, in D. Briggs and M. P. Seah (eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Wiley, New York, 1984, p. 247. 19 K. Wandelt and C. R. Brundle, Phys. Rev. Lett., 46 (1981) 1529. 20 F. F. Abraham, N. H. Tsai and G. M. Pound, Surf. Sci., 83 (1979) 406. 21 R. I. Hegde and A. P. B. Sinha, Surf. Sci., 133 (1983) 233.
-
139
133
SURFACE SEGREGATION IN Cu,0Zr30 METGLASS: AN AUGER ELECTRON SPECTROSCOPY STUDY S. BADRINARAYANAN, A. B. MANDALE and S. SINHA Special Instruments Laboratory, Physical Chemistry Division, National Chemical Laboratory, Pune-411 008 (India) (Received January 27, 1987)
Summary The surface enrichment of metglass Cu70Zr~ was studied by Auger electron spectroscopy at various temperatures in high vacuum. Segregation of zirconium to the surface was analysed on the basis of bond breaking and elastic strain theories. The heat of segregation was estimated. 1. Introduction The rapid development of surface analytical techniques such as X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) has made it possible to determine the nature of surface atoms, their oxidation states and to a large extent their relative concentration in the first few layers. In particular, AES is a useful technique for elucidating the surface composition and its depth distribution in alloys. In this technique, the escape depth of Auger electrons is one of the important parameters in estimating the surface composition. According to the literature [1], the escape depth of Auger electrons is nearly 15 20 A in the energy range up to 1000 eV. Hence this technique is ideally suited for surface segregation studies. In the past few years, a growing amount of evidence has been produced concerning surface segregation from multi-component systems [2 8]. This phenomenon, which manifests itself as a difference in composition between the surface and the bulk or between an interface and the adjoining phase, plays an important role in several branches of materials science such as mechanical behaviour, kinetics of phase transformation, corrosion and the catalytic properties of alloys [9 11]. In this communication, we report our results of the AES studies on the surface composition of the Cu70Zr30 metglass system as a function of annealing temperature. A suitable calibration procedure was adopted to determine the surface composition. The results are analysed in the light of the existing theories on segregation. Metallic glasses are a class of materials which are produced by rapidly cooling the liquid metals. The cooling rate is often of the order of 106 K s~. -
-
-
0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
134
Under such rapid cooling, the material has no time to arrange itself in the crystalline configuration commonly found in solid-state metallic materials. The resulting structure is amorphous and possesses some unusual properties. The high rates of cooling are achieved by spreading a thin layer of liquid in good contact with a highly conductive substrate, e.g. metals and sapphire. The development of metglass alloys (or amorphous alloys as they are sometimes called) is displacing some of the conventional materials used in transformers, brazing foils, high-resolution printers, power supplies etc.
2. Experimental procedure The metglass was in the form of a thin ribbon. The sample was mounted on a suitable holder along with its pure metal components. The present AES study was performed with a V.G. Scientific ESCA-3 MKII electron spectrometer provided with a preparation and an analyser chamber separated by a gate valve. The base pressure in the analyser chamber was about 3 X iD_b Torr. The energy of the primary excitation beam was 5 kV and the modulation voltage used was 2 V (peak-to-peak). The peak-to-peak amplitudes of the Auger signals in the dN(E)/dE vs. E spectra were taken as a measure of the amount of the elements present on the surface of the sample. This proposition has been generally accepted, as long as the peaks retain their shape in the course of treatments. All the spectra were recorded at the same target current and with identical spectrometer parameters. The line positions of the Auger peaks were obtained from the maximum in the high energy lobe of the dN(E)/dE vs. E curve following the method of Bishop and Revière [12]. Spectra from the as-prepared sample showed the presence of small amounts of carbon and oxygen on the surface. Hence some surface cleaning was essential to remove these contaminations. The sample was etched by argon ion bombardment and the AES spectra were recorded between etchings. After obtaining a clean surface, the sample was annealed in vacuum at various temperatures for 1 h and the AES spectra were recorded after the completion of annealing at various temperatures. However, all the AES spectra were recorded at room temperature. To interpret the peak height of the Auger features quantitatively, a suitable calibration procedure is essential. There are several methods of calibration documented in the literature [13] for surface segregation studies in binary alloys. The most widely used calibration procedure in studying segregation in alloys is to fracture a piece of homogenized stable intermetallic compound in an ultrahigh vacuum and analyse the spectra obtained from the fractured surface. The fractured surface is presumed to contain elements of the compound in the proportions given by the stoichiometric formula. Auger spectra obtained in this way give better information about the intensities of the characteristic elemental peaks within the probing depth. This type of calibration was not possible in the present study, as the
135
samples were in the form of thin ribbon. Hence the sample surface was cleaned by argon ion sputtering and this cleaned surface was assumed to represent the initial composition of the sample. Generally, sputtering of an alloy surface by argon ion bombardment leads to preferential enrichment in a particular element of the alloy. We found that there was a surface enrichment of zirconium after prolonged sputtering. Hence we carried out the sputtering just to remove the carbon and oxygen and the surface composition obtained is taken to represent the initial composition of the metglass. The most widely used normalization technique is to record the Auger spectra of pure components together with that of the sample under study and then normalize the Auger signals from the sample by dividing by the intensities of the pure components. This will give the surface composition (within the probing depth) directly, provided that the cross-section and line shape are the same as those for as the pure components. The calculation procedures are as follows. The Auger peak intensity (peak-to-peak height) is proportional to the number of emitting atoms n. Hence IzrRiflzr
(1)
IçjuR
(2)
2fl~u
where ‘Zr and ‘Cu are the Auger peak-to-peak heights of zirconium and copper respectively from the metglass, and R1 and R2 are the Auger signal intensities obtained under identical conditions for pure zirconium and copper respectively. If X~ris the surface composition of zirconium, then ~Zr
—
Xzr
— -
fl7~+ fl~
______________ ________
Izr/Ri + IZrIR2
‘Zr
=
=
(1? 1/R 2)ICu R1/1?2.
‘Zr +
where a
=
Xzr (at.%)
=
r IZr+aICu
3
‘Zr ‘Zr +
X 100
aIcu
( (4)
Table 1 gives the surface composition of zirconium at various temperatures of annealing calculated using eqn. (3). We used Zr148 and Cu920 Auger peaks for the peak-height calculations (Figs. 1(a) and (b)).
3. Results and discussion The cleaning procedure prior to annealing reduces the carbon and oxygen contaminants to below the detectable limit for AES analysis. The disadvantage with argon ion bombardment is that it results in selective sputtering of a particular component. To check this, the sample was subjected to
136 TABLE 1 Relative intensities of zirconium and copper Treatment
Temperature (°C)
Argon ion cleaning
~Zr]/[Cü]
30
Heating in vacuum for 1 h
30 70 110 150 200 250 300 350 400 500
Surface composition (at.% Zr)
1.2 1.5
0.40 O.4 6a
1.9
050b
1.2 3.3 5.6 6.5 9.0 11.8 14.9 18.0 21.3 27.2
0.40 0.65 0.76 0.79 0.84 0.87 0.89 0.91 0.94 0.95
aAfter 1 h sputtering. bAfter 3 h sputtering.
1 a
I
HI~~j~r 200°
A
~
200
1/.0
80
950
KINETIC ENERGY (eV)
*—
890
830
KINETIC ENERGY (eV)
Fig. 1. AES spectra of Cu 7oZry~metglass for (a) zirconium region and (b) copper region at various temperatures.
argon ion etching for nearly 3 h and the spectra were recorded at intervals of 1 h etching. The surface concentration of zirconium was found to increase with the length of sputtering (Table 1), indicating that copper was preferentially sputtered. It is well known that the sputter yield is different for various metals at the same ion energy. It was also observed that the metal with the lowest heat of sublimation has the highest sputtering rate. The heats
137
of sublimation of copper and zirconium are 80.9 kcal mo11 and 145.8 kcal mol~ respectively [14] and hence the sputtering rate of copper will be higher than that of zirconium. The experimental samples were subjected to a minimum argon ion etching to remove the surface contaminant, but not to cause segregation of zirconium on the surface. All the vacuum-heated samples showed an increase in the ratio [Zr]/[Cu] which implies preferential segregation of zirconium on the surface. Figure 2 shows the plot of surface concentration of zirconium vs. the annealing temperature and shows a linear relationship between them. 28-
o
-
0
j20cc C)
0
-
0
~
-
I
I
100 200 300 400 500 TEMPERATURE C Fig. 2. Plot of concentration ratio [Zr]/[Cu]
us. temp~rature.
The surface enrichment of a constituent element can be explained on the basis of well-known thermodynamic parameters, the primary driving force being the reduction of surface free energy. The experimental results from several binary alloy systems have led to the formulation of some generally applicable rules [15 18]. Surface energy is proportional to the heat of sublimation and hence the element with the lower heat of sublimation of a binary system will enrich the surface. On interaction with active gases, the surface will become enriched in the element with the higher heat of formation. The lattice strain theory explains the surface segregation characteristics, taking into account the differences in atomic radii; the strain produced in the surface lattice on account of such differences is assumed to control the surface enrichment. Differences in the size and compressibility of the atoms in the lattice are the important parameters. We now analyse the preceeding data for CuZr in terms of bond breaking and elastic strain theories. As pointed out earlier, the heat of sublimation for copper is low compared with that of zirconium and hence, according to bond breaking theory, copper is expected to enrich the surface. The atomic radii for zirconium and copper are 1.6 A and 1.28 A respectively. Therefore, the lattice strain theory predicts the segregation of zirconium to the surface, and in fact seems to explain the present experimental results. The surface segregation in dilute alloys may be related to bulk concentrations through [19, 20] -
XBS X~~’ ~~—~exp~—
/
Z~Hseg\
RT)
(5)
where XAS and XBS are the atom fractions of component A and B in the surface, XAb and X 5” are the corresponding quantities in the bulk and L~Hseg in
138
the heat of segregation. Equation (5) is strictly valid for very dilute alloys, and shows that, for positive values of ~H, the element B will segregate to the surface. Though strictly valid for dilute alloys, eqn. (5) has been used to explain the segregation behaviour in alloys with higher concentrations of the constituent elements [19, 21]. The ratio XzrS/X~uScan be considered proportional to ‘Zr/’Cu when ‘Zr and ‘Cu are the Auger peak-to-peak heights obtained from the experimental sample. Figure 3 shows a plot of log ‘Zr/’CU vs. lIT. From the slope of the resulting straight line, the heat of segregation was calculated to be about 3 kcal moF’. We have not attempted to elaborate our results any further, since our system is not a dilute alloy.
1/T 2~~o_3 Fig. 3. Plot of log [Zrl/[Cu]
us.
l/T.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
M. P. Seah and W. A. Dench, Surf. Interface Anal., 1 (1979) 2. P. Braun and W. Farber, Surf. Sci., 47 (1975) 57. S. H. Overbury and G. A. Somorjai, Surf. Sci., 55 (1976) 209. K. Watanabe, M. Hashiva and T. Yamashina, Surf. Sci., 61(1976) 483. A. Jablonski, S. H. Overbury and G. A. Simorjai, Surf. Sci., 65 (1977) 578. F. J. Kuijers and P. Ponec, Surf. Sci., 68 (1977) 294. G. D. Parks, Appl. Surf. Sci., 5 (1980) 92. V. I. Nefedov, Y. V. Salyn, V. A. Makcev and V. I. Zelenn, J. Electron Spectrosc. Relat. Phenom., 24 (1981) 11. V. Ponec and W. M. H. Sachtler, J. Catal., 24 (1972) 250. J. J. Burton and P. L. Ganten (eds.), Advanced Materials in Catalysis, Academic Press, New York, 1977. M. W. Delgass, G. L. Haller, R. Kellerman and J. H. Lowsford, Spectroscopy in Heterogeneous Catalysis, Academic Press, New York, 1979. H. E. Bishop and J. C. Revière, Surf. Sci., 17 (1969) 462. F. F. Abraham and C. R. Brundle, J. Vac. Sci. Technol., 18 (1981) 506. G. V. Samsonov (ed.), Handbook of Physicochemical Properties of the Elements, Plenum, New York, 1968. R. A. Vansanten and M. A. M. Boersma, J. Catal., 34 (1974) 13. E. L. Willims and D. Nason, Surf. Sci., 45 (1974) 377.
139 17 R. Bouwman, L. H. Toneman, M. A. M. Boersma and R. A. Van Janten, Surf. Sci., 59 (1976) 72. 18 M. P. Seah, in D. Briggs and M. P. Seah (eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Wiley, New York, 1984, p. 247. 19 K. Wandelt and C. R. Brundle, Phys. Rev. Lett., 46 (1981) 1529. 20 F. F. Abraham, N. H. Tsai and G. M. Pound, Surf. Sci., 83 (1979) 406. 21 R. I. Hegde and A. P. B. Sinha, Surf. Sci., 133 (1983) 233.