Ion beam analysis of niobium-vanadium alloys

Nuclear Instruments and Methods 191 (1981) 275-281 North-Holland Pubhshlng Company

275

ION BEAM ANALYSIS OF NIOBIUM-VANADIUM ALLOYS P J. MARTIN *, C M LOXTON, R F GARRETT, R J MACDONALD Department of Physics, Faculty of Scwnce, Austrahan Nattonal Umvers~ty, Canberra, Austraha and W O HOFER Kern]orschungsanlage, Juhch, Germany

A series of nlOblum-vanadmm alloys were investigated using the ion bombardment analytical technNues of ion scattering spectrometry, sputter reduced photon spectroscopy and secondary ion mass-spectrometry The measurements show that long term transmnts in signal strengths were present in all three types of analyses and that extreme care must be exercised in obtaining reproducible data The effects of oxygen leakage on both photon and secondary ion analysis were also investigated and showed that the analysis was dependent upon the background oxygen pressure at whmh analysis was performed

1 Introduction Many techniques are available today for quantitative and semi-quantitative surface analysis of materials and there exist many reviews in the literature detalhng the relative merits of each approach In general they may be sub-grouped according to the nature of the primary probing beam, 1 e electron, X-ray or ion and consequently have varying degrees of depth and spatial resolution, element sensitivity and are Intrinsically destructive or non-destructive Ion beams are widely used in conjunction with other probes to facihtate sample cleaning and to provide a depth profiling capability The effects of ion beams on multi-element samples has created much interest since the phenomena of preferential sputtering, recoil implantation and atomic mixing have substantial influence over the measured element concentrations [ 1 - 3 ] However little work has been published on the capabilities of ion-based methods applied to well defined systems such as binary alloys and the problems encountered in ion beana analysis As a comparative study of the capabilities of ion beam techniques we have analysed a series of N b - V alloys using ion scattering spectrometry (ISS), sputter induced photon spectroscopy (SIPS) and secondary ion mass spectrometry (SIMS) The N b - V system is a convenient system to study since atomic mass separation IS sufficient to avoid mass overlap in SIMS * Present address CSIRO, Division of Applied Science, Sydney, Austraha 2070 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 75 © 1981 North-Holland

and there are strong spectral emission lines sufficiently separated in wavelength to avoid spectral overlap m SIPS Furthermore the sputtering yields for He*, Ne + and Ar ÷ bombardment of the pure elements are very slmtlar [4,5] so that major preferential sputtering effects are not anticipated in the measurements

2 Experimental The targets used were metal discs polished to a mirror finish and characterlsed for bulk concentrations using electron probe mlcroanalysls (EPMA) to +0 5% (table 1)

3. Ion scattering measurements (ISS) The theoretical basis of ISS is well described in the literature [6,7] obviating the need for a detailed discussion here and we confine ourselves to the method of calculating surface concentrations only The normahsed intensity of an ISS peak can be written as I A/Io = Q)ANoO A(E)~IR ,

(1)

where I A is the count rate for the peak due to scatterlng of the Incident beam Io by atomic species A, occupying a fraction 0 A of surface sites, OA(a3 IS the cross section for elastic scattering of the incident ion through angle 0 by element A at energy E, r/the neuVl SURFACE SCIENCE

276

P J Ma;tm et al / lon beam anah,sls

trahsatlon probability, No number density of atoms and R a surface roughness and shadowing factor For a two element system we can write [8] Oa=

SA/BIA SA/BI A + I B '

(2)

where SA[ B lS a correction factor for the relative sen-

sitivlty of the detector to elements A and B This t'actor can be expressed in terms of signals f from pure reference samples as IA'iao(B) ~2

SA/B

=IBB'\~!

'

(3)

where ao is the lattice constant Using pure V and Nb standards to find SA/B at each probe energy enables the surface coverage to be calculated independent of N0, R and r/ assuming that the surface roughness and neutralisatlon probability are the same for Nb and V Ion scattering experiments wele pertormed Ul a Leybold Heraeus system using a 3M mimbeam Ion gun to produce a probe beam with a 300/~m spot on the target surface and currents of around 0 2 #A of He* and Ne ÷ No attempt was made to mass analyse the probing beam but only research grade gases were used and the source was operated under conditions which did not produce multiply clnarged ions The system was turbomoleculal pumped and aided by titanium subhmation pumping to produce a background pressure of less than 1 × 10 l0 Torr rising to 1 × 10 -6 Torr of inert gas with the 3M gun in operatlon The targets were mounted in batches ot 3 and could be individually heated by electron bombardinent to about 800°C Additional target cleaning was achieved using a rastered 3 keV AI + beam from a Colutron ion gun The electrostatic analyser was operated In the AE constant mode (constant energy resolution) with a pass energy of 200 eV such that greater detection senSltlvlty was achieved in the low energy region lot monitoring secondary ~on y~elds and hence contam~nats than IS possible in the more conventional A E / E constant mode A scattering angle of 135 ° was employed giving adequate resolution of the Nb and V peaks when He* was used as a probe

4 Results of ISS analysis When the alloys were annealed to mound 800°C for 30 mm and subsequently bolnbarded with the

probe beam it was found that marked changes occuired in the peak heights of both V and Nb wltla time indicating the existence of a V rich layer on the surtace From esnmates based on sputtering ylekts foi the pure elements and current densllmS we calculate the thickness of this layel to be as nmch as 2 3 monolayers in some runs Continued bombardment led to an apparent stablhsatlon of the Nb to V peak raiio alter approximately 30 mm although the peak heights continually decreased up to 1 h aftel commencement of irradiation This effect was found for all the investigated samples and the pine elements also indicated a peak decay of up to 30% Oxygen was detected on the surtace at all times and could not be completely removed through heating and sputter cleaning The origin of the oxygen contaminant is most likely diffusion of oxygen from the bulk to the surface Nb IS known to be exceptionally difficult to clean and oxygen free surfaces can omy be produced in vacua better than I0 -~° Tolr and tempelatures above 1700 K [9] Our sample surfaces then contamed some constant oxygen impurity and measulements were only taken when stable signals were obtained The decay of V and Nb peak signals was invest> gated further by monitoring the integrated secondary ion signal ovm 0 100 eV and the oxygen peak with irradiation time afte) anneahng A composite plot of all signals is shown in fig 1 We see no transient behawour in the oxygen signal but an initial decrease m the seconda U ion level In the first 10 rain These observations indicate that the decay effect IS not merely due to a sub monolayer of oxygen present on the sulface aftel anneahng and its subsequent removal by sputtenng Further, the secondary ion behavlour IS conslstenl wlth a surface changing trom V rich to N b - V when decreases in secondary ion yields are expected [10] The long term nanslents are then posslbly effects due to sm face ~oughness developing with tnne or neutrallsatlon processes associated w~th unplanted primary pmtlcles m the smface layers Fig 2 shows a plot ofmeasmed vs bulk percentage Nb t\~ samples 4, 5, 6 7 and 8 obtained using 0 5 , 10, 15 and 2 keV He + a n d ( 0 5 , 1 0 , 2 0 , 3 0 keV) Ne + probe beanls Agreement with bulk values is bettel for Ne + than He + and is within +5(/~ The He + data is consistently hlghel by as nmch as 10% for most samples Thls result is dltflcult to leconcfle with a sunple sputtering model based upon the sputtenng yields of

P J Martin et al /Ion beam analysts

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t lg 1 The changes observed in the ISS peaks of Nb, V and O and the sum of the secondary 1on yield from 0-100 eV with time after annealing

the pure elements winch are essentially the same, for 1 keV He + and Ne + bombardment [4,5], such that preferential sputtering In the alloy IS not anticipated Such an approach is generally successful for binary alloys [11] but may be complicated by differences in surface binding energy between the bulk elements and the alloys A possible reason for the discrepancy between the He + and Ne + data may be a variation in the neutrahsatlon probability between the bulk ele-

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There have been many attempts to use sputter Induced photon spectroscopy for analytical purposes [ 1 2 - 1 6 ] mostly in quantitative analysis of doped glasses and insulators where the band gap has been assumed to exclude non-radiative transitions allowlng all excited states to decay radlatlvely Merlaux [12] has reported the SIPS analysas of Cu/N1 alloys although the conditions for the study were probably insufficient to maintain surface cleanhness SIPS analyses were performed under ultra high vacuum and also under background oxygen conditions (since oxygen is commonly employed to enhance signal sensitivity) to investigate the accuracy obtainable and the influence of surface cleanliness The experimental arrangement has been described elsewhere [17] and is only mentioned briefly here Basically it involves the bombardment by 55 keV Ar ÷ of a series of the Nb V alloys and pure element standards at normal incidence Photon emission from sputtered pamcles was observed parallel to the target surface with a scanning spectrometer of resolution 1 A Care was taken to allow the signals to stablhse before analysis proceeded as long term transients were found during initial cleaning and after the targets were heated The 4059 A Nb I and 4111 A V I emission lines were chosen as standards fo~ analysis since both are intense and free of spectral overlap UHV analysis was performed with the total background vacuum less than 10 -9 Torr The background oxygen conditions were set for an oxygen pressure of 7 × 10 -6 Torr with a beam current density of 0 5 mA/cm 2 which corresponds to the peak in the intensity increase with oxygen exposure for alloy no 8 [17]

6 Results of SIPS analyses

I 20

40

BULKNb

60

CONC

80

100

(7o)

Fig 2 Plot of the measured concentration against the EPMA determined concentration for tte+ and Ne+ ISS data (+2%) Key (a) He+ (keV) 0 5 e, 1.0 c~, 1 5 A, 2 0 v, (b) Ne+ (keV) 05-,10v, 20-,30 A

The results of the analysis performed under UHV conditions for the V and Nb concentrations are shown In table 1 In each case, the mtenslty of the hne was assumed proportional to the atomic concentrauon and this concentration determined by compaVI SURFACE SCIENCE

278

P J Martm et al / lon beam analysts

Table 1 Analysis of Nb/V alloys using the NbI 4059 A and the V 4111 A hne under UHV condltmns Target

No No No No No No

9 8 7 6 5 4

Nb concentratlon using Nb I 4059 A %

V concentratmn 4111 A

V + Nb %

EPMA Nb concerttratlon %

98 -+ 2 81 _+ 2 55_+4 31 ± 4 17 4- 4 8_+3

2_+ 3 16_+ 3 44-+4 70 _+3 82 -+ 2 91_+2

100 97 99 101 99 99

98 80 455 24 13 65

n s o n o f the line i n t e n s i t y f r o m t h e alloy to the standard As s h o w n in table 1 the Nb c o n c e n t r a t i o n appears high a l t h o u g h similar to the E P M A values ( e x c e p t for alloy n o 7) The results a p p e a r Internally c o n s i s t e n t with t h e sum o f t h e c o n c e n t r a t m n s b e i n g close to 100% To u n d e r s t a n d the effects o f o x y g e n c o n t a m i n a t i o n o n t h e analysis results t h e b e h a v l o u r o f t h e line intensities f r o m the alloys w i t h o x y g e n c o n t a m i n a t i o n m u s t b e considered T h e Nb I 4 0 5 9 A i n t e n s i t y changes w i t h o x y g e n e x p o s u r e for the pure Nb and the n o 8, n o 7 a n d n o 5 alloys are s h o w n in fig 3 A similar curve m a y be o b t a i n e d w i t h the V 4111 A line T h e c o n d i t i o n u n d e r w h i c h the o x y g e n b a c k g r o u n d analysis was p e r f o r m e d as i n d i c a t e d As the i n t e n s i t y changes are n o t c o n s i s t e n t for t h e different

Nbl

405

Target

Uncorrected Nb concentration %

V concentratlon %,

No No No No No No

98-+2 64_+3 20 _+2 12_+ 2 3-+ 1 5 2-+ 1

11_+3 69_+3 79_+ 2 102-+ 2 63_+ 3 68-+ 3

9 8 7 6 5 4

Correctcd Nb concentratlon %

V concentratmn 5"/~

71_+12 43 _+ 15

22_+ 6 50_+ 10

18_+ 15

100_+ 15

metal alloys, analysis m u s t also include a f a c t o r derived f r o m fig 4 w h i c h takes into a c c o u n t the difference in the i n t e n s i t y increases for the o x y g e n e x p o s u r e used These factors have b e e n d e t e r m i n e d for the V 4111 A and Nb 4 0 5 9 A h n e s for t h e n o 5, no 7 and n o 8 alloys w i t h oxygen e x p o s u r e [17] T h e corrected and u n c o r r e c t e d analyses are s h o w n In table 2 As e x p e c t e d , t h e u n c o r r e c t e d analysis shows p o o r a g r e e m e n t w i t h the E P M A results w i t h the s u m o f the t w o c o m p o n e n t s b e i n g in the range 66 133% The c o r r e c t i o n t e r m , a l t h o u g h leading to a relatively h~gh u n c e r t a i n t y In the c o n c e n t r a U o n , p r o d u c e s values w h i c h are m o r e c o n s i s t e n t with those o b t a i n e d u n d e r UHV condmons

7 Secondary ion mass spectrometry (SIMS) measurements

9 nm

S e c o n d a r y ion mass s p e c t r o m e t r y is a well estabh s h e d t e c h n i q u e , and is used r o u t i n e l y for surface, b u l k and d e p t h analysis [10,18,191 T h e ion ymld o f a specms A w d h surface c o n c e n t l a t l o n ~A lS given b y

Nb

#8 #7 - - m

Table 2 Analysis of Nb/V alloys usmg the Nb 4059 A and V 4111 A hnes under oxygen contamination conditions at the intensity peak for the No 8 alloy

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I'18

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- 5

Fig 3 Intensity changes for the Nbl 4059 A line ,~lth oxygen exposure for the bombarded Nb and no 8, no 7 and no 5 alloys (pressure in Tort and the current density in uA/ mm 2) The conditions at which the SIPS analysis were performed with oxygen exposure is shown

(4)

w h e r e lp is the p r i m a r y b e a m c u r r e n t , S A is the sputtering yield o f A, 3A IS ItS l o m z a t l o n c o e f f i c m n t and ~I(A) is an i n s t r u m e n t f u n c t i o n In t h e present analysts ot b i n a r y alloys w i t h pure s t a n d a r d s o f b o t h elem e n t s avadable, the usual p r o b l e m o f d e t e r m i n i n g the i o n i z a t i o n coefficient can be m r c u m v e n t e d A s s u m i n g n o v a r i a t i o n o f the s p u t t e r i n g yield S or the lomzat l o n coefficient/3 w i t h m a t r i x , 1 e w i t h N b / V c o n c e n -

279

P J Martm et a l / Ion beam analysts

t r a n o n (this point is discussed later), then the ratio of the ion yield of element A from the alloy in question to that from the pure standard gives ~bA,the surface concentration of A In the alloy If no chemical enhancement is present and as preferential sputtering effect are not expected, then the surface concentration equals the bulk concentration of the alloy The secondary Ion spectrometer has been described previously [20] The spherical electrostatic analyser was operated in the AE constant mode, with AE = 3 eV Mass analysis was performed by a quadrupole mass analyser Only ions from the central region of the bombarded spot were accepted by the spectrometer The target chamber was od diffusion pumped to a pressure of 1 × 10 -8 Torr The primary ion beam was 53 keV Ar ÷, at a current density of about 60/IA/cm 2 The alloys were bombarded at 45 °, and the secondary 1on spectrometer axis was along the target surface normal The n i o b i u m - v a n a d i u m alloys nos 4, 6, 7, 8 and 9 and the pure Nb and V samples were analysed under the residual vacuum and oxygen pressure of 5 × 10 -6 Torr, which was sufficient to saturate the 1on yields of both elements Several targets, notably the vanadium rich alloys no 4, no 6 and pure vanadium, required excessive bombardment times of >5 h to stabilize the ion yields

8 SIMS results and discussion The five alloys exhibited rich mass spectra, with clusters of the form NbmVn [(0 ~< m), (n ~< 6)] being detected [21 ] The V* and Nb ÷ 1on yields from each alloy and the standards were integrated over the range 0 - 2 0 0 eV The analysis results are shown in table 3 labeled "total yields". The results are internally consistent,

with the Nb and V concentrations summing to 100 -+ 3% However, they are consistently vanadium enriched over the EPMA concentrations of table 1 Fig 4 shows the vanadium concentration to be enhanced approximately hnearly with niobium concentration, wath noblum showing the reverse trend The concentration ratio plotted in fig 4 is identical to the relative ionization coefficient used by many workers to quantify matrix effects [19,22,23] Fig 4 is very simtlar to plots produced by Plvin et al [23] for matrix effects in FEN1, FeCr and N1Cr alloys However, such a matrix effect should also be apparent m the SIPS results (table 2) which show good agreement with the EPMA concentrations We believe the vanadium enhancement to be due to oxygen contamination As mentioned in section 4, oxygen is always present in association with niobium which results In an increase in oxygen contamination, and hence m the Nb ÷ and V ÷ yields, with niobium concentration Because the V + yields are compared with the yield from the pure vanadium target which is not enhanced by flus means, the vanadium concentrations are enhanced as shown in fig 4 The Nb ÷ yields are compared with pure niobium which is most contammated, resulting in reduced niobium concentrations Oxygen contamination usually preferentially enhances the low energy portion of the secondary ton yield [24], and indeed the low energy peak in both the V+ and Nb ÷ spectra becomes more dominant with Increasing niobium concentration The average energy of the V ÷ yield for example decreases from about 45 eV for pure vanadium ro 20 eV for alloy no 9 With regard to this effect, we have also analysed the alloys by integrating the ion yields in the ranges 0 - 5 0 eV and 5 0 - 1 0 0 eV The results appear in table 3 The results from the 0 - 5 0 eV window, which encompasses the dominating low energy peak in the spectra,

Table 3 SIMS analysis results Errors are statlsncal counting errors plus 5% pnmary 1on beam variation Alloy

No No No No No

9 8 7 6 4

Cleantotal yield

Clean 0-50 eV

Nb%

Nb%

93 -+6 68 -+5 33 -+ 3 153-+ 1 5 36-+05

V%

45-+06 29 -+ 3 69 -+5 85 -+ 7 96 -+7

98 -+7 74 -+ 5 29 -+ 3 150_+ 15 33-+05

Clean 50-100 eV V%

34_+05 28 -+ 7 69 -+ 5 88 -+ 7 102 -+7

Nb%

95 -+8 87 -+ 8 57 -+ 5 19 -+ 3 61_+1

Oxygen saturated total yield

V%

37-+1 24 -+ 3 64 -+ 6 85 _+7 91 -+7

Nb%

V%

94 -+5 50 -+ 3 1 7 6 -+ 1 94-+05 21-+02

107_+07 53 ± 3 75 -+4 102 -+5 98 -+5

VI SURFACE SCIENCE

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P J Martm et al / Ion beam analysts

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are slnnlar to the total yield concentrations The results using the higher energy window, which is less affected by oxygen enhancement, show good agree° ment with the EPMA concentrations The results obtained with the ion yields saturated by oxygen adsorption appear in table 3 A typical enhancement curve is shown in fig 5 Tile concentrahons show a greater vanadium enhancement than the clean results from the total yields, and also a greater scatter with the sum of V and Nb varying between 92 and 111%

9 Summary The results o f the ISS, SIPS and SIMS analyses of tile N b - V alloys are summansed m fig 6

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big 6 Summary of the ISS, SIPS and SIMS determined Nb concentratmns for the alloys plotted against the EPMA detertamed concentranons Key • 3 keV Ne + ISS, o, UHV SIPS, • O2 contaminated SIPS, ~ corrected 02 contaminated SIPS, A UHV SIMS, • 02 contaminated SIMS and n UHV (X 5 0 100 eV) SIMS

The ISS analysis results show good agreement w]th bulk concentrations However the transient effects observed COlnpllcate the analysis, as well as any study o f surface enrichment ol preferential sputtering effects 111 the inonolayer range Reasonable results were also obtained with SIPS at UHV, whereas at fixed oxygen exposme the agreement wtth the bulk concentrations is poor A correction term to compensate for the varying enhancement factors at tile fixed oxygen pressure must be calculated trom exposure curves to obtam agreement This reqmres that clean surfaces be produced initially for the measurement o f tile exposure curves Tile SIMS data demonstrates the hlgtl sensltwlty of this techmque to low levels o f oxygen contamlnatnin, In contrast to SIPS where the n]obmm associated contamlnatmn did not s]gnlflcantly enhance the photon yields (also compare the enhancements in figs 3 and 5) The choice o f energy w i n d o w in SIMS has a strong influence on the results in the Desence o f such contaminatlon It is to avoid this effect that many workers saturate the ton yields in SIMS, however hme tlre oxygen saturated results are in poorer agreement than the clean data In conclusion we find that ISS, SIPS and SIMS, as apphed here, can provide reasonable quantatatwe the

P J Martm et al ~Ion beam analysts

analyses b u t m u s t b e used w i t h c a u t i o n , partxcularly if s p e c i m e n h e a t i n g a n d / o r o x y g e n b a c k f l l h n g is emp l o y e d , w h e r e surface c o n c e n t r a t i o n s a n d s p u t t e r i n g y M d s can change We wish to t h a n k Mr N Ware o f t h e Research S c h o o l o f E a r t h Sciences, A N U , for the E P M A mea s u r e m e n t s and Mr A C r a w f o r d for t e c h n i c a l assls tance

References [1] N Q Lam, G K Leaf a n d H Wiederslch, J Nucl Mater 88 (1980) 289 [2] P S Ho, J E Lewis and W K Chu, Surface Scl 85 (1979) 19 [3] N J Chou and M W Shafer, Surface Scl 92 (1980) 601 [4] J Roth, J Bohdansky and W Ottenberger, IPP Report Garchmg 9/26 (1979) [5 ] H H Andersen and H L Bay, in Sputtering by ion bombardment, e d , R Belmsch (Unlv Aarhus Press, 1980) ch iv [6] D P Smith, J Appl Phys 38 (1967) 340 [7] E Taglauer and W Hedand, Proc 7th Intern Vac Congr and 3rd Intern Conf Sohd Surfaces (Vienna) 1977 [8] T A t lalm, Research Pubhcatlon GMR-1942, Research Laboratories, General Motors (1975)

281

[9] M Grundner and J Halbrltter, J Appl Phys 51 (1980) 397 [10] A Bennlnghwen, Surf Scl 53 (1975) 596 [11] J W Coburn, Thin Solid Fdms 64 (1979) 371 [12] J P Menaux, Thesis, 1971, Umverslty Claude Bernard, Lyon, trance [13] C W White, D L Simms and N H Tolk, in Characterization of solid surfaces, Eds, P F Kane and G R Larrabee (Plenum, New York, 1974) [14] CW White, D L Simms and N H Tolk, Scxence 177 (1972) 481 [15] I S T Tsong and A C Mclaren, Spect Act 30B (1975) 343 [16] I S T Tsong and R B Llebert, Nucl Instr and Meth 149 (1978) 523 [ 1 7 ] C M Loxton, P J Martin and R J Macdonald, these Proceedings, p 269 [18] HW Werner, Surf Scx 47 (1975)301 [19] G Blaise, in Material characterization using ion beams, eds J P Thomas and A Cachard (Plenum, New York, 1978) [20] A R Bayly and R J Macdonald, J Phys E 10 (1977) 79 [21] R F Garrett and R J Macdonald, these Proceedings, p 308 [22] H Rodrlgues-Murcla and H E Beske, Berlchte der Kernforschungsanlage, Juhch, No 1292 (1976) [23] J C Plvln, C Roques-Carmes and G Slodzlan, Int H Mass Spectrom Ion Phys 26 (1978) 219 [24] M A, Rudat and G H Momson, Int J Mass Spectrom Ion Phys 30 (1979) 233

VI SURI'ACE SCIENCE