A cylindrical-mirror electron spectrometer for studies of gases and metal vapours

Journal of Electron Spectroscopy and Related Phenomena, 16 (1979) 423-433 @ Ekevler Sclentlfic Pubhshmg Company, Amsterdam - Prmted m The Netherlands

A CYLINDRICAL-MIRROR ELECTRON SPECTROMETER FOR STUDIES OF GASES AND METAL VAPOURS

J VAYRYNEN

and S AKSELA

Department of FVaysacs,Unzversgy of Oulu, SF-90100

Oulu 10 (Fmland)

(Received 17 November 1978)

ABSTRACT A detalled descrlptlon of the reahzatlon and performance of a cylmdmcal-mirror anztlyzer with a resJstance-heated oven for measurements of Auger and photoelectron spectra from gases and metal vapours 1s presented The best energy resolution observed without preretardatlon was better than 0 05% Oven temperatures of 900°C were easily reached An efficient electron gun producmg beam currents of up to 3 mA and a simple Al Ka X-ray tube operating at 800 W are described The posslblllty of measunng electron spectra from solid surfaces condensed contmuously from vapour IS also outlrned The overall performance of the spectrometer 1s demonstrated with the L2,3Mz,3Mz,j Auger spectrum of argon and the 3d,,,,, pho&electron spectrum of cadmium vapour The high-resolution argon spectrum 1s decomposed into line components and the relative mtensltlea are compared with recent expenmental and theoretical results For the inherent width of the argon L~,~MZ,~M~,S lines 0 12 k 0 02 eV has been obtained

INTRODUCTION

The Auger and soft X-ray photoelectron spectra of metal vapours have become the obJect of mtenslve research durmg the last few years’-” . This has been brought about by the need to mcrease the number of elements which can be studled as free atoms Expenmental free-atom Auger and photoelectron data are of fundamental unportance for theoretical studies due to the fact that they are free from all solid-state and molecular effects On the other hand it IS extremely unportant to accurately determme atomic spectra so as to act as references for studies of molecular and solrd-state effects, e g extra-atomic relaxation shlfts’1-19 and hne broadenmgs As the mstrumentatlon’*S*10 for the measurements of Auger and photoelectron spectra from metal vapours 1s very new and commercial spectrometers are not yet available for the soft X-ray energy region, we will descmbe m some detrul our expenmental set-up It has been implemented succesfully to measure Auger spectra from zmc, cadmium, magnesium,

mdmm, silver, antunony and tellunum vapours Quite recently a soft X-ray tube has also been constructed and has been employed to measure photoelectron spectra from vapours. The expenmental set-up for direct measurements of energy shti between vapour and solid&ate electron spectra will also be bnefly described

EXPERIMENTAL

SET-UP

Eiectron energy analyzer The electronenergy analyzer 1s a cyhndncal-mirror analyzer20-23, with the electron optrcal source and usage shts on the surface of the inner cylinder This analyzer has been used by our research group at the Unwerslty of Oulu for measurements of Auger spectra of solid surfaces24*2s The purpose of the present work has been to construct a vapour oven which IS not only demountable but also easy to clean for this analyzer The analyzer presented m Fig. 1 IS of the first-order-focusmg type mth the mner- and outercyhnder radu of 49 mm and 136 mm, respectively. The distance between the two arc shts on the surface of the mner cylmder 1s 146 mm. The azunuthal openmg IS restrxted by the slits to 60’ The oven, which has a watercooled housmg, IS located mslde the inner cyhnder, and the electron gun 1s attached to rt by means of a supportmg rmg The prunary electron beam from the gun travels along the symmetry 8x1s of the cylinders through the oven and 1s monitored by a Faraday cup (FC) . The Auger electrons to be analyzed are emltted at a mean emlsslon angle of 54 5* This emlsslon angle has the special advantage of mmnnlzmg the effects of the possible angular dlstnbutlon of the emitted electron?* 27 The electrostatic stray fields at the ends of the analyzer are elunmated with the aid of brass rings of loganthmlcally mcreasmg radu The rmgs are connected to each other by resistors of 10 MS2 The voltage of the outer cylmder 1s controlled by a multi-channel analyzer (MCA) through whrch the channel number, via the dlgrtakmalog converter, IS connected to the voltage amplifier This amplifier Dves the step-sweep voltage below the base voltage adJusted from the preclslon high-voltage supply The stability of the supply

ANALVZER

Flgure I SchematIc representation of the experimental set up

425

IS better than 50 mV h-l and the npple VP,(rms)< 20 mV External control of the MCA by a burst generator 18also possible when one wishes to read the voltages correspondmg to the deszed channel numbers. For an electron detector we have used a channel electron multlpher (Mullard 419B). The spectra are collected and stored m the memory of the MCA and output deuces consut of a XY-plotter and a tape puncher (TTY) The punched tape can be fed mto the computer for further data-handlmg of the spectra. The vacuum station consists of a 600 1s-l oil dlffuslon pump combmed with a rotation pump The vacuum urlth respect to the volume of the analyzer dunng the measurements 1s higher than lO+ torr as measured by an lomzatlon manometer The earth’s magnetic field 1s compensated for by Helmholz colls and a cylmdncal Permalloy magnetic shldd The residual field m the re@on of the electron paths m the analyzer 1s of the order of 2mG Vupour oven

The h&-temperature oven whrch produces the vapour target 1sshown m Fig 2 It 18deslgned m such a way that It can be easily mserted into the analyzer The pnnclple behmd the system3p4*6 pa 1s that the pnmary electron beam goes through the oven and the Auger electrons to be analyzed are brought through an openmg m its cyhndncal wall. This set-up 1s advantageous because much lower temperatures are required than for nozzlebeam-oven types The mam disadvantage of this type 1s that the oven has three rather mde opemngs, through which vapour IS part&y diffused rnto the surroundmg space. In order to shorten the path of Auger electrons m the oven and thus reduce the melastlc scattermg probablllty, the oven has been located asymmetncally mth respect to the symmetry axis of cylmders The holes for the pmnary beam are 2 mm m drameter, and the openmg for Auger electrons IS 1 5 x 6 mm2, which means that the angular divergence of analyzed electrons for the flight plane IS about 2O. The oven cylmder 1smade from stamless steel, and 1sheated by a bLfi1a.rchromlum--nlckel heatmg wire mth a resistance of 20 J-2(Sodem Thennocoax) The temperature 1s mamtamed so as to afford a vapour pressure m the range of 10-4-10-3 torr A

Flgure

!T -& --

temperature

oven and electron

gun

426

heatmg power of O-60 W produces temperatures from 20-900°C The stray magnetic field of the heatmg element inside the oven IS less than 1 mG when the heatmg current corresponds to a temperature of 900°C and no broadening of the Auger lines of the rare gases has been observed. The temperature 1s measured by a Chromel-AIumel thermocouple. Cylmders of thm stamlesssteel of 0.1 mm thickness have been used as heat radiation shields around the oven Electron gun The electron gun wrth its three-element focusmg lens system 1salso deprcted m Fig 2 In pnncrple, It 1s a modlfled Stelgerwald gun The cathode IS a duectly-heated tungsten filament, 0.15 mm m diameter, modelled on a harrpm or simple spmal-tip form The Wehnelt cylinder IS electncally connected to the filament by a resistance of 100 ksL and therefore the cylinder has a negative potential with respect to the filament To some extent this potential focuses the charge cloud emitted from the filament into the anode hole The purpose of the three-element electron lens 1sto focus the beam so as to relay the maximum electron beam to the target regron. Usually the voltage of the mrddie element LSvaned m order to maximize the beam current, whrle the other two are grounded For low accelerating voltages it LSalso possible to use the lens system mth asymmetrical voltages to decrease the effect of the space charge around the filament, the first element IS then set at a posltlve voltage, and the electrons are decelerated to the deslred energy by the other two elements. The lens elements and the cathode with the Wehnelt cyhnder are supported msrde a copper tube by means of lsolatmg alummmm oxrde rods The power supply for the filament of the electron gun LSHV-Isolated and stabrhzed to gwe a constant current through the filament Typical voltage and current values are 3 V and 3 A, respectively The maximum electron current of the gun through the oven 1s about 3 mA by the acceleratmg voltages greater than 15 kV Cundensatron of s&d samples The reliable measurement of the Auger electron spectra from clean metal surfaces m a vacuum of lo-’ torr 1simpossible because contammatlon of the surface 1s much too rapld To overcome this dlfflculty a partially cooled copper rod was mtroduced inside the oven and the spectrum was taken from the surface which contmuously condensed on this rod from surroundmg vapour A pressure difference of about two orders of magnitude between vapour and residual gas makes it possrble to secure the signal from the clean surface The pnmary electron beam was directed at a grazing mcldent angle to the surface of the sample The expenmental set-up IS portrayed m Fig. 3. The Auger spectra of solid cadmium, zmc and magnesium have been measured using this arrangement I1 However, the high background of melastlcally-

427

ELECTRON COOLIN CONTAC

Flgure 3 Set up used for condensation of metal vapours on partially cooled copper rod m oven

0 Anode @ Filament @

At wmdow

@

water

Q @

Sample cell Heat radlallon

Q)

Heatrng

@I @

Gas I” let Analyzer

coolrng shreld

resistance

Figure 4 X-ray tube and vapour oven

scattered pnmary electrons from sohd surface usually prevented the snnultaneous observation of the correspondmg vapour-phase Auger spectrum X-ray source For measurements of the photoelectron spectra of metal vapours the AlKa source was substituted for the electron gun and the oven construction shghtly modified The fixst version of the X-ray tube presented m Fig 4 xs very elementary The f&unent IS directly above the anode so the desved electron flux onto the anode IS easily obtamed mthout any space-charge hnutatlons The disadvantage of this arrangement IS the slow tungsten deposit onto the anode Until now the vacuum mslde the tube has been the same as m the spectrometer, only 104-10-5 torr, so contamination from other sources 1s also rather rapid The contammatlon was observed to be lowest when the operatmg temperature of the anode was near the meltmg pomt of alummlum The power supply of the Soviet-made soft X-ray spectrometer (RSM-500, max 10 kV, 300mA) was used to dehver 700-1000 W power to the X-ray tube, the anode bemg at a high posltrve voltage with the filament near ground potent&. An alummlum wmdow, 5 pm thick, separates the volume of the oven from the X-F:- ?be Because the temperature of the endow ISabout the same as

that of the oven, contammatlon of the sample on it has been ehmmated The vapour pressure of 10m2-10-l torr of the sample has been used m the measurements of photoelectron spectra. By usmg the above dlscussed condensation method of sohd samples we have been able to simultaneously record vapour and sohd state photo- and Auger electron spectra This has made the essentially lower background mtenslty from solid samples v&h X-ray excltatlon possible.

FEATURES

OF THE SPECTROMETER

Resolukon The observed resolution of the analyzer has been typically 0 05% with sht wrdths of 0.05 mm. The resolution 1s good enough for most Auger measurements, because the line broademng caused by the analyzer 1s the same or smaller than the mherent spectral-lme mdths The Auger spectra of vapours can be measured vnth moderate mtenslty (10 3 -lo4 c k’ ) when the high Intensity electron gun, with the focusmg system, ~3 used as a pnmary lomzatlon source The background mtenslty ansmg mamly from multiple melastlc scattermgs of pnmary electrons IS also rather low. For the Auger spectra of solsds the background 1svery high and the ratio of mam peak mtenslty to the background 15 about 4: 3l’ or less. Fluctuations m the prunary-electron current or the target-gas density may cause undesirable vanatlons m detected mtenslty However, usmg the rapld sweep technxque, these effects m the sum spectrum can be dununshed. The shape of the pulses from the multiplier allows a m&mum pulse frequency of up to 5 x lo4 c C1 However, frequencies of 10 3 -lo4 c s-l have been noted to be usually sufficient and safe in practice Energy calrbmtwn The energy cahbratlon of the Auger spectra excited by an electron beam can be done with high accuracy by using the known Auger lines of rare gases as reference lines. For most purposes the useful cahbratlon lmes are Ne KL~,3L~,3(‘D) (804 557 * 0 017eV)28 and Ar&&3i&j(L&,) or ( 1 D2 ) lines with enewes 201 IO + 0 05 eV and 203 49 + 0 05 eV, respectlvely29 The same ener@;resfor these lines can also be obtamed from the photoelectron measurementsz9 combmed mth the optically known double hole final state ener@es3*, so it 1s possible to consider these the best reference lines at present In addition to the energy cahbratlon, rare-gas spectra can be used convemently to test the resolution of the spectrometer From the murture of t+e sample gas or vapour and the rare gases the desired spectra can be measured and the analyzer voltages approximately correspondmg to the spectral lmes can be read from a high-precision d@al voltmeter (DVM) using the burst generator to control the MCA sweep Accu-

429

rate voltages for the line pos~tlons can then be calculated vvlth the ad of the precise channel numbers of the lme posltlons obtamed from the least-squares fits The spectrometer constant has been determined from the reference-lmevoltage values and the known kmetlc energres An accuracy of about 0.1 eV can be reached for the eneraes of the spectral hnes

PERFORMANCE

In order to demonstrate spectrometer performance the L2,sMt, 3M2, 3 spectrum of argon ls presented m Fig. 5 The experunental spectrum 1s shown by dots and the sohd curve represents a least-squares fit of standard lmes to the data The vertical bars @ve the pos&ons and relative mtensltles of the components As the standard lme shape we have used the measured L3 M2,3 M2,3 ( 1So ) hne or a Vo~gt function and the relative posltlons of the lmes are vmed or fixed to the optically known values30. Our expenmental values, averages from five different f&s, are gven m Table 1 with the experimental vaIues of Mehlhom and Stalherm31, Werme et aI 32, and Ridder et al 33 Also m the mixed couplmg scheme calculated values of Mehlhom and Stalherm3’ are shown As pomted out by Ridder et al.33 the relative mtensltles of drfferent 3P components provide a cntlcal test for the vahdlty of mixed couplmg calculation scheme where mltlal state 1s treated m II coupling and final state m LS couplmg The mtensltles of tnplet components are m very good agreement with the expemnental results of Ridder et al 33

5

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4

O-

f=7RCXIlN

O-

O-

O-

O-

ENERGY

CEVl

Figure 5 L~,JM 2, 3M 2,~ Auger spectrum of argon excited wrth 3-keV electrons The solld curve and vertical lines represent a least-squares fit of standard hnes to the data Labels wlthout parentheses Identify transltlons orlgmatmg m the L3 shell and labels m parentheses those orlgmatmg m the Lz shell

3p2

3p, 3h

ID2

%

L2M2,3”2,3

3p2

3h 3p1

92

“SO

L3M2,3M2,3

State

_~_.

121 489 95 1891390 10 6

108 448 28 102 447 317I

This work

78 232 80

23 112 312

Ridder et al 33

81 499

81 499

Theory”

368

147 485

424

156 420

Wene et al32

396

124 480

418

122 460

Mehihmnet al 31

TABLE1 EXPERIMENTAL RELATIVEINTENSITIES OF L2,3M2,3M2,3 AUGERSPECTRAOFARGON WITHTHE CALCULATED !! LINESOFRIDDERETALARENORMALIZEDTO VALUESOFMEHLHORNETALTHETOTALINTENSITIESOF3Po,,,g OURVALUESFORDIRECTCOMPARISON

431

and mth the theorel~cal values confirmng the apphcabtity of the mxedcouphng scheme. Our values for other components agree better with the values of Mehlhom et aL3* than with those of Werme et al.32 Also the hne width es&nation for the ArL&f2,31W~,3 hnes has been produced. Recent mh resolution measurements have shown that the mherent mdth of the diagram Auger lines of argon 0.16 4 0 02 given by Mehlhom et al.34 1s substanttiy too high Ridder et al.33 and Kondow et aLss have obtamed 0 10 f 0.02 eV deconvoluted from the spectrum which has been measured usmg preretardatlon m the spectrometer. Nordgren et al 36 have determmed from the L X-ray spectrum of argon mdth of 0.12 eV for &, 3-M, transitions. For determmatlon of the mherent mdth of argon lines the resolution function of the spectrometer has been determmed from the narrow argon For its mherent mdth the value satellite hne (L ~,3Mz,3(303)-M32,3(2P)) 0.04 eV36 has been used. The total measured mdth IS 0 14 eV which gwes a resolubon of 0.07%. With this resolution the measured mdthof L2, 3M2,31M2, 3 hnes has been 0 23 eV from which after the deconvolutlon of the spectrometer contibutlon the mherent mdth of 0 12 + 0 02 eV has been obtamed the value stated by Nordgren et al 36 As a standard lme m the fittmg of the spectrum the convolution of the Gaussian spectrometer lme of mdth 0 15eV and the Lorentzlan mherent lme of width 0.12 eV has been used The spectrum of argon has also been retarded by means of very sunple electrodes to an energy re@on of 50 eV where the spectrometer contnbutlon IS small Then we find that the upper lunlt for the width has to be 0 14 eV because the fktmg with the pure Lorentzlan of this mdth mthout the spectrometer contnbutlon gives the best fit. Some spectra of metal vapours and also spectra of metals measured by the contmuous condensation method have been pubhshed previously” The 3d3,,, 512 photoelectron spectrum of cadrmum vapour excited by the Al Ka 12-

CRDM

I UM

3%2

3d

23

312

x

-eOS

BINDiNG

Flgure atlon

6

ENERGY

3d3,&-photoelectron

(EV)

spectrum of cadmmm vapour excited by Al

Ka radl-

432

radiation ls depleted m Fig. 6. The peak to background ratxo was 10 1 m thus measurement. In addition to the photoelectron lmes, extra structure can be seen at distances of 5.4 eV on the low energy side from the mam lmes. These ongmate most hkely from melaslzc energy loss of photoelectrons by excitation of 5s electron to 5p 1eveP’ m cadmm vapour. On the ha energy side, hnes excited by Al I@ 3,4 satelhte lmes can also be seen.

CONCLUSION

A low-cost and snnple cyhndrical-mrrror electron spectrometer wrth the high-temperature oven for measurements of Auger and photoelectron spectra from gases and vapours has been outhned. Transmmlon of the energy analyzer LSnot very high because of the first order focusmg type and the part& use of the whole 27r cucular openmg m a transverse duectlon. Lower transmlsslon can e&y be compensated for by the use of a high-mntensltyelectron gun and thm presents no semous mtenslty problems for Auger measurements. - -__3 has also been proven that photoelectron spectra can be measured adequately mth this system. The use of a sector of about 60° m a transverse drrecbon m essence srmphfres problems wrth the mechamcal adJustment and the elunmatlon of the effects of disturbmg residual magnetic field. Hence a high energy resolution (0 05%) can be obtamed rather easily w&out preretardatlon. This resolution IS usually good enough to resolve the detailed fine structures of the spectra. The posslbfity of measurmg snnultaneously wrth X-ray excltatron both vapour and solid-state photoelectron and Auger spectra has been found very promismg. The accuracy of the determination of the sohd-state shifts m photo- and Auger spectra can be essentially unproved by this method.

ACKNOWLEDGMENT

The authors thank R Kumpula for his help m the construction of the X-ray tube and m the measurements of the photoelectron spectra. REFERENCES 1 2 3 4

5 6 7

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