DEPTH CONTROLLED EXAFS AND NEAR EDGE SPECTROSCOPY TO STUDY SURFACE LAYER STRUCTURE

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DEPTH CONTROLLED EXAFS AND NEAR EDGE SPECTROSCOPY TO STUDY SURFACE LAYER

STRUCTURE

F. Thornley, G. Antonini, G. Greaves, N. Barrett

To cite this version:

F. Thornley, G. Antonini, G. Greaves, N. Barrett. DEPTH CONTROLLED EXAFS AND NEAR

EDGE SPECTROSCOPY TO STUDY SURFACE LAYER STRUCTURE. Journal de Physique Col-

loques, 1986, 47 (C8), pp.C8-883-C8-886. �10.1051/jphyscol:19868171�. �jpa-00226076�

DEPTH CONTROLLED EXAFS AND NEAR EDGE SPECTROSCOPY TO STUDY SURFACE LAYER STRUCTURE

F.R. THORNLEY, G.M. ANTONINI", G.N. GREAVES*' and

. ^{BARRETT* *}

Department of Applied Physics, University of Strathclyde, GB-Glasgow G4 ONG (Scotland), Great-Britain

*CNSM and Physics Department, University of Modena, 1-41100 Modena, Italy

* * S E R C Daresbury Laboratory, GB-Warrington W A 4 4AD, Great-Britain

t t *

Physics Division, JRC, I-21020 Ispra ( V A ) , Italy Abstract

Calculated and experimental intensity values are presented and compared for EXAFS spectrometry at grazing incidence with fluorescence or reflectivity detection, and samples in the form of single or double metal films evaporated onto flat glass substrates.

The limited and controllable penetration

X-rays incident at grazing angles onto flat samples provides scope for the investigation of surface regions. An EXAFS and near-edge X-ray spectrometer with grazing incidence and fluorescence or reflectivity detection has been develope4 at the SRS of the Daresbury Laboratory. The instrument has been used t o study actinide behaviour during the water corrosion of borosilicate glasses for high-activity radioactive waste disposal (11.

The effect of the corrosion may be the formation of a layered structure on the surface of the glass; this possibility may introduce ambiguities into the interpretation of the results. There was therefore interest in looking at the behaviour of known layered structures. This paper describes some tests of the instrument with samples in the form of single or double films evaporated onto polished flat borosilicate substrates.

The angular scale of such measurements is set by the critical angle

for total external reflection, approximately given by the expression vF 1.64

mrad, where A is the wavelength in A and

is the density in g ~ m - ~ . For grazing angles

<< ^{vc the depth of}

penetration of the X-rays, Z, is approximately

A, independent of absorption or wavelength. This is what is meant by 'surface region1 -

perhaps 4 atom layers for dense materials, 20 atom layers for light rnater ials . ^As

is increased,

increases, to a value of (k/47~fi(r)

for

9 =

vC, where

is the X-ray absorption coefficient. Thus Z is about 200 A for

200 cm-l. For

and increasing, Z increases rapidly and tends to the geometric value of (v2 -

How do these changes in Z appear in the experimental measurements? For a homogeneous material, with fluorescence detection, the intensity is approximately proportional to (1-R), where R is the reflectivity [2,1]. The observed increse in fluorescence intensity as v is increased from zero arises because a greater proportion of the incident intensity is able to penetrate into the sample, rather than being reflected. The variation of the penetration depth of the radiation with v does not affect the intensity directly.

How then can the depth effect be checked?

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868171

C8-884 JOURNAL DE

Consider a double f i l m , where t h e lower f i l m h a s a h i g h e r e l e c t r o n d e n s i t y t h a n t h e upper f i l m , f o r example P t below Cu, on a f l a t g l a s s s u b s t r a t e . How w i l l t h e i n t e n s i t y of P t f l u o r e s c e n c e depend upon t h e g r a z i n g a n g l e

Fluorescence w i l l be observable o n l y i f

is l a r g e enough f o r t h e p e n e t r a t i o n depth through t h e upper f i l m t o be g r e a t e r t h a n t h e f i l m t h i c k n e s s . For f i l m t h i c k n e s s e s of a few hundred

t h i s means

j u s t g r e a t e r t h a n t h e c r i t i c a l a n g l e f o r t h e upper f i l m .

Thus

is n e c e s s a r y t o consider t h e X-ray o p t i c s f o r a t h i n f i l m of Cu w i t h vacuum on one s i d e and a more dense medium ( e . g . P t ) on t h e o t h e r s i d e . The t h i c k n e s s of t h e P t l a y e r w i l l be assumed t o be i n f i n i t e . Suppose X-rays a r e i n c i d e n t a t a g r a z i n g a n g l e

on a Cu f i l m of t h i c k n e s s h , with r e f r a c t i v e index n , , on a P t s u b s t r a t e with r e f r a c t i v e index n,. The q u a n t i t i e s t o be c a l c u l a t e d a r e t h e r e f l e c t i v i t y

I r

and T

where r and t a r e t h e r e f l e c t i o n and t r q s m i s s i o n c o e f f i c i e n t s ( i . e . amplitude r a t i o s ) f o r t h e f i l m . Expressions f o r r and t a r e given, f o r example, i n [ 3 ] . Some c a l c u l a t e d v a l u e s of R and

f o r a range of v a l u e s of h and v a r e shown i n Fig. I.

The m a t e r i a l c o n s t a n t s used were f o r

1.05

( j u s t above P t

edge). The c r i t i c a l a n g l e s f o r t h e pure m a t e r i a l s a r e 5 . 1 mrad f o r Cu and 8.0 mrad f o r P t .

The q u a n t i t y T g i v e s a measure of t h e p e n e t r a t i o n of t h e X-ray beam i n t o t h e P t , s o

w i l l be r e l a t e d t o t h e magnitude of t h e P t f l u o r e s c e n c e s i g n a l . For

-

^{mrad -}

^.e. i n t h e v i c i n i t y

t h e

c r i t i c a l a n g l e - a s t h e f i l m t h i c k n e s s h is i n c r e a s e d , T d e c r e a s e s . The P t is being s h i e l d e d by t h e Cu f i l m . For l a r g e r v a l u e s of

some s t r u c t u r e s t a r t s t o appear b o t h i n

and i n T. T h i s behaviour

R(9) a t f i x e d h is f a m i l i a r . For r e l a t i v e l y s m a l l h , t h e r e i s a d i p near t h e Cu c r i t i c a l a n g l e , t h e n an i n c r e a s e a s t h e X-rays p e n e t r a t e t h e Cu and a r e r e f l e c t e d by t h e P t followed by a second drop near t h e P t c r i t i c a l angle. For l a r g e r v a l u e s of h t h e r e a r e K i e s s i g o s ~ i l l a t i o n s r e p r e s e n t i n g multiple-beam i n t e r f e r e n c e . The p r e s e n t c a l c u l a t i o n s show t h a t t h e s e two e f f e c t s blend i n t o one a n o t h e r .

P l g .

1 Calculated v a l u e s of

and

f o r X-rays (X=1.05A) i n c i d e n t a t an

a n g l e

on a f i l m of Cu of t h i c k n e s s h with a P t s u b s t r a t e .

L I I I absorption edge; (b) X-ray energy 50 eV below absorption edge.

Measurements were made with a sample consisting of 500A Pt then 200A Cu evaporated onto a polished (A/10) flat borosilicate glass substrate. The instrument, described in [l], was used on experimental

9

SRS

Daresbury Laboratory. The X-ray

was set t o be above the absorption edge for Pt LI I1 radiation. Fig.

2(a) shows the variation of the reflected and the fluorescent intensity with grazing angle

The oscillatory structure appearing in Fig. 1 is present in both signals

the general resemblance to the theoretical values is good. Fig. 2(b) gives the corresponding lfluorescence' measurement at an energy below the Pt L I I I absorption edge. Thus it

indicates the angular dependence of the background scattering and fluorescence. The oscillatory structure is absent; this effect has been found with other samples also. The probable reason is that the

scattering comes mostly from the Cu film.

Fig. 3. Fluorescence i n t e n s i t y f o r d i f f e r e n t g r a z i n g a n g l e s 9 from P t f i l m cover- e d w i t h 500 A Cu. The a b s o r p t i o n edge observed i s t h e P t L I I I edge a t

11.56 kev. ( a ) 'P/rPc = 0.77 ; (b) 'P/cPc = 0.84 ; (c) 9/qc = 1.4, where y c i s c r i t i c a l a n g l e f o r P t .

C8-886

DE PHYSIQUE

2 . 8 8 - .

. . . . . ^. . .

I 1

1 I

Pig 4 Variation of the increase in fluorescence

>

intensity at the Pt LIII

absorption edge with grazing angle for Pt (squares) and for

..

_. Pt covered with Cu (crosses).

The intensities are scaled to

.

be equal for 9/qc pt

1.4.

The vertical dashed lines indicate the critical angles for Cu and Pt.

8.8a 8.32 8.68 8 . 8 8 1.28 1.58

phl/Pt s r l t l c o l onale

Pig. 2(a) was used to choose angles for energy scans where the intensity of the fluorescence from Pt was varying rapidly. Some measurements are shown in Fig. 3. The variation of the height of the observed Pt LIII absorption edge with

is shown in Fig. 4. For comparison, the same quantity has been measured for a 500A film of Pt alone; this is also shown in Fig. 4. The shielding effect of the Cu film is shown clearly. The Pt fluorescence intensity varies more rapidly with

when shielded by Cu, and is suppressed completely in the vicinity of the critical angle for Cu.

What is the significance of these results? There is useful information contained in angular scans at fixed energy, if the sample is suspected to be non-homogeneous. Because vc ia so small, measurement of the R(9) curve is not easy. However, to obtain reliable and known control of the penetration depth by varying 9 , it is necessary. The fluorescence signal for the present samples had to come from the Pt film. However, the same element could be distributed within a layered structure. Thia is a possibility for the glasses described in 113; a surface layer of different density may be formed. Both the surface layer and the substrate will contain U, the element of interest. If the form of the layering can be established, then suitable values of

may be chosen for the measurement required. Even for non-homogeneous samples it would then be possible confidently to assert that a particular value of

corresponded to a certain penetration into the material.

Ref erencee

[11 Thornley F R, Barrett N T, Greaves G N and Antonini G M 1986 J.

Phys. C to be published.

Barrett N T, Antonini G M, Thornley F R and Greaves G N these proceedings.

Greaves G N these proceedings.

[ 2 }

Becker R S, Golovchenko

A and Pate1 J R 1983 Phys. Rev. Lett.

50 153.

[31 Born M and Wolf E 1980 Principles of Optics, Sixth Edition (Pergamon, Oxford).

143

International Tables for X-ray Crystallography, Val

1962

(Kynoch Prsas , Birmingham) .