TECHNIQUES IN SOFT X-RAY SPECTROSCOPY

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TECHNIQUES IN SOFT X-RAY SPECTROSCOPY

S. Luck, D. Urch

To cite this version:

S. Luck, D. Urch. TECHNIQUES IN SOFT X-RAY SPECTROSCOPY. Journal de Physique Collo-

ques, 1987, 48 (C9), pp.C9-63-C9-67. �10.1051/jphyscol:1987907�. �jpa-00227202�

JOURNAL DE PHYSIQUE

Colloque C9, suppl6ment au n012, Tome 48, decembre 1987

TECHNIQUES IN SOFT X-RAY SPECTROSCOPY

S.R. LUCK and D . S . URCH

Chemistry Department, Queen Mary College, University of London, Mile End Road, GB-London El 4NS, Great-Britain

R&sitm& GrLce A l 'emploi des mcrlticouches ou des cristaux organiqctes de grandes valeurs de Zd, on peut faire les expiriences spectroscopiques de rayon-X de grandes longueurs d 'ondes avec un di sposi tif commerci a1

.

Pour l'excitation on fait usage d'un tube A fenetre oitverte. Les spectres N K a, Re K a et Si L=,JM ont &ti observis pour une skrie des composes et, meme pour les multicouches, les deplacements de pics sont apergus. Le bombar-dement d 'un ichanti 11 on de 1 'a1 uminium anodi SO par un f aisceau Clectronique d 'inergie variable permit 1 'analyse chimique, en fonction de profondeur, de la couche superf icielle de 1 'oxide.

Abstract Using multilayers or organic crystals with large 2d spacings it is possible to carry out soft X-ray spectroscopy using commercially available equipment. fin open window X-ray tube is used for excitation.

N K a, Re K a and Si L=.sM X-ray spectra have been observed from a wide range of samples and, even for the multilayers, peak shifts were observed. The bombardment of a sample of anodised aluminium with a beam of electrons whose energy could be varied, permitted the depth profile chemical analysis of the thin oxlde layer.

1. Introduction

The development of highly reflecting multilayer devices C11 and the use of an open-window gas discharge X-ray tube'has brought soft X-ray spectroscopy from being an esoteric research topic to the ordinary X-ray laboratory. Mu1 ti layers have (as will be shown below) poor resolution (E/AE -40) but quite sufficient to distinguish the

characteristic X-rays from diferent elements. But if higher resolution i 5 required a range of organic crystals is now available with 2d

spacings from 4 to 12 nm C 2 1 . The purpose of this note is to

demonstrate that commercially available X-ray equipment can be used to detect radiation of wavelenghths of up to 15 nm (150 8 )

-

without any special modificatons and that by the use of a suitable multilayer nitrogen K a can be observed without difficulty.

Furthermore if electron excitation is used the depth of analysis can be modified by varying the energy of the electron beam. Thus soft X-ray spectroscopy can now be used for elemental analysis, for bonding studies and for the depth profiling of surface layers.

2. Experimental Procedure.

The spectra were measured using a Philips FW 141a X-ray spectrometer fitted with a CGR (Compagnie G&n&ral de Radiation) Elent-10

open-window gas-discharge X-ray tube (normal operating conditions 3mA, SkV).The X-rays were diffracted either by means of an OVONYX

multilayer or an organic ester crystal:

OV-0404 Zd

-

OV-200H 2d -1958 Mo/b,C mu1 t i 1 dyer OHM Zd a, 638 octadecyl maleate

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987907

C9-64 JOURNAL DE PHYSIQUE

X-rays were detected in the usual way by means of a proportional counter (argon/methane at atmospheric pressure, 1 pm window), followed by Harwell 2000 series electronics. Spectra were measured either by step-scanning or by digitising a chart recording of the ratemeter output.

The OHM crystal was grown slow1 y (cooling rate *0.4"C day-*) from toluene using caref ull y purified starting material C23.

Other spectra were measured using a Telsec Betaprobe in which it i s possible t o vary the energy of the bombarding electron beam from 3 t o 15 k:V.

3. Results and Discussion

(a) Nitrogen K a A comparison of the results that can now b e obtained for nitrogen K a using a multilayer and those previously measured using synchrotron radiation and a grating spectrometer E 3 J are shown for the nitrate anion in Fig. 1. Although the resolution is inferior for the multilayer vestiges of structure are apparent.

F i g u r e 2 s h o w s s i m i l a r c o m p a r i s o n s b u t b e t w e e n t h e m u l t i l a y e r a n d OHM for thiourea and for hexamminonickel(I1) chloride. The 2d spaciny of OHM i s too large for good resolution, the N I:; a peak being observed at only 60" 20. Even s o it i s clear that the crystal is capable of better resolution than the multilayer. Work i s in progress t o grow a crystal wl.th a 2d spacing of about 458 which should give improved resolution for nitrogen.

Despite its limitations the multilayer can show structure in the nitrogen peak profile and does show shifts in peak position that result from chemical changes. In Fig. 3 the N K a spectra from a series of XNOZ species ( X = 0-, para-diphenyl and Co++) and also from ammonium in NH4CL and ligand ammonia in CNi(NH,),ICC=, are compared. Chemical effects' c a n b e seen with mctltilayers.

(b) Beryllium K a and Silicon Lz.=M Other esperiments were carried out using a multilayer with a much larger effective Zd spacing, a nominal value of 200 A. Surprisingly it proved possible t o detect silicon L radiation with n o difficulty, Fig. 4. Prevlous

measurements E43 using grating spectrometers have revealed two clear peaks at about 89 and 93 eV. This structure i s reflected in the spectra of Fig. 4 a s a shoulder on the low energy side of the main peak. Resolution i s adequate enough, however t o pick up the

satel 1 ite at 75 eV (a1 though unequivocal identification is

compromised by the possibility of fourth order carbon K a at about the same angle). This satellite reflects Si 3s (and possibly 3d) character in orbitals that are mostly 0 Zs, whilst the main doublet indicates the presence of Si 3s and Si 3d respectively in the bonding molecular orbitals C53.

Silicon spectra have also been measured for Si, Sic, Si=N4 and a range of silicates; in all cases changes in peak profile and position were found.

Two beryllium compounds were also studied, the sulphate and the carbonate, Fig. 5 . The former shows an intense peak at 77" 2 0

<-10ZeV) fallawed by a pair of peaks at 96- and 1020. Adjustment of the pinlse height selector for pulses corresponding t o 150

-

X-rays (rather than 80 - 130) indicated that these two peaks were due t o S L=.,M from sulphate t63, second order. By contrast the B e K a from ReCOs is quite simple, too simple: there i s n o evidence of either or 4*" order carbon at -81° and -1 18- respective1 y

(previous experiments had shown that carbon K a was observed even when the pulse height selector had been optimised for He K a). A s

ber-yIl.ium carbonate is thermally the least stable of the Group I1 carbonates it is probable that this sample had decomposed under irradiation leaving beryl1 ium oxide.

(c) Depth Prqf ile Analysis Figure fi shows AE K @ spectra from

al~tminium metal (i)

,

spectrometer. Whilst (i) and (iv) were measured with an electron energy of SkV (ii) and ( i i i were obtained using energies of 12 and S kV respectively. As can be seen the effect of decreasing the bombarding energy is pr-odu.ce spectra from the anodised sample that change from 'half-metallic' to, 'oxide' reflecting the diminishing penetrating power of lower energy electrons. It is clear that a spectroscopic technique based on variable enenergy electron bombardment has a great potential for the surf ace analysis C71.

4. Conclusi on5

The results presented in outline above clearly demonstrate the potential nf soft X-ray spectroscopy for chemical state analysis and for depth profile studies. The use of an open-window X-ray source and either an organic crystal or a m~iltilayer for diffraction enable this potential to be realised using commercially available

eqc-(ipment

.

The authors wish to thank Nuclear and Silica Products, Pye Unicam Ltd., and the SERE for a CASE award; they also thank the Royal

Society, the Central Research Fund of London University and the SERC for financial support for the prsrchase of equipment. The a~tthors are also most grateful to Ovonic Synthetic Materials Inc, Troy, Mich, USA, for the loan of the multilayers used in this work.

References

C13 ARAR, T . , RYON, R. W. and SHOI, T. Adv. i r t X-ray A n a l . , 28 (1985) 137

C 2 3 ARHER, M.

,

,

,

Growth, in press (1987)

C33 KOSUCH, N., TEGELEH, E., WXECH, G. and FASSLER, A. J. Elec.

Spec. Rel. Phenom., 1 3 (1978) 263

C41 WIECH, G . , Pfikrnchim. Acta Supp. 6 (1975) 295

C51 AL-KADIER, M. A., TOLON, C. and URCH, D. 5 . J , Chem. Soc.

-

Faraday Trans. I I 80 f 1484) 669

E61 HENEE, B. Adv. X-ray Anal 9 (1965) 430

C71 ROMAND, M. J., HADOR, R., DESUZINGES, C., CHARRONNIER, M., ROCHE, A. and GAILLARD, F. J - de Physique 45 (1984) C2-371

JOURNAL DE PHYSIQUE

F i g u r e 1. Nitrogen K a

L

!l (a) multilayer

f

G

380 390 400

PHOTON E N E R G Y CrV3

F i g u r e 2. Nitrogen K a XE s p e c t r a f r o m t h i o u r e a

(dotted lines) and h e x - amminonickel ( I I f c h l o r i d e

(solid l i n e s ) (a) multilayer ( b ) O H M crystal

F i g u r e 3. Nitrogen K a XE s p e c t r a using a multi- 1 d y e r :

(a) NH4CE

( b ) CNi (NHa) 6 l C E s (c) Na3CCo (NO=) 6 1 (d) KNO,

(ef para-nitro diphenyl e t h e r

F i g u r e 4. Si L ~ . J M XE s p e c t r a f r o m si 1 i c a uslng a mu1 ti 1 ayer (-2008)

F i g u r e 5. Beryllium K a s p e c t r a f r o m BeS0- (dotted l i n e ) a n d 'BeCOs' (solid line)

F i g u r e 6. Aluminium K p s p e c t r a f r o m f I ) AE metal, (111 & ( 1 1 1 ) a n o d i s e d a l u m i n i u m and (IV) alumina. E l e c t r o n beam settings:

(11) 12kV 0.lmA, (111) 5 k V 0.2mA