UHV apparatus for electron-stimulated desorption: experimental procedure and characteristics

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UHV apparatus for electron-stimulated desorption : experimental procedure characteristics

and

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1040 Sofia, Bulgaria

S G Kasabov,

A UHV apparatus for electron-stimulated desorption studies of solid state surface ions is described. The bakeable stainless-steel vacuum system is equipped with a high-sensitivity nonconventional singlefocusing magnetic sector mass spectrometer for mass determination of the desorbing ions. A retarding field energy analyzer is placed between the sample and the mass filter for simultaneous measurements of mass and ion kinetic energy distributions. The absolute scale of the energy analyzer is periodically obtained in situ by sodium ions of known energy produced by surface ionization. Results from electron bombardment of oxygen and sodium co-adsorbed on a single crystal W( 700) surface are presented to demonstrate the basic parameters of the instruments.

1. Introduction Bombardment of adsorbate-covered solid surfaces with lowenergy (l@-1000 eV) electrons can lead to excitation of the adsorbate-substrate system, and, as a result, to desorption of charged and neutral species. This phenomenon is known as electron-stimulated desorption (ESD) and is the subject of increasing interest for many researchers’. Usual ESD equipment contains a mass spectrometer and an energy analyzer for direct determination of the masses and energies of the desorbing particles. Positive ions are mostly studied. High sensitivity of ESD registration is needed since the ion desorption cross-sections are very small (usually in the range 10-‘8-10-24 cm*), and ions with a yield < 10s9 ion electron-’ should be detected. ESD is highly sensitive to the top surface monolayer and for this reason measurements must take place in ultrahigh vacuum (UHV). The apparatus should also meet a series of additional requirements such as surface cleanliness, temperature control of the samples and formation of pure adsorbates. The present paper contains a description of a UHV apparatus for ESD which answers the above requirements. Some results of ESD measurements of positive ions from a tungsten single crystal with oxygen or oxygen+ sodium adsorbed on the (100) surface are presented. Tests of the apparatus and comparative characterization of its basic parameters with other ESD equipment illustrates the performance of the system. 2. Experimental

system and procedure

The apparatus is similar to that constructed by Ageev et al*; a magnetic type mass spectrometer and the same semi-cylindrical energy analyzer (with some changes) for ESD are used. Details of the vacuum pumping system, electronics and some procedures are different for our apparatus. 2.1. Vacuum system. The bakeable stainless steel UHV system is pumped by a diffusion oil pump (Metrovac 033C, AEI Ltd) with

Santovac 5 oil, baffled and cold trapped, an ion pump P 300 (AEI Ltd) and a home built titanium sublimation pump (with a Leybold Heraeus Ti sublimator) cooled by LN2. The total gas pressure is measured by a Leybold Heraeus IE211 Bayard-Alpert ionization gauge. Residual gas spectra are taken by a magnetic mass spectrometer calibrated against an ionization gauge. After bake-out at temperatures up to 25O”C, the background pressure attained is < 1 x lo-” mbar (N, equivalent). 2.2. Electron-stimulated desorption positive ion detection : mass and energy analysis. The single-focusing magnetic sector mass spectrometer (20 cm mean radius flight tube and 90” deflection) is used to analyze positive species. The election impact gas ionization source is a standard ‘nude’ Nier-type design with electrostatic focusing of the ionization beam. The ions with different masses are focused on the exit slit of the mass-spectrometer by varying the magnetic field (constant accelerating voltage set at N 1.2 kV). A U-shaped semicylindrical electrostatic energy analyzer is placed in front of the ion source with two grids and a collector mounted around the ribbon-shaped sample to measure the total surface ion current emitted during electron bombardment and to determine absolute ion yields. The electron beam source for ESD is a 0.01 cm diameter tungsten filament, resistively heated and situated parallel to the ribbon along the cylindrical symmetry axis. The ESD ions desorbed from the sample are accelerated through the grids and are retarded to the collector where the total ion current is measured by an electrometer. A small number of the ions are directed to the mass spectrometer through a grid-covered slit in the collector for determination of the ion masses. The second electrostatic diode-type retarding field energy analyzer (with retarding grids formed by the mass spectrometer ion source focusing grids) is used for determining both the masses and the kinetic energies of the desorbing ESD ions. The slit widths are changed according to previous work16. The acceptance semi-angle of the mass spectrometer and the diode energy analyzer is about 4” around the surface normal, i.e. only ions 1035

S G Kasabov:

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emitted normally to the surface in a cone with a full angle of about 8” are measured. Ion energy distributions are obtained by sweeping the retarding field at a linear rate and electronically differentiating the ion currents with respect to time. The energy scale is calibrated periodically in situ by sodium ions with a known energy produced by positive surface ionization on tungsten (see Section 3.2.). The dependence of the ion yield on the electron energy and the threshold energy for ion desorption is determined by varying the bombarding electron energy. The electrode dc potentials for operating in different modes are supplied by stabilized power units and an electrical scheme allowing quick and convenient switching. Each mass spectrometer has its specific optimum dimensions of slits to be used in registering ESD ions because the sector magnetic field ensures no energy focusing. For energy spreads of about 8-10 eV, typical of the kinetic energy range of ESD ions, the chromatic aberrations lead to a considerable mass amplitude discrimination during mass analysis of ions. The slit widths of the mass spectrometer are determined by taking into account the energy dispersion D, = roAE/Vo so that for ion energies and energy spread AE N 10 eV there is a negligible discrimination only and the mass resolution is satisfactory. The mass spectrometer output ion currents are measured by a magnetically screened channel electron multiplier, an electrometer, a counting system and a recorder. The full-scale current the measuring range is 1O-s-1O- I3 A for the electrometer; maximum counting rate, 10’~ s-’ for the counter; the noise level, < 1 x IO- I3 A (< 1 count s- ‘) at a multiplier gain of - 10’.

2.3. Sample, admission of oxygen and sodium deposition. The design of the sample holder allows mounting polycrystalline and single crystal samples. The sample used was a tungsten single crystal (18 x 2 x 0.3 mm) with a (100) surface orientation. The crystal was prepared in the A F Ioffe Physico-Technical Institute in Leningrad according to a standard procedure including cutting and mechanical and electrolytic polishing3.14. The orientation determined by X-ray diffraction was within 0.5”. Resistive heating of the crystal by alternating or direct current from a voltageregulated stable dc supply (IO V x 50 A, peak current 80 A) with a time stability < 0.1% h-’ at mean current values was possible“. The temperature was measured by a tungsten-rhenium thermocouple (0.005 cm diameter wires) fixed at the crystal end by spot-welding and an optical micropyrometer at high temperatures (>, 1000 K). The sample was cleaned by prolonged heating under oxygen (10e6 mbar) at temperatures up to 2000 K and flashing for a short time up to 2500 K under high vacuum. All hot filaments, as well as the sample were cleaned by high temperature oxygen treatment. The central part of the tungsten ribbon (N 11 mm) was bombarded with electrons. Oxygen was admitted to the UHV system via a silver diffusion leak and a bakeable variable valve (RIBER). A Knudsen effusion source of pure Na atom beams obtained by reduction of sodium molybdate with zirconium’ is used. Potassium was the main metal admixture in the sodium beam, its concentration being about 5 x 10m3%. The angle between the direction of the incident atom beams and the ribbon normal was about 40”. The beams were switched on or off by a shutter.

ization gauge at an oxygen pressure in the range 10-6-10-7 mbar and a residual gas pressure < 1 x lo-” mbar. It is assumed that the gauge sensitivity is the same for O2 and N2. The partial pressures of the other gases are estimated on the basis of the ion currents of the mass spectrometer, the N, relative sensitivity correction factors being taken into account. Figure 1 shows the residual gas spectrum in the well outgassed vacuum system. An ionization beam current of 2 x 10W6A and a multiplier gain of N lo6 are used. To reduce errors resulting from non-controllable outgassing and pumping processes of the ionizer, a weak ionizer current (with a maximum of 1 x low4 A) is used. From the integral current spectrum in Figure 1 it is seen that the total background pressure does not exceed 7 x lo-” mbar (Nz equivalent). Control measurements have indicated HzO, CO and CO2 to originate mainly from the electrodes of the mass spectrometer ion source. The 19, 35 and 37, 39 mass peaks are attributed to ESD of 19F+, “Cl+ and “Cl*, “K+ within the spectrometer ionizer. ESD has also contributed to the 16 and 17 mass peaks of i60+ and CH:, and OH+ ions. The contribution of ESD ions to the residual gas spectrum can be estimated6 but this is not our task. Mass resolution m/Am measured for two adjacent peaks of equal magnitude at half maximum is about 70. Oxygen pressures in adsorption experiments usually range from lo-’ to lo-* mbar. Mass analysis has shown the oxygen partial pressure in this range to be 99% of the total pressure, the main admixtures being CO and CO2 whose partial pressures do not exceed 0.5% with an initial residual pressure < 1 x lo-” mbar. Figure 2 shows an ESD mass spectrum of ‘dirty’ tungsten (clean tungsten exposed to more than 50 L oxygen, then allowed to stand for 24 h in a residual gas atmosphere at a pressure about 5 x 10e9 mbar). The spectrum is recorded at an energy of 120 eV with an incident electron beam current of 5 x 10e6 A. Mass peaks l(H+), 16(0+), 17(OH+), 18(H20f), 19(F+) and 28(CO+) are recorded.

1 2

3. Measurement and apparatus performance 3.1. Mass analysis. The mass spectrometer used as a conventional pressure analyzer is periodically calibrated against an ion-

gas

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Figure 1. Gas phase mass spectrum at total gas pressure p = 7 x IO-” mbar : V, = 75 eV; i, = 2 x 10m6 A ; multiplier gain - 106.

S G Kasabov: UHV apparatus for electron-stimulated

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Mass number Figure 2. Mass spectrum of ions under electron bombardment

of ‘dirty’ tungsten (> 50 L O2 exposure of a clean sample followed by contamination under 5 x 10Y9mbar residual pressure for 24 h). Bombardment with a 120 eV, 5 x 10m6A electron beam.

3.2. Energy analysis: calibration of analyzer scale zero and resolution determination. The exact determination of the energy of

charged particles with respect to the vacuum level (absolute energies) is a specific experimental problem since the existing contact potentials displace the zero of the energy scale. The analytical correction of the scale based on the work function of the analyzer electrode materials can lead to considerable errors. The scale should be periodically corrected experimentally in situ using particles with known energies. These are ions formed during surface ionization (SI) which have a Maxwellian energy distribution with a half-width AE,,, = 2.54 krand a most probable energy corresponding to the distribution maximum E, = kT, where k is the Boltzmann constant and T is the emitter temperature’*“. We used Na+ ions to calibrate our energy analyzer. To utilize these thermal ions as a check point in determining the absolute zero of the scale, one has to exactly measure the emitter temperature and record the distribution. When the emitter work function is known (it can be measured by standard methods), it is possible to determine, during calibration, the zero displacement which is due to the work functions of the analyzer electrodes whose surfaces are inaccessible to direct control. Since the distribution width is affected by the analyzer instrumental width and the structure of the emitter surface, the maximum may not be clearly discernible or even shifted. A more precise determination of the correction is achieved using the undifferentiated retardation curves whose plateau and initial drop are extrapolated to straight lines from the interception of which the contact potential difference shifting the zero is determined. Thus, the scale is periodically calibrated by means of Na+ thermal ions from clean tungsten with a known work function. The resolution R, = AE/E of the diode-type energy analyzer is determined as follows. At a constant temperature of the tungsten sample (1150 K) and a constant incident evaporated sodium flux, the distribution of Na+ ions accelerated to different energies is recorded. To reduce the effect of secondary emission from the ends of analyzer electrodes and the different ionization effects of the residual gases, the ions are accelerated with low voltages ranging from 5 to 12 eV. This is the energy range within which most ESD ions are desorbed’.

.

6

’ 8

’

’ 10

’

0 12

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voltage (v)

Retarding

Figure 3. Dependence of the energy distribution full width at half maximum (FWHM) of Na+ ions produced by surface ionization on clean tungsten on retarding voltage (position of peak maxima). The constant sample temperature is T, = 1150 K and the constant flux of the incident sodium atom beam is _ 10” cm-* s-‘.

The variation of the Na+ surface ion distribution full width at half maximum (FWHM) as a function of primary energy (position of peak maxima) is shown in Figure 3. From the slope of the straight line, the resolution R,,2 = AE,,,/E N 6% is evaluated. One of these energy distributions is shown in Figure 4 to illustrate the energy resolution. The ions are accelerated to 8 eV before they enter the diode energy analyzer. The half-width is 1.2 eV. 3.3. ESD measurements. This section contains

results from our ESD measurements which illustrate the basic parameters of our apparatus and permit comparison with other equipment. The “Of mass signal of tungsten reaches saturation after an oxygen exposure of about 100 L at 300 K and has the value of about 2 x lo4 counts s-’ with a 120 eV, 1 x 10m6A bombarding

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Figure 4. Energy distribution of Na+ surface ionization ions (one of the curves in Figure 3). The ions are accelerated to 8 eV before entering the diode energy analyzer. The half-width is 1.2 eV. The curve illustrates the energy resolution. 1037

S G Kasabov:

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Na+from

Na/O/WflOO)

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Figure 5. Energy distribution of 0 + ions from fully oxygen-covered W(lO0) exposed to > 100 L 0, at 300 K. Bombardment with a 120 eV, 1.6 x 10m6A electron beam. The energy scale zero is corrected for the contact potential between the sample and the analyzer grids. The distribution has a maximum at 8.5 eV and a half-width of 2.6 eV. electron beam. On the basis of the total ion current measured on the semi-cylindrical collector it is established that the ion yield -‘. from this state is q + = i+/& 21 low6 ion electron The energy distribution of O+ ions desorbed from fully oxygen-covered tungsten is presented in Figure 5. The bombarding electron energy is 120 eV and the electron current is 1.6 x 10d6 A. The correction for the contact potential difference between the clean tungsten and the energy analyzer electrodes is determined using Na+ thermions formed during SI according to the procedure described in Section 3.2. A correction of 1.6 eV for the increase in work function of W(100) fully covered with oxygen at 300 K (on the basis of previous work10,‘2.‘3) is made. With this correction, the maximum of the energy distribution curve is established at 8.5f0.5 eV and has a half-width of 2.6 eV. As is known from coadsorbed oxygen and sodium layers on tungsten 14,“, 160+ and 23Na+ ions are observed during ESD. Figure 6 shows the energy distribution of 23Na+ ions of such a layer. Oxygen is adsorbed at 300 K on clean tungsten until saturation is achieved (above 100 L exposure). After pumping, a definite amount of sodium is deposited on this layer until the appearance of a noticeable Na+ ESD signal. The distribution is recorded with an electron energy of 120 eV and an electron current of 1 x 10v3 A. The shoulder at the beginning of the distribution curve is characteristic of high electron currents (above 3 x low4 A). On the basis of the Na+ thermal ion peak position for this layer (marked by SI), in the case of surface ionization (electron current off; hot ribbon), one can state that the shoulder is due to a not well resolved peak of Na+ ions which are desorbed simultaneously with ESD Na+ ions owing to the heating of the sample with stronger currents and irradiation of the hot filament. The distribution in Figure 6 shows a direct resolution of ESD ions and thermal ions with the same mass which cannot be distinguished from one another except on the basis of their kinetic energies. The zero of the energy scale is not corrected. 1038

RETARDING

ENERGY(eV)

VOLTAGE(V)

Figure 6. Energy distribution of Na+ ions desorbed upon electron impact. The tungsten was exposed at 300 K to oxygen for > 100 L with subsequent sodium deposition. Bombardment with a 120 eV energy and 1 x IO-’ A; the position marked with SI is attributed to thermally released ions. The peak at 3.5 eV is due to ESD ions. The curve illustrates the direct observation and resolution of thermally desorbed and ESD ions (simultaneous desorption upon high-current electron impact). The energy scale zero is not corrected.

4. Discussion Some peculiarities of the ESD measurements will be discussed in this section. The basic parameters of the apparatus will be compared with those of other equipment on the basis of the ESD measurements. The mass spectrometer can detect partial pressures < lo-l6 mbar and partial pressure ratios of 6 lo-“. The gas pressure sensitivity is several orders of magnitude higher than that of conventional mass spectrometers and comparable with the sensitivity of the magnetic mass spectrometer of Lichtman and McQuistan’a”7 which allows a detectable limit of about lo-l6 torr. An important parameter of each ESD apparatus is its sensitivity during the registration of ESD ions. Usually authors give no information on the transmission of their apparatus and for this reason we shall compare on the basis of the ‘“0: signal levels measured on different equipment during ESD from tungsten fully covered with oxygen at 300 K. With a 1 x lO-‘j A bombarding current (current density 4.5 x 10m6 A cm-*) and an electron energy of 120 eV, our apparatus gave an 0: signal of 2 x lo4 counts SC’. With the same current and an energy of 160 eV, Prigge et aI9 obtained a value of 2.6 x lo4 counts SC’ using a QMS. With the same kind of mass spectrometer, a current of 0.9 x 10m6 A (current density 2.5 x 1O-6 A cmm2) and an energy of 100 eV, Madey” measured a current of 6 x lo-* A which is equivalent to 4 x lo4 counts s-’ assuming an electron multiplier gain of 10’. Nishijima and Propst” used their cylindrical spectrometer at a current of 1 x 10m6 A (density 1.2 x 10m4 A cm-‘) to measure a current of 3 x lo-* A with an electron multiplier gain of 10’. When reduced to a density of4.5 x 10m6 A cm-*, this current is equivalent to about 7 x lo4 counts s-‘. This comparison shows that our apparatus has a sensitivity towards ESD regis-

S G Kasabov:

UHV apparatus

for electron-stimulated

desorption

tration which is comparable with known ESD equipment and can detect ions with a yield of 10-9-10-‘o ion electrons-‘. The energy distributions give important information on the different adsorption states and potential energy characteristics of the adsorbed species and for this reason they should be measured correctly with a maximum accuracy, i.e. analyzers with a good resolution and calibrated scales should be used. As was shown in Section 3.2., our apparatus allows in situ calibration of the zero scale ; the resolution, experimentally determined on the basis of the half-width of the peak is -6%. The absolute resolution at half maximum in the range 5-12 eV is - 1 eV. Nishijima and Propst” obtained a theoretical resolution of 4%, i.e. the best among all nonconventional ESD analyzers. In previous works9,2’, commercially available analyzers with a cylindrical mirror analyzer (CMA) have been used. Theoretically they ensure a mean resolution of about 1% for energies ranging from 10 to 1000 eV2’. A regime of constant pass energy of the CMA within the range 75-100 eV is needed for the registration of ESD ions to optimize the signal strength. This ensures absolute resolution of about l-2 eV2’ which is comparable with that achieved by us. We shall again make comparison with experimental ESD results. The half-width of the 160f energy distribution for emission into a cone having about +4” angle aperture around the surface normal is shown in Figure 5. Under conditions close to ours (emission normal to the surface), Prigge et aI obtained a distribution half-width of about 2.7 eV. Nishijima and Propst” and Traum and Woodruff”, collecting the ions in cones with semiangles of 38” (+4” angle aperture) and 42.3” ( f 6” angle aperture), respectively, obtained distribution half-widths of 3 eV. Madey and YatesZ2 and Ashcroft et aZ23gave quite different results: 3.7 and 4.5 eV, respectively. The measurements were made with hemispherical electrostatic analyzers which differed from the semi-cylindrical one used here only in electrode symmetry. It should be pointed out that using this analyzer we also obtained a higher width value (24 eV). Let us discuss the reasons leading to a broader distribution of the ESD ions with this type of energy analyzer. Above all, the regions of ion formation and retardation are not resolved and for this reason the retarding field between the sample and the collector influences the real trajectories of the ions. As a result, additional tangential components of the ion velocities parallel to the grid surfaces appear. They are perpendicular to the retarding field and are not retarded. Additional analysis errors are due to different transmissions of the analyzer grids with respect to the angles, formation of secondary species originating from the grids, and scattering of the ion beam by the space charge near the collector. The hemispherical and semi-cylindrical analyzers with a large acceptance angle retard all ESD desorbing ions. The ESD ions have a real angle distribution’8.‘9 and the ions with tangential velocity components differing from zero lead not only to broadening but also to shifting of the distribution. The deconvolution method applied previouslyz3 is a compromise solution because i.hermal ions which have, in principle, an angle distribution different from that of the ESD ions are used for the determination of the analyzer response. It may be concluded that distributions obtained with largez2.“, and small acceptance angles 9~“,2’should not be compared. In the case of small angles, as well as in our case when a diode-type energy analyzer is used, ions with almost parallel velocities only are analyzed. When the energy analyzers are not calibrated experimentally, errors and considerable differences in the results on the maxima

of the energy distributions by different authors are observed. For instance, the values of the 0,’ peak range from 6 to 9 eV. On the contrary, when the scale is calibrated, the results are close irrespective of the analyzer type. We found the above peak at 8.5 eV which is in good agreement with results previously obtained”*2’~23 reporting this maximum at 8.8 and 8.0 eV, respectively. 5. Conclusions The control measurements made with a view to determining the principal parameters of the apparatus and comparing them with other ESD equipment have shown comparable performances. More attention is paid to experiments and methods used for the correct measurements of ESD ion kinetic energies with the hope that the data would be of interest for other authors. Our results on ESD of oxygen adsorbed on tungsten are in good agreement with the results of other authors which allows the conclusion that ESD is correctly used with our apparatus. This was in fact the purpose of the present work. Acknowledgements The author should like to thank Professor V N Ageev for helpful discussions and, through him, the Ioffe Physicotechnical Institute in Leningrad for some financial support. References

’ Reviews on ESD. (a) M J Drinkwine and D Lichtman, in Progress in Surface Science, ~018, p 239. Pergamon Press, New York (1977). (b) E Baier, J Elec SpectrosC Rel Phenomena, 15, 119 (1979). (c).V N Ageev, Pouerkhnost, 4,l (1982). (d) D Menzel, J Vat Sci Technol, 20,538 (1982). (e) N H Tolk, M M Traum, J C Tully and T E Madey, Desorption Induced by Electronic Transifions. Springer, Berlin (1983). (f) G B Hoflund, Scan EIec Microsc. 4. 1391 (1985). (a) T E Madev and R Stockbauer, in Methods of Experimental Surface-Science, 2, p 37. Academic Press, New York (1985). *V N Ageev, S T Dzhalilov, N I Ionov and D Potekhina, Sov Phys Techn Phys, 21, 596 (1976). 3V N Ageev and S G Kasabov, Pouerkhnost, 12,3 1 (1982). 4 S Kasabov and B Ponov. to be published. ’ S G Kasabov, J Phys E :‘Sci Ins&m, 19,369 (1986). 6P A Redhead, J Vat Sci Tecimol, 7, 182 (1970). ’ E Ya Zandberg and N I Ionov, Surface Ionization. Israel Program for Scientific Translations, Jerusalem (1971). *J A Simson, in Methods of Esperimental Physics, vol 4a, p 124. Academic, New York (1967). 9 S Prigge, H Niehus and E Bauer, Surface Sci, 75,635 (1978). “T E Madey, Surface Sci, 33, 355 (1972). ’ ’ M Nishijima and F M Propst, J Vuc Sci Technol, 7,420 (1970). ‘*B J Hopkins and K R Pender, Surface Sci, 5, 155 (1966). “D A Gorodetskii and Yu P Melnik, Bull Acad Sci USSR, Phys Ser, 35, 1978 (1971). 14V N Ageev S G Kasabov and Ts S Marinova, Proc Fourth Int Conf Solid Surfaces, vol 2, p 1263, Cannes (1980). “V N Ageev and B V Yakshinski, Fiz Tverd Tela 27,99 (1985). 16T D Tandyrev and Yu F Baranov, Pribory i Tekhnika Eksperimenta. 3, 143 (1984). “D Lichtman and R B McQuistan, S/ow Electron Interaction with Adsorbed Gases, Progress in Nuclear Energy, ~014, p 95. Pergamon Press, Oxford (1965). “T E Madey and J T Yates, Jr, Surf Sci, 63,203 (1977). I9H Niehus, Appl Surface Sci, 13,292 (1982). ” D Roy and J Carette, Electron Spectr for Surface Analysis. Springer, Berlin (1977). 2’ M M Traum and D P Woodruff, J Vuc Sci Technol, 17, 1202 (1980). 22T E Madey and J T Yates, Surface Sci, 11, 327 (1968). *‘K W Ashcroft, J H Leek, D R Sandstrom, B P. Stimpson and E M Williams, J Phys E: Sci Instrum, 5, 1106 (1972). 1039