Coadsorption studies by Auger electron spectroscopy: Nitrogen with oxygen and carbon on W {100}

Surface Science 62 (1977) 93-105 © North-Holland Pubhshmg Company

COADSORPTION STUDIES BY AUGER ELECTRON SPECTROSCOPY: NITROGEN WITH OXYGEN AND CARBON ON W {100} Michael HOUSLEY * and Dawd A KING The Donnan Laboratories. The University, P 0 Box 147, Liverpool L69 3BX, England Received 13 July 1976, manuscript received in final form 5 October 1976

Coadsorptlon experiments on polycrystallme tungsten showed that the mmal sticking probabdlty and the total uptake of oxygen are unaffected by the prior adsorption of 6.5 X 1014 atoms cm -2 of mtrogen Desorpt~on spectra revealed that mtrogen ~s not displaced it, the process and that the adsorption energy for mtrogen is relatively unaffected by the presence of coadsorbed oxygen. AES peak-to-peak height lntensRles were absolutely calibrated as a function of mtrogen coverage on W{100 ), and compared with prevaous cahbratlons for C and O. It was found that during oxygen adsorption on a mtrogen-saturated surface both the W and the N Auger signals were attenuated Flash desorptlon again shows that nitrogen is not displaced from W {100), and it is concluded from the degree of attenuation of the N Auger signal that the N adatoms are buried ~3 6 A beneath a pseudomorphlc layer of tungsten oxide. A structural model is proposed which is consistent wRh the results The growth of a carbon layer from electron beam interaction with adsorbed CO was also examined on a N-presaturated W {100} surface, and a similar model proposed to explain the results

1. Introduction Coadsorptlon studies of oxygen on CO-precovered tungsten surfaces revealed that the CO adlayer apparently minblts the oxidation of tungsten beyond the initial rapid uptake of a monolayer [1,2], whereas preadsorbed mtrogen has little effect on the subsequent oxidation [2] process The strongly adsorbed ~-CO state on tungsten was n o t displaced by oxygen at room temperature, but the two ~ peaks found in the desorption spectra for CO from clean W were found to converge into a single peak in the presence of coadsorbed oxygen [1 ]. Tins convergence was attributed to lateral interactions between adatoms in the overlayer [3] By contrast, Yates and Madey [4] reported that coadsorptlon of oxygen caused the two ~ peaks of nitrogen from polycrystalhne W to move apart, the/~2 peak sinftlng towards Ingher temperatures and the/31 towards lower temperatures. The present investigation, winch includes a preliminary flash desorptlon and electron stimulated desorptlon study of coadsorptlon on a polycrystalhne tungsten * Present address CNRS, 45600 Vtllers-Nancy, France. 93

94

M Housley, D A King / Coadsorptton studws by AES on I¢

filament, was undertaken to determine the location o f nitrogen adatoms m the surface region after oxidation, using Auger electron spectroscopy (AES) In the presence o f carbon monoxide, a carbon deposit o f ca 12 × 10 I4 atoms cm -2 can be formed on a W {100] surface [5] The effect o f nitrogen preadsorption on this process and the location o f the nitrogen adatoms m the mixed overlayer thus formed were also investigated

2. Experimental The experiments were performed In an all pyrex glass ultrahigh vacuum system, described elsewhere [5] Auger electron spectra were recorded from a self-constructed cylindrical mirror energy analyser, operated with ~1% resolution Desorptlon spectra were recorded (where appropriate, using mass 30 isotopic mtrogen) on a quadrupole mass spectrometer The prehmlnary experiments on polycrystalhne tungsten were conducted in an apparatus described elsewhere [6] The tungsten single crystal, cut and polished to within 1° o f the (100) plane, was cleaned by heating to 1700 K in oxygen at 10 -6 Torr for several hours, and subsequently flashIng in vacuum to 2500 K This left no trace o f impurity, as monitored by AES

3. Studies on polycrystalline tungsten 3.1 Sttckmg probabihtws, coverage and desorptton spectra Sticking probabthtles and surface coverages were determined by the flash filament uptake technique. For reasons previously discussed [6], the absolute accuracy of this technique is generally rather poor, but relative values are good. In fig 1 we show the uptake curves for oxygen on 0) a clean polycrystalhne tungsten wire, and (II) the same surface after an exposure of 6 × 10 - s Torr sec to nitrogen. The clean surface results have been reported elsewhere [6], and show two stages in the adsorption process as s drops to 10 -2, the second stage being marked by an lnflexion in the uptake curve. The mItlal sticking probability is 0 7 and the uptake when s has fallen to 0 02 is 8 × 1014 molecules cm -2 After presaturatIon with nitrogen, the Initial stickang probability for oxygen and the amount o f oxygen adsorbed are identical to those for the clean surface The only effect o f the nitrogen overlayer IS to remove the reflection in the uptake curve, which occurs at a coverage of 4 X 1014 molecules cm -2 on the clean surface DesorptIon spectra are shown In fig 2 for mass 30 isotopic nitrogen for (1) an itherwise clean tungsten surface, after a nitrogen exposure o f 1.7 × 10 - s Torr sec, (11) the same exposure to nitrogen, and then to oxygen to a coverage of 4 X 1014 molecules cm -2, and 0n) the same exposure to nitrogen followed by oxygen to a coverage o f 8 X 1014 molecules cm -2 Only the /32 nitrogen peak is observed at

M Housley, D A King / Coadsorptzon studtes by AES on W

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M Housley, D A King / Coadsorptton studies by AES on W

these exposures, with a peak temperature o f 1430 K at the scan rate o f 46 K sec -1 After oxygen coadsorptlon, the peak temperature shifts shghtly to lower temperatures, reaching 1400 K with 4 × 1014 oxygen molecules cm -2 and 1350 K with 8 × 1014 oxygen molecules cm -2 This corresponds, roughly, to a fall in adsorption heat o f about 25 kJ mo1-1, or less than 10% Thus we note the very surprising result that more than a monolayer of oxygen atoms adsorbed onto a tungsten surface covered with nitrogen has very httle effect on the nitrogen desorptlon spectra, and hence, by reference, on the N adatom binding energy Since oxygen is only desorbed at temperatures above 1600 K [6] (as oxides and as oxygen atoms), coadsorbed oxygen is still present on the surface durmg nitrogen desorptlon Further, nitrogen preadsorptlon has very httle effect on the subsequent rate of oxygen uptake Smce there can be slgmficantly more than a monolayer present on the surface ( ~ 6 X 1014N atoms cm -2 and ~ 1 6 × 1 0 1 4 0 atoms cm-2), we conclude that the surface layer must be reconstructed The problem tackled m this work is the location o f the nitrogen atoms in this surface layer 3.2 Electron snmulated desorpnon (ESD)

The ESD cross secuon for nitrogen on tungsten is very low, with oxygen on tungsten the cross section is again low for most of the adsorbate, but a high cross section species, designated 31, has been observed [7] The latter is formed during adsorption at coverages above ~ 4 X 1014 molecules cm -2 and constitutes between 5 and 10% of the total adlayer. It has been described as a non-dlssoclatlvely adsorbed species, though it does not desorb as such The appearance during adsorption and the disappearance of the 31 state during heating appears to be associated with the appearance and desorptlon of a set of oxide states observed by desorptlon spectra [8].

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M Housley, D A Kmg / Coadsorptton studtes by AES on W

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The growth of an ESD O ÷ 1on current durmg oxygen adsorption on a clean, polycrystalhne tungsten wire is compared m fig 3 with the growth during oxygen adsorption onto the same surface after exposure to nitrogen for 1.7 × 10 -s Torr sec The conditions correspond exactly to those for the uptake curves m fig. 1. We note that after mtrogen adsorption the ESD-actwe 131 state begins to be populated at lower oxygen exposures, and the final O ÷ current is higher. This lmphes either a higher/31 populatmn,.or a higher ESD cross sectmn The earher appearance of the /31-oxygen state was also observed after CO preadsorptlon [ 1]

4. Calibration of Auger intensities 4 1 Intensity against coverage mtrogen on tungsten

In the first part of ttus work [5], calibrations of Auger peak-to-peak derivative intensities were reported for C and O chemlsorbed on W(100} The peak-to-peak intensities, sff, were normahsed with respect to the peak-to-peak height, sff (W), of the tungsten substrate peak at 350 eV. Here we extend the results to mtrogen, coverages being determined independently by (a) the flash uptake technique, using a W ribbon, and (b) integration of desorptlon spectra, using a W ribbon and a (100)-oriented W crystal The peak-to-peak helght ratio, s~ (N)/s~(W), is plotted against surface coverages determined by these methods in fig 4. The coverage at saturation (6.5 X 1014 atoms cm -2) on the W ribbon correlates closely with the results of King and Wells [9], using a more accurate molecular beam techmque A good straight hne is obtamed, and from the slope we have

~(N)

NN = 2 95 X 1014 X ~

atoms cm -2

(1)

4 2. Comparison ofcahbrations for C, N and 0

The Auger peak intensities for C, N and O should be correlated to the K-level ionization cross sechon, QK, for each element As the cahbratlons have been performed for data In the derivative mode, any comparison of peak heights between elements must depend on the asumphon that the halfwldths of the Auger peaks for the different elements are the same We can test this assumption by writing the general expression

1

_~(X)

N x =B QK,~ ,~(W) '

(2)

where Nx is the coverage of element X at the surface, Qr~x is the K shell lomzatlon cross sechon for X, and B IS the proportmnahty constant. In the cyhndrlcal m~rror

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] _~(x) 3 s o N x = B QK,~ -~ (W) E x

(3)

Ionlsatlon cross secuons obtained from the calculations of Gerlach and Ducharme [10] based on a model proposed by Burhop [11 ] are given m table 1, together w i t h . the coverages and peak intensity ratios from the present work, for C, N and O, and In the final column, values for B from eq (3) The three values for B are only m fair agreement The difference between N on the one hand and C and O on the other may arise either from hneshape differences, or from errors m the calculated cross sections Clearly, the general expression (3) cannot be used where accuracy better than 50% is required, separate cahbratlon of each element to be studied is necessary Table 1 Element

Surfacecoverage, N X (atoms cm -2)

QK,X ( c m 2 )

C N O

4 3X 1014 65 X 1014 4 3 X 1014

3 X 10 -19 1 5 × 10-19 0 7 × 10-19

~(X)/~ (W) 295 22 145

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B

272 381 511

34 X 10 -5 4 8 2 × 10-5 303X 10 -5

M Housley, D A Kmg / Coadsorptton studws by AES on I¢

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5. Oxygen adsorption on nitrogen-precovered W {100} Auger derivative spectra were continuously recorded during the adsorption of oxygen onto a W{100} surface prevaously saturated with nitrogen (exposure 3 × 10 -4 Torr sec), checking the constancy of the primary beam current at frequent intervals by flipping the beam into a Faraday cup (see ref [5]). Peak-to-peak heights for oxygen, nitrogen and the tungsten 350 eV transition are plotted as a function of oxygen exposure in fig 5. Both the tungsten and the mtrogen peaks are attenuated during oxygen adsorption. The oxygen signal increased asymptotically to a value corresponding to a coverage of 10 × 1014 atoms cm -2, determined from the calibration discussed above w~thout correcuons for Incorporation, and the lnlUal slope gives a StlCkang probabdlty of 0 5 The more accurate molecular beam technique gives an initial sticking probabdlty of " 0 9 and an uptake of 12 × 1014 atoms cm -2 for oxygen onto clean W{100} [12,13]. Here, the estimated oxygen uptake Is a lower hnut, since incorporation of oxygen below the surface layer would gwe rise to inaccuracies m the use of cahbratlon constants due to the finite escape depth of the Auger electrons, as discussed later Thus, as with polycrystalhne tungsten, the rate and amount of oxygen uptake on W{100} is not appreciably affected by the presence of 5.5 × 1014 atoms cm 2 of preadsorbed nitrogen. Attenuation of the mtrogen signal may be due to either displacement of mtrogen from the surface or incorporation of the mtrogen adatoms below a surface oxide film To distinguish between these possibilities, desorptlon spectra were obtamed (fig 6), again using mass 30 isotopic mtrogen to avoid confusion with CO, for 0) nitrogen adsorbed to saturation, and (u) after subsequent exposure to oxygen Within the accuracy of the technique, in this case no change was observed in either

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the peak temperature or the mtrogen coverage Attenuatton o f the nttrogen Auger stgnal ts thus due to burial o f the N adatoms below the surface during oxygen adsorptton The depth to which they are buried can be estimated by the following procedure. The ratio of the Auger signal where the nitrogen is in the uppermost layer, a~ (N)0, to the signal with a layer of oxide d A thick over the nitrogen adlayer, ~(N)a, is given by s~ (N)o/~(N)a = exp(d/h sm a ) ,

(4)

where ~. is the escape depth of the 381 eV Auger electrons through the tungsten oxide film, and sm 0 is the correction to k for non-normal emission From fig 5, the Auger signal ratio o f ~ (N)0/s~(N)a , after an oxygen exposure of 10 -4 Torr sec, ts 1 8, with ~. = 9 A, eq. (4) gpves an oxide layer thickness d of 3 6 A Of course, the major source of error here hes m the value assumed for the escape depth ;k Thas was taken from the "universal" escape depth curve [14] from wtuch the scatter indicates an unceramty of about 50%. A structural model is proposed for the mixed overlayer based on data in the literature for the mteractlon of oxygen with tungsten, which is consistent with all the data presented here. lOng et al. [6] reported desorptton spectra for oxygen on a polycrystalhne tungsten wire which showed that the amount of oxygen adsorbed at room temperature reached a plateau at exposures between 10 --4 and 3 X 10 -3 Torr sec At these exposures the desorptlon spectra are composed of oxide states deslg-

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M Housley, D A Kmg / Coadsorptton studtes by AES on W

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/

Fig 7 Model for the mixed overalyer formed by adsorption of nitrogen on W (100) followed by by exposure to oxygen Large ctrcles, W atoms, small open ctrcles, oxygen atoms, small filled ctrcles, nitrogen atoms nated as WO3(1700) and WO2(1700), being fragmentation products of an incipient oxide layer, and oxygen atoms desorblng at 2100 K The oxide film is considered to possess the WO3 structure, the most stable oxide of tungsten at 300 K, a description o f its structure is gwen by Avery [15] The lattice resembles a cubic subshell with lattice constant 3.65 + 0 2 A, with the oxygen atoms occupying sites at the midpoints o f the 12 edges of the cube and a tungsten atom roughly in the centre. Fig 7 shows the proposed model, which is consistent with the oxygen and nitrogen coverages and with the AES escape depth data, as chscussed below. The nitrogen atoms are shown poslUoned in the fivefold co-ordinate, fourfold s y m m e t n c sites which they are believed to occupy on the clean surface [16], except that they are now buried beneath a pseudomorptuc layer of WO3, slightly distorted due to the difference in lattice constants between W ~100) and the oxide Consistency with the AES data is demonstrated as follows 0) In ttus model, the N atoms are buried to a depth o f 3.7 A, clearly in close agreement with the above escape depth calculation. (n) The tungsten signal should be less than for a clean W ( 1 0 0 ) surface, since although there are 10 is atoms cm -2 in the top layer, there are none in the second layer A layer-by-layer summation calculation based on eq (4), with an escape depth ~ = 8 A, for the W signal from the structure in fig 7 compared with clean W ( 1 0 0 ) gives a calculated attenuation of 25%, which is the experimentally observed attenuation o f the 350 eV W peak (fig 5) (in) The total oxygen coverage for the structure in fig. 7 is 15 × 1014 atoms cm -2, consisting of 5 × 1014 atoms cm -2 in the surface layer and 10 × 1014 atoms cm -2 in the second layer at a depth o f 1 85 A. Again using the layer-by-layer method based on eq. (4), and with

102

M Housley, D A Kmg / Coadsorptton studzes by A E S on I¢

3, = 10 A, the oxygen to tungsten signal ratio .¢/ (O)/s~ (W) should be 4 35 The experimental value (fig 5) is 3 1 Although this'may be over-mterpretlng the model given the uncertainty in ~,, the discrepancy can be attributed to a deficiency of O atoms in the uppermost layer With 2 × 1014 atoms cm -2 m this layer, the predicted ratio -~ (O)/M(W) Is 3 35, and the total oxygen coverage would then be m agreement with the hterature [12,13], 1 e 12 × 1014 atom cm -2 This oxygen deficiency m the structure would not affect the above calculauons for nitrogen and tungsten Auger signals Since the desorptlon temperatures of both the oxide film (1700 K)and oxygen atoms (2100 K) are higher than for nitrogen, it was anticipated that nitrogen might reappear m the surface layer prior to desorptlon Heating the W(100) crystal m small steps, however, did not bnng about a change in the mtrogen Auger signal, until a 1000 K desorptlon occurred and the N signal began to decrease The slmllarlty between nitrogen desorptlon spectra with and without oxygen for both the polycrystalhne filament and the W(100} crystal indicate that the adatom bmdmg energy is effectively unaltered by the existence of the oxide film The proposed structure (fig 7) does give the N adatom the same fivefold coordination to substrate atoms as on the clean surface, a simple fact which may explain the observed lnvarlance of the binding energy

6. Beam-assisted carbon accumulation on a nitrogen adlayer In the prewous paper [5] we have made a quantltatwe analysis of the electronbeam-assxsted formation of a carbon deposit on W{100 } in the presence of carbon monoxide. It was concluded that the beam interacts with the a2-CO state to eventually produce a carbon film at the surface contamlng approximately 12 × 1014 carbon atoms cm -2 Our purpose here Is to examine the effect of preadsorbed nitrogen on the carbon accumulation process, and also to attempt to locate the N adatoms in the mixed overlayer so formed Ragby [17] reported a reduction in the total carbon monoxide coverage of ~30% when mtrogen was preadsorbed on a polycrystalhne filament The mtrogen desorptlon spectrum was broadened, but nitrogen was not displaced Desorptlon spectra obtained from the {100} crystal in the present work are m agreement with Ragby's results In particular, only a shght reduction m the c~-state population was observed when mtrogen was preadsorbed, I e. the ~ state alone is blocked by the N adatoms No displacement of mtrogen was observed Nitrogen was adsorbed onto the clean W{100} crystal to saturation The gas was then pumped out, the Auger electron beam switched on, and carbon monoxide was introduced to the cell while recording peak-to-peak amphtudes for C, O, N and the 350 eV W transmon. The results are shown m fig 8, and can be compared with fig 9, which is a similar experiment carried out without preadsorptlon of nitrogen. Dunng the first four minutes of adsorption the C and O peaks grow in the raho

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2 1, corresponding to the sto]ctuometnc uptake of CO. After ~ 5 mln, however, the O signal passes through a m a m m u m and begins to decrease The C signal contlnues to increase monotomcally, approaclung an asymptonc value after " 5 0 mm correspondmg to a coverage of 1.6 × 101 s C atoms cm -2. We note that a major difference between the results in figs. 8 and 9 ]s the maximum value of the O s]gnal, it has a tugher value where mtrogen was not preadsorbed and occurs at a lower exposure This is m accordance with the desorptlon spectra, the ratio of the H-state population with and without preadsorbed mtrogen is 1.6, whale the raUo of the mamma m the O Auger signal Is 1.3. The reducnon m s~ (O) is thus attributable to the reduced

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104

M Housley, D A Kmg / Coadsorpnon studies by AES on I¢

population of the ~ state However, the a m o u n t s of oxygen adsorbed (determined from ~f (O) and the cahbratlon presented earher) at the maxima are 2 67 × 1014 cm -2 without preadsorbed nitrogen and 2 2 × 1014 atoms cm -2 with mtrogen preadsorbed, both significantly below the/3 state population of 4 3 × 1014 molecules cm -2. Since C accumulation results from beam interaction with the a2-CO state, it is concluded that the 12 state begins to populate before completion of the/3 state A comparison of figs 8 and 9 show that the rate of carbon deposition is independent of the presence of nitrogen m the adlayer, thas is consistent with the postulate [5] that the rate is proportional to the 12 population and the observed insensitivity of the latter to preadsorbed nitrogen The amount of carbon formed at the asymptote xs significantly tugher in these experiments than in the work described in the previous paper [5] In that work the/3-CO state was allowed to saturate before turnlng on the electron gun, the additional chermsorbed O atoms present thus possibly inhibiting the growth of the carbon film. This may also explain higher carbon coverages achieved by Chesters et al [19] The nitrogen Auger signal is eventually reduced by ~20% as the carbon deposit IS formed, and, as with coadsorbed oxygen, this is attributed to burial of the nitrogen adatoms The escape depth calculation based on eq. (3), and on the assumption that the N layer is uniformly covered by a ttun film, yields a depth d = 2.05 A Tins may either be a graphmc film (for which the thickness would be approximated by the covalent diameter of the carbon atom, ~1 5 A, in perhaps fortuitous agreement with the above value) or a tungsten carbide layer In contrast to oxygen, incorporation of the small C atom into the tungsten lattice leads to only a small expansion of the lattice, the density of W atoms in WC being only 17% less than that in tungsten Uslng eq. (4), attenuation of the W Auger peak was calculated layer-by-layer, and the summation compared with a calculation for clean tungsten, tins yielded a decrease of ~5%, which is easily accommodated wlttun the uncertamty m measured values for s~ (W). A graphltlC film overlayer would however, decrease the W signal by ~20%, which is well beyond the uncertainty in the ~ (W) measurement reported in fig. 8 and 9. The AES results for C and N are thus consistent with the type of structure proposed In section 5 for O and N, In winch a tungsten carbide layer resides over the N adatoms which may again be assumed to remain in the five-fold co-ordinate site The most puzzling feature of the results is the initial lnvarlance of the N Auger sagnal during carbon deposition. Apparently, dunng the initial stages of tins process the N atoms remain in the surface layer and only begin to be submerged at a critical C atom coverage' This may point to a cooperative, structure sensitive reconstruction of the adlayer, but the mechanism is far from clear.

7. General comment

We have not examined here the process by which an oxide or carbide layer is formed above the ongmal mtrogen overalyer It is possible, however, that a M o t t -

M Housley, D A King / Coadsorptzon studws by A E S on W

105

Cabrera mechamsm [18] is apphcable, with &ffuslon of tungsten Ions occurring across the original interface, facihtated by the electric field estabhshed between the metal/overlayer interface and adsorbed oxygen at the vacuum interface as a result of the transport of electrons to the adsorbed oxygen The ESD results reported for polycrystalllne W (section 3.2) show that the/31 state builds in at the onset of oxygen adsorption when mtrogen is preadsorbed at the surface, the normal induction period for its formation,not being observed Smce the fll-O2 state is in some way associated with oxide formation at the surface [8] it would appear that mtrogen preadsorption precludes the necessity for the formation of a chemlsorbed oxygen layer to precede oxide film growth This is borne out by the AES results, since, as shown in fig. 5, the nitrogen Auger signal is attenuated as soon as oxygen is introduced to the system. Interestingly, since it would appear that the preadsorbed nitrogen is held in the interface between the oxide film and the bulk metal, attenuation of the nitrogen Auger signal prowdes a means of determining the oxide film thickness during ox~datlon, and hence a simple alternative to elhpsometry for followmg oxidation kanetlcs for relatively thin films

Acknowledgements The authors thank G. Plquard for prepanng the diagrams for pubhcatlon. The award of an SRC studentshlp to Michael Housley is gratefully acknowledged.

References [1] C G Goymour and D.A King, Surface Scl 35 (1973) 246 [2] V N Ageev and N I lonov, Flz Tverd Tela 12 (1970) 1573 [3] C G Goymour and D A King, J Chem. Soc Faraday 1, 69 (1973) 736, 749 [4] J T YatesandT E T.E Madey, J Chem Phys 45 (1966) 1623 [51 M Housley and D A King, Surface Scl 61 (1976) 000 [6] D A King, T E Madey and J T Yates, J Chem Phys 55 (1971) 3236 [7] T E Madey and J T Yates, J Vacuum Scl Technol 8 (1971) 525 [8] D A King, T E. Madey and 1T Yates, Surface Scl 68 (1972) 1347 [9] D A King and M G Wells, Surface Scl 29 (1972) 454 [10] R.L Gerlach and A R Ducharme, Surface Scl 32 (1972) 329 [11] E H S Burhop, Proc Cambridge Phil. Soc 36 (1940) 43 [12] T E Madey, Surface Scl 3 (1972) 355 [13] M G Wellsand D A King, J Phys C (Sohd State Phys ) 7 (1954) 4053 [14] For example C.R Brundle, J Vacuum Scl Technol 11 (1974)212 [15] N R Avery, Surface Scl 33 (1972) 107 [16] D L Adams and L H Germer, Surface Scl 27 (1971) 21 [17] LJ Rlgby, Can J Phys 42 (1964) 1256 [18] N CabreraandN F Mott, Rept Progr Phys 12 (1949)163 [191 M A Chesters, B J Hopkins, A R Jones and R Nathan, J Phys C (Sohd State Phys ) 7 (1974) 4486