The N45N67X Coster-Kronig transitions of gold

and Related Phenomena, 12 (1977) 11 l-l 18 @ Elsevler Sclentdic Pubhshmg Company, Amsterdam - Prmted m The Netherlands

Journal of Electron Spectroscopy

THE

N, 5-N6 ,X COSTER-KRONIG

F P LARKINS

Department

and A

TRANSITIONS

OF GOLD

LUBENFELD

of Chemrstry,

Monash

Unrverslty,

Clayton,

Vrctona,

(Austraba)

(Received 21 March 1977)

ABSTRACT

An experimental and theoretical study of the N, 5-N67N67 super-CosterKromg and the N 4 5-N6 ,O Coster-Kromg transltlons of gold has been undertaken The results are analysed wlthm an mtermedlate couplmg framework on the basis of the semi-emplncal Auger electron model developed by Larkms The most significant findmg IS that the transition probablhty calculations by McGulre based on an atomic system overestimate the relative Importance of the N, 5-N67N67 processes m the solid system The mam de-excltatlon processes are the N 4 5-N67 V transltlons mvolvmg the conduction band of gold INTRODUCTION

It has been well-estabhshed that mtra-shell non-radiative processes are the predominant de-excitation pathways for hole states when they are energetically allowed For gold, transition probablllty calculations on the atomic system by McGuu-e1 predict that the Na5 hole states decay rapidly predominantly via the N4 5-N6 7N, 7 super-Coster-Krontg processes (ca 97 %) and the N, 5-N6 7O CosterKromg processes Consequently, the calculated width of the emlsslon lines 1s at least 8-9 eV Some N-NN Coster-Kromg lmes m several systems have been previously reported m electron-induced low-resolutlon dlfferentlal spectra* An analysis of a high resolution Al K, X-ray induced spectrum of the gold system IS reported herem The analysis 1s based upon theoretical predictions using a semi-emplrlcal method formulated wlthm an intermediate couplmg framework3 The experlmental and theoretlcal approaches are outlmed m the next section The results presented and discussed m the final sectlon are thought to provide the first detailed analysis of N, 5-N6 7X Coster-Kromg processes

112 METHOD

Expermental

High purity gold foil was mounted on a stamless steel sample holder cleaned and pohshed before bemg transferred to a modified AEI-ES100 spectrometer The sample was heated at 200°C for 2 h in vacuum prior to commencement of the experlment The spectrum was induced using Al K, radiation with the sample held at 200°C m the presence of a liquid nitrogen cryotlp m the mam chamber to mmlmlze contammatlon levels A bias of - 1000 V with respect to the spectrometer earth was apphed to the sample using a Fluke 415B high voltage power supply This procedure effectively compensated by over an order of magnitude for the otherwise low sensltlvlty of the spectrometer m the kmetlc energy range 100-300 eV Information from a study of the varlatlon m the lme width of the gold 4fphoto-electron peaks under various biasing condltlons was used to deduce that the broadening of the electron emlsslon lines investigated here as a result of the apphed bias was not slgmficant relative to the mtnnslc width of the Auger profiles Typically the broadening was less than one percent of the FWHM value The spectrum was calibrated relative to the gold 4f7,2 line measured with the apphed bias and chosen to have the value 84 0 eV Cahbratlon spectra were run before and after the Coster-Kromg spectrum to ensure that no shift m the bias had occurred throughout the duration of the experiment The spectrum reported resulted from data accumulated over 78 scans controlled by a PDP8/e computer system using a channel width of 0 2 eV The number of electron counts per channel ranged from 3 x 1051 1 x lo6 Theory A general semi-emplncal method has been developed to calculate Auger hne energies wlthrn an rntermedlate-couphng frdmework3” The Auger electron energy for the W,-X, Y,, 2Sf ‘L, p recess may be determined within an mtermedrate-coupling model using the expression

where EE, 1s the bmdmg energy of an electron m a selected subshell W, of an atom with atomic number 2 and A’(“+ ’LJ) JS a correction term It has been shown for solids 3b that A=(2s+xL_,)

= ’

2Sf1LJ -

rcz(x,yl)

+ SAz

(2)

where ’ 2s+ IL, denotes the interaction energy of a pair of holes m the orbltals X, and Y, of an atom m the 2s+ ‘E, multiplet state x(X, Y,) is an atomic adiabatic relaxation term and - 6Az 1s the solid state correction term These energy terms are evaluated within the framework of an atomic Hartree-Fock model as outhned m previous theoretical papers 3 This model 1s well suited for the analysis of

113 the Auger spectrum associated with the NbS subshells of gold since conslderatlon of both spm-orblt mteractlon and LS couphng effects IS Important for a detalled analysis of the data RESULTS

AND

DISCUSSION

Theory The followmg electron subshells of the gold system .sNs352 0 eV, ~~~ 107

8 eV,

bmdmg

energies

have been

used for the requn-ed

&Ng333 9 eV,

&N687 8 eV,

Ed, 84 0 eV,

so1

co3 58 7 eV,

&OdI 2 5 eV (ref

71 1 eV,

4)

The expresslons required for the mteractlon energy of a pan- of holes were determmed from standards works 5 The Fk and Gk SIater integrals required m the evaluation of the 2s+ ‘L, expresslons were obtained by least-squares polynomial fits to values derived from calculations for selected closed she11 elements using a numerlcal relatlvlstlc HF procedure The values used for these mtegrals along with the atomic adiabatic relaxation terms are presented m Table I The sohd-state correction factor for gold IS 8 3 eV (ref 3b) The kinetic energies determmed for the N,-N,,X Coster-Kromg series are hsted m Table 2 The N,-N, 7X energies are not tabulated They are 18 1 eV lower m energy than the correspondmg N,-N6,X values Wlthm the mtermedlate coupling model the J value 1s the only good quantum number, therefore, the lme assignments shown m the tables, with the exception of the .7 value, are m many cases only notlonal The Inadequacy of the U-type notation arises because of a strong couplmg of like J states

TABLE

1

LEAST-SQUARES-FITTED AUGER ENERGIES (ev) W4f4f) W4f4f)

F2C4f $1

P(4fSd) G*(4f Sp) G5(4f5d) P(5s5d) P(5pSd) P(Sd5d) G2(Ss5d) K(4S4S)

50 1 11 5 82 19 27 10 18 9 18 2 79 69 28 1

TERMS

REQUIRED

m4f4f> FO(4f 5s) P(4j5d) G3(4f5s) G 1(4f Sd) P(5s5s) FY5P5P) f=(SpW) P(5d5d) Py5pSd) K(4SSS)

FOR

THE

25 0 31 4 20 6 41 14 25 9 22 7 89 52 10 5 13 2

EVALUATION

W4f4f > F°C4f SP) P(4fSd) G2(4f Sp) G3(4f Sd) FO(5sSp) F’(~P~P) FO(5d5d) Gl(5sSp) G3(SpSd) K(SSSS)

OF

GOLD

15 9 28 1 4s 32 13 24 2 12 1 15 6 162 64 82

114 TABLE

2

SEMI-EMPIRICAL

COSTER-KRONIG

Transrtron

Theory

Transrtfon

153 3” 161 7 163 0

N4-NmOz3

N4-N67N67

1So

3PZ 3Pl ‘16

* Ns--Na7X TABLE

lDz 303

lG4 3G3

1639

FOR GOLD

(eV)

Theory

Transztzon

185 7 186 6 189 6

N4-Nw045

Theory

3H4 3D2 lF3 3G3

189 8

6Po lD2

163 9 165 3

301

198 2

3G4

198 5

lG4

165 7

302

199 3

lP1

3F3 3F4 3F2

168 5 168 9

3F3 3F2

2002 201 4

169 2 170 0

3G5

202 9

IF3 3F4

203 4 203 6

3P2 3H5 3D3 ID2 3H6

SW5 3H4 N 4-N 670 1

LINES ENERGIES

1722

3F2 3G4

3H6

173 4

3P1

261 5

3F3

lF3 3F2

146 6 147 2

3Po II35

261 8

3G5

262 2

lG4

3F3

150 3

301

262 3

3F4

3F4

1509

N4-Nw045

lmes are 18 1 eV lower in energy than the correspondmg Nw-NwX

hnes

3

SEMI-EMPIRICAL Transrtion

AUGER

LINE ENERGIES

Theory

Tram&on

N4-0

FOR GOLD

Transttron

lD

237 9

N4-023045

30 ‘Pl lF3

2406 274 S 275 1

N4-0

10 I

1so

127 0”

1023

lP1 3Po 3Pl

1643 170 2 178 2

3P2 ‘SO

183 2 201 8

3P2 3D2

276 9 279 5

lD2 3Po 3P2

215 7

3Pl 3D3 301

N4--023023

226 4 229 9

1045

N4-023045

(eV)

Theory

N4-0

Theory

3F2

289 7

3Po 3F3

289 7

ID2

292 1

3F4 W

292 9 3441

286 8

IG

347 2

287 3 289 6

3P 1D

N4-045045

3F

& N5-00

262 4 262 5 262 8 263 1 263 2 2640 265 3 265 6 265 9 266 2 266 5 266 5 266 7 266 8 266 8 267 1

lmes are 18 1 eV lower In energy than the correspondmg N4-00

291 1

347 7

347 9 349 3

lines

The values for the N 45-OO normal Auger hnes are presented m Table 3 Previous mvestlgatlons with other systems suggest that for accurate bmdmg energy data the calculated values should be reliable to within l-2 eV In the present work the exception to this guide must be transltlon series mvolvmg the 5d and 6s conduction band levels The calculations have assumed a discrete value of 2 5 eV for the 5d

115 level, however, rn the solid the conduction band extends to nearly 8 eV below the Fermi level6 Hence, m addition to lifetime broadening further srgmficant broadening of lines mvolvmg electrons from the conductlon band IS to be expected Unfortunately, the pubhshed calculations by McGulre ’ do not provide detalled Intensity data for the mdlvldual Coster-Kromg processes and the many multlplet components Expermen tal The raw data corrected for the varlablhty of analyser sensltrvlty throughout the range and smoothed using an eleven-point parabohc least-squares fitting IS shown m Fig 1 The broad Coster-Kromg lines are superimposed upon an mtense sIopmg background of low energy melastlcally scattered electrons The background was removed m two parts The kinetic energy region from 130 to 175 eV was assumed to have a Imear background, whereas the background region from 175 to 310 eV was determmed by fittmg a fourth-order polynomial to points m the regions 175-8, 193-4 and 302-l 1 eV This procedure should have the effect of essentially removing much of the melastlc energy loss contrlbutlon associated with the Coster-Kromg processes m addltlon to the background from other scattered electrons The resultant spectrum 1s shown m Fig 2 The spectrum may be mterpreted using the theoretical findings grven m 100 l--r\

80 -

20 -

0

110

I

130

I

150

I

170 KlNETfC

I

190

I

210

1

230

1

250

I

270

I

290

1

310

ENERGY eV

Figure 1 The N 45~N67X Coster-Kromg Lines of gold superimposed on a background of low energy melastlcally scattered electrons The background that was subtracted IS shown by the dotted lme

116

100

0

f

110

I

t30

1

150

I

170

I

190

KINETIC

I

I

I

I

r

1

210

230

250

270

290

310

ENERGY eV

Figure 2 The N45-Ng7X Coster-Kromg lmes of gold after background subtraction (see text) The posltlon of the multlplet lines calculated usmg eqn (1) are shown Numerical values and label ldentlficatlons are given m Tables 2 and 3

Tables 2 and 3 The posltlon of the predicted series of lines are also shown on the diagram For clarity, the mdlvldual multlplets have not been labelled as their order may be readily deduced from Tables 2 and 3 No adJustment of the theoretlcal predlctlons to fit the observed values has been made It 1s evident that the agreement between theory and experiment 1s excellent for all peaks with the exception of transltlons directly mvolvmg the conduction band electrons m the final state In particular there 1sa discrepancy of some 6 eV between the predicted values and the peak posltlons associated with the N as-N6 7V levels This variation results primarily from the inadequacy of a single energy value of 2 5 eV to describe the valence band The overlap of the various transltlon series m the energy range considered 1s apparent Meaningful deconvolutlon of the broad peaks 1s precluded because of this strong overlap, the density of multlplet states and the lack of detalled theoretlcal mtenslty data for the transltlons m the kmetlc energy region There 1s a contrlbutlon to the profile from the carbon K-LL Auger spectrum m the kinetic energy range 280-210 eV While it IS not possible to accurately assess this contrlbutlon It 1s estimated to be only ca 4% of the total Intensity m the region from conslderatlon of the mtenslty of the Auger spectrum of graphite, corrected by comparison of the carbon IS photo-electron lme mtensltles for graphite and the

117 contaminant on the sample, with the observed spectrum Other factors supportmg low carbon contammatlon levels on the specimen are the followmg (1) the experiment was conducted under high vacuum condltlons with the sample held at 200°C m the presence of a liquid nitrogen cryotlp, (11) the relative strength and prmclpal peak separation associated with the N,,-N6,V series 1s consistent with the correspondmg ratios and peak separations, as far as they can be assessed, In other regions of the spectrum, (m) when the sample was deliberately contammated a new dominant peak grew m on the high kinetic energy side at the posltlon shown and the shape of the profile changed slgmficantly, (iv) an mdependent mvestlgatlon of the carbon Auger spectrum from a graphite layer ylelded a dlstmctly different profile m this region It 1s concluded from the spectrum shown m Fig 2 that the predominant decay processes for N,, vacancies m the solid are the Nd5-N6 ,V Coster-Kromg series rather than the super-Coster-Kromg N,,-N,,N67 series predicted by atomic calculations 1 The atomic calculations are expected to be more rehable m predlctmg the relative transition probabdltles for processes not mvolvmg conduction band electrons Hence, m mterpretmg the observed spectrum the normal Nq5-00 Auger processes mvolvmg non-conduction bands levels are assumed to be much less important than the Coster-Kromg transItIon processes and to contribute little Intensity to the spectral region below kmetlc energy 210 eV However from a conslderatlon of the mtenslty of the profile centred near 291 eV It 1s clear that normal Auger-type processes mvolvmg the conductlon band are Important de-excltatlon pathways The relative areas of the prmclpal peaks A-F shown m Fig 2 are as follows 1 0, 0 5, 0 2, 0 1, 10 7, 0 2 Peak A IS prmclpally asslgned to the N,-Ns 7N67 series with the shoulder near 150 eV resulting from an addltlonal contrlbutlon from series It 1s of mterest to note N4-N6,01 series and the lS, hne of the N4-Ne7N6, that the bmdmg energy of the O1 Ievel 1s lower than that for the N, and N, levels The principal lines of the N,-N, ,N 67 series dommate peak B but there IS also a small from contrlbutlon from multlplets of the N 5-N6 ,O,, series Peak C has contrlbutlons series, while peak D orlgmates multlplets of the N5-N67023 and the N,-N,7023 from components of the N4-NG70,, series The profile E 1s a convolution of several transltlon series lnvolvmg the conductlon band electrons Its extreme width results because the contrlbutmg transittons are inherently very broad owing to the short hfetlme of the mltlal hole and the breadth of the conductlon band The Na5- N, ,V series 1s of most importance series also make a contrlbutlon but the N45-01045 series and the N,-0,3045 Peak F results from multlplets of the normal Auger N4-023045 series From this mvestrgatlon It 1s concluded that the N,, hole states m gold decay mamiy via Coster-Kromg processes mvolvmg the conduction band This result underlmes previous observations for other systems that transltlon rate data based upon an atomic model may be unrehable when conductlon band levels are involved

118 Although the multlplet fine structure was not resolved, the N,,-N,,X spectrum gold can be satlsfactorlly explamed using an mtermedlate couphng model

of

REFERENCES 1 2 3a b 4 5a b

E J McGulre, Phrs Rev , A, 9 (1974) 1840 T W Haas, J T Grant and G J Dooley, Whys Rev , B, 1 (1970) 1449 F P Larkms, J Phys B, 9 (1976) 47 F P Larkms, J Whys C, 10 (1977) 2463 K D Sevler, Low Energy Electron S’ectrometry, Wiley-Intersclence, New York, 1972 J C Slater, Quantum Theory ofAtomzc Structure, Vol 1, McGraw-HIII, New York, 1960 E U Condon and G H Shortley, The Theory Atomzc Spectra, Carnbrldge Unlverslty Press, London, 1951 c G Racah, Physzca (Utrecht) , 16 (1950) 651 6 K Slegbahn, Phzl Tram Roy Sot London A, 268 (1970) 33

of