Conversion Electron MÃ¶ssbauer Spectroscopy and Its Recent

3 Conversion Electron Mössbauer Spectroscopy and Its Recent Development

1

Downloaded by UNIV OF MONTANA on April 4, 2015 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch003

M. J. TRICKER

Department of Chemistry, Heriot-Watt University, Riccarton, Currie, Edinburgh EH14 4AS

The principles and applications of conversion electron Mössbauer spectroscopy (CEMS) are reviewed in detail. Consideration is given to the surface selectivity and sensitivity of C E M S , and experimental and theoretical aspects of the method are discussed in depth. Applications of C E M S in areas such as the oxidation of iron and steels, surface treat ment of steels, metallurgy, measurements of surface stress, ion-implantation, thin films, inorganic solids and minerals, and archeological materials are described.

n p h e m a j o r i t y o f Môssbauer s p e c t r o s c o p i c experiments a r e p e r f o r m e d -••in a transmission geometry

a n d involve the detection

r a d i a t i o n t r a n s m i t t e d t h r o u g h t h i n absorbers.

of g a m m a

I n this m o d e a w e a l t h o f

d a t a r e l a t i n g to t h e b u l k p r o p e r t i e s of solids m a y b e o b t a i n e d .

However,

i f i n f o r m a t i o n r e l a t i n g t o t h e surface p r o p e r t i e s o f solids i s sought, t h e use of t r a n s m i s s i o n m e t h o d s

i s r e s t r i c t e d t o r a t h e r s p e c i a l absorbers.

These m a y b e microcrystallites, either freely supported o r dispersed o n h i g h - a r e a i n e r t substrates, r a t h e r s p e c i a l solids w i t h h i g h i n t e r n a l surface areas s u c h as zeolites o r c l a y m i n e r a l s , o r s i m p l y stacks o f v e r y t h i n absorbers.

T o c i r c u m v e n t these restrictions a n d a l l o w t h e s t u d y o f t h e

surface a n d near-surface regions o f solids, t h e r e has b e e n a significant increase i n interest i n t h e p a s t f e w years i n b a c k s c a t t e r i n g t e c h n i q u e s b a s e d o n t h e d e t e c t i o n of c o n v e r s i o n electrons e m i t t e d f r o m t h e surface f o l l o w i n g t h e o c c u r r e n c e o f a resonant event i n t h e absorber.

Because

these electrons a r e a t t e n u a t e d r a p i d l y i n m a t t e r , o n l y those

electrons

Current address: The British Petroleum Company, Ltd., BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN, England. 1

0065-2393/81/0194-0063$09.50/0 © 1981 American Chemical Society In Mössbauer Spectroscopy and Its Chemical Applications; Stevens, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

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MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L APPLICATIONS

p r o d u c e d i n regions close to t h e surface escape t h e surface, a n d t h e r e s u l t i n g M o s s b a u e r s p e c t r u m is w e i g h t e d t o w a r d s t h e surface regions of the absorber.

A l t h o u g h , i n p r i n c i p l e , m a n y isotopes are a m e n a b l e to

s t u d y b y c o n v e r s i o n e l e c t r o n M o s s b a u e r spectroscopy studies to date h a v e i n v o l v e d either

5 7

F e or

1 1 9

(CEMS),

most

S n , a n d w o r k w i t h these

isotopes forms t h e m a i n c o n t e n t of this r e v i e w . T w o basic types of C E M S experiments m a y b e p e r f o r m e d .

The

first

of these i n v o l v e s the d e t e c t i o n of the t o t a l flux of b a c k s c a t t e r e d electrons w i t h o u t e n e r g y r e s o l u t i o n . T h i s a p p r o a c h w i l l b e r e f e r r e d to as i n t e g r a l CEMS.

I n the s e c o n d t y p e of e x p e r i m e n t the flux of b a c k s c a t t e r e d elec


trons is e n e r g y - r e s o l v e d , a n d M o s s b a u e r s p e c t r a are a c c u m u l a t e d u s i n g selected b a n d s of e l e c t r o n energies.

I t w i l l e m e r g e that t h i s t e c h n i q u e

a l l o w s the surface regions of solids to b e p r o b e d as a f u n c t i o n of d e p t h . A c c o r d i n g l y , t h i s t e c h n i q u e w i l l be r e f e r r e d to as d e p t h - r e s o l v e d v e r s i o n e l e c t r o n M o s s b a u e r spectroscopy CEMS

(DCEMS).

con

S i n c e t h e a r e a of

has b e e n r e v i e w e d i n the past ( 1 , 2 , 3 ) , emphasis h e r e w i l l

g i v e n to recent d e v e l o p m e n t s .

p r i n c i p l e s of the t e c h n i q u e a n d g i v e a b r o a d o v e r v i e w of t h e ments i n the area u p to a b o u t m i d - 1 9 7 6 . recent d e v e l o p m e n t s

be

H o w e v e r , t h e first section does o u t l i n e t h e develop

L a t e r sections w i l l d e a l w i t h

i n i n s t r u m e n t a t i o n , t h e o r e t i c a l aspects

and

data

r e d u c t i o n , a n d a p p l i c a t i o n s of C E M S .

Basic Principles and Overview of CEMS Internal Conversion and Backscattering Experiments.

F o r many

M o s s b a u e r n u c l i d e s the d e c a y of t h e e x c i t e d n u c l e a r s p i n state is h i g h l y internally converted (4). fied

T h e process o f i n t e r n a l c o n v e r s i o n is e x e m p l i

b y reference to T a b l e I , w h e r e t h e events t h a t o c c u r d u r i n g the

d e c a y of the I =

3 / 2 excited

s p i n state of

i n t e r n a l c o n v e r s i o n coefficient a for t h e 7 = i r o n is large, a n d o n l y a b o u t 1 0 %

5 7

F e are s u m m a r i z e d .

3/2 - » I =

The

i t r a n s i t i o n of

of the d e c a y events o c c u r b y t h e

e m i s s i o n of a 1 4 . 4 - k e V g a m m a p h o t o n . T h e p r e d o m i n a n t event is one of i n t e r n a l c o n v e r s i o n w h i c h results i n the e j e c t i o n of a 7 . 3 - k e V K - c o n v e r s i o n e l e c t r o n together w i t h s u b s e q u e n t A u g e r electrons a n d x - r a y p h o t o n s . I n t e r n a l c o n v e r s i o n also occurs i n the L - a n d M - s h e l l s , b u t w i t h l o w e r p r o b a b i l i t y , a n d leads to t h e p r o d u c t i o n of f u r t h e r c o n v e r s i o n electrons, A u g e r electrons, a n d x - r a y p h o t o n s . 2 3 . 8 - k e V t r a n s i t i o n of

1 1 9

A s i m i l a r s i t u a t i o n is f o u n d f o r the

S n , a l t h o u g h h e r e t h e K - c o n v e r s i o n process is

e n e r g e t i c a l l y f o r b i d d e n a n d t h e m a j o r i t y of electrons are 1 9 . 6 - k e V L c o n v e r s i o n electrons ( T a b l e I ) . I n v i e w of t h i s p h e n o m e n o n , i t is p o s s i b l e to r e c o r d M o s s b a u e r spectra i n a backscatter geometry b y detecting either the backscattered photons or electrons r a t h e r t h a n b y the m o r e u s u a l p r a c t i c e of d e t e c t i n g t h e

In Mössbauer Spectroscopy and Its Chemical Applications; Stevens, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1981.

3.

TRICKER

Conversion

Electron

Mossbauer

65

Spectroscopy

Table I. Summary of Major Events D u r i n g the Decay of 1=3/2 Excited-Spin States of F e and Sn° 57

Energy (keV)

Fe


57

119

Number (per 100) Absorption Events

y-photons X-x-rays K - c o n v e r s i o n electrons L - c o n v e r s i o n electrons M - c o n v e r s i o n electrons KLL-Auger electrons LMM-Auger electrons

14.4 6.3 7.3 13.6 14.3 5.4 0.53

9 27 81 9 1 63

Sn y-photons X-rays L - c o n v e r s i o n electrons L M M - A u g e r electrons

23.8 3.6 19.6 2.8

17 9 83 74

Approximate Maximum Range

250 n m 900 n m

119

2.4/im

° The maximum electron ranges are calculated using the Bethe-Bloch expression and are taken from Ref. 29.

transmitted g a m m a radiation ( F i g u r e 1 ) . I n a backscatter geometry, the r e q u i r e m e n t of a t h i n a b s o r b e r is r e m o v e d a n d t h i c k samples c a n b e e x a m i n e d i n a n o n d e s t r u c t i v e f a s h i o n . I f t h e b a c k s c a t t e r e d p h o t o n s are d e t e c t e d , i n f o r m a t i o n p e r t a i n i n g to t h e b u l k of t h e s o l i d o r r a t h e r t h i c k overlayers of a surface p h a s e w i l l b e o b t a i n e d , as t h e p a t h l e n g t h s of t h e g a m m a - a n d x - r a d i a t i o n are at least o n t h e o r d e r of m a g n i t u d e of m i c r o n s . H o w e v e r , because electrons are m u c h m o r e r a p i d l y a t t e n u a t e d i n m a t t e r , the b a c k s c a t t e r e d C E M s p e c t r u m w i l l b e w e i g h t e d t o w a r d s t h e o u t e r m o s t

Figure 1. Schematic of possible geometries for Mossbauer experiments with detection of (1) transmitted gamma-rays, (2) backscattered electrons, and (3) backscattered photons, x-rays, and/or gamma-rays


66


surface regions of the s a m p l e , as o n l y electrons p r o d u c e d close to t h e absorber surface w i l l escape the surface. T h e d e p t h s e l e c t i v i t y of a g i v e n e x p e r i m e n t w i l l therefore d e p e n d o n the e n e r g y s p e c t r u m of the electrons p r o d u c e d d u r i n g the d e c a y of t h e e x c i t e d n u c l e a r l e v e l of t h e u s e d , a n d o n t h e e n e r g y of the electrons d e t e c t e d .

isotope

A n i n d i c a t i o n of t h e

m a x i m u m ranges of the v a r i o u s electrons u s e d is g i v e n i n T a b l e I. Surface Selectivity and Sensitivity of C E M S . p r e v i o u s section t h a t C E M S

It follows f r o m the

opens u p t h e p o s s i b i l i t y of s t u d y i n g the

surface regions of l o w - a r e a solids b y M o s s b a u e r spectroscopic t e c h n i q u e s . I n the s i m p l e s t i n t e g r a l C E M S e x p e r i m e n t , first p e r f o r m e d b y S w a n s o n


a n d S p i j k e r m a n ( 5 ) , t h e b a c k s c a t t e r e d electrons are d e t e c t e d efficiently w i t h a 2?r c o l l e c t i o n g e o m e t r y b y m o u n t i n g t h e s a m p l e i n s i d e a H e / C H flow

p r o p o r t i o n a l counter.

significantly s m a l l e r t h a n i n t r a n s m i s s i o n e x p e r i m e n t s p e r u n i t strength.

4

H e r e the e l e c t r o n c o u n t rates a r e u s u a l l y source

H o w e v e r , s t r o n g sources c a n b e u s e d w i t h o u t fear of c a u s i n g

s a t u r a t i o n effects e i t h e r i n t h e detector or the c o u n t i n g electronics. t h i c k samples c o n t a i n i n g a n a t u r a l a b u n d a n c e of

5 7

For

F e , t h e signal-to-noise

ratios are c o m p a r a b l e to o r less t h a n those o b t a i n e d i n t r a n s m i s s i o n e x p e r i m e n t s . T h e b a c k g r o u n d arises l a r g e l y f r o m photoelectrons ejected f r o m t h e absorber enriched i n

5 7

a n d w a l l s of

the detector.

However,

if

samples

F e are u s e d , p e r c e n t a g e effects o n t h e o r d e r of h u n d r e d s

of a p e r c e n t m a y b e o b t a i n e d (6),

a n d d a t a a c c u m u l a t i o n times m a y b e

r e d u c e d to t h e o r d e r of m i n u t e s . A q u a l i t a t i v e i n d i c a t i o n of t h e surface s e l e c t i v i t y of

5 7

F e C E M S is

s h o w n i n F i g u r e 2 i n w h i c h the i n t e g r a l C E M s p e c t r u m of a n u n e n r i c h e d i r o n f o i l that h a d b e e n exposed b r i e f l y to m o i s t H C 1 v a p o r is s h o w n Apart from

t h e s i g n a l r e s u l t i n g f r o m t h e substrate, a d o u b l e t

(7). with

p a r a m e t e r s c o r r e s p o n d i n g to a h i g h - s p i n ferrous phase is c l e a r l y seen. T h i s d o u b l e t w a s not o b s e r v e d i n the t r a n s m i s s i o n m o d e after a c o m parable counting time. T h i s experiment demonstrated that

5 7

Fe

CEMS

is c a p a b l e of r e v e a l i n g t h e presence of surface phases t h a t w o u l d h a v e g o n e u n d e t e c t e d i f t r a n s m i s s i o n m e t h o d s h a d b e e n u s e d alone. f r o m this o b s e r v a t i o n , o t h e r studies u n e q u i v o c a b l y

Apart

demonstrated

the

p o t e n t i a l of C E M S as a surface t o o l i n s u c h areas as m e t a l l u r g y

(8),

ion-implantation chemistry

(9-16),

c o r r o s i o n a n d o x i d a t i o n (7,17-20),

a n d geo

(21,22).

O t h e r experiments e s t a b l i s h e d that the p r o b i n g d e p t h of

5 7

F e integral

C E M S is a p p r o x i m a t e l y 300 n m , a n d t h a t 6 6 % of t h e electrons in a H e / C H

4

c o u n t e r arise f r o m w i t h i n 54 n m of n a t u r a l i r o n foils

detected (5,19).

T h e s e n s i t i v i t y of the m e t h o d is s u c h t h a t a b o u t 10 n m of a n e w surface p h a s e m a y b e d e t e c t e d , a n d i t has b e e n d e m o n s t r a t e d t h a t i t s h o u l d b e p o s s i b l e to detect a m o n o l a y e r of substrate (23).

5 7

F e present o n a M o s s b a u e r i n e r t

A n o m a l i e s c o n c e r n i n g t h e m e a s u r e d m e a n - f r e e p a t h of


3.

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Conversion

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67

Spectroscopy


m a o o

i

I

f

i t „ VI LOCI TV MM I

•

J

1 _ Surface Science

Figure 2. T / i e F e CEM spectrum of an iron foil after brief exposure to HCl (7). The inner four peaks of the spectrum of the iron substrate are seen together with a new doublet arising from the surface species. 5 7

the

5 7

F e c o n v e r s i o n a n d A u g e r electrons w e r e r e s o l v e d w i t h the d i s c o v e r y

of a n u n e x p e c t e d c o m p o n e n t i n C E M s p e c t r a (24). ( J ) t h a t t h e m e a s u r e d p r o b i n g d e p t h s of

5 7

It h a d been noted

F e C E M S were larger than

e x p e c t e d w h e n c o m p a r e d to c a l c u l a t e d v a l u e s b a s e d o n p r e v i o u s l y m e a s u r e d e l e c t r o n m a s s - a b s o r p t i o n coefficients ( 2 5 ) .

T h e o r i g i n of this " e l e c

t r o n t a i l " is c a u s e d b y t h e p r o d u c t i o n of s o - c a l l e d g a m m a - a n d x - r a y c o r r e l a t e d photoelectrons

( X P E s and G P E s , respectively), produced i n

surface regions of the absorbers b y t h e M o s s b a u e r s p e c t r u m of g a m m a a n d x-rays b a c k s c a t t e r e d f r o m d e e p w i t h i n t h e s a m p l e .

For

5 7

F e , the

X P E s a n d G P E s c o n t r i b u t e a b o u t 1 0 % to t h e t o t a l flux of b a c k s c a t t e r e d electrons

(26).

T h e presence

of a s i m i l a r c o m p o n e n t

in

1 1 9

Sn C E M

spectra r e c e n t l y has b e e n c o n f i r m e d e x p e r i m e n t a l l y ( 2 7 ) . Depth Resolution by D C E M S .

It was noted earlier that H e / C H

detectors d o n o t p e r m i t the e n e r g y s p e c t r u m of the b a c k s c a t t e r e d

4

elec

trons to b e r e s o l v e d , a n d i n t h i s sense t h e y c a n b e r e g a r d e d as i n t e g r a l detectors. H o w e v e r , i f the electrons are e n e r g y - a n a l y z e d a n d s p e c t r a are accumulated w i t h

selected

e l e c t r o n energies,

each

of

the

individual

D C E M s p e c t r a w i l l b e w e i g h t e d t o w a r d s a p a r t i c u l a r d e p t h i n the s a m p l e , thus p r o v i d i n g t h e p o s s i b i l i t y of d e p t h p r o f i l i n g t h e i m m e d i a t e surface


68


> UJ


m i c r o ns Figure 3. Relation between energy and range of electrons of initial energy ((a) 7.3 keV, (b) 13.6 eV, and (c) 19.6 keV) calculated using the Bethe-Bloch expression (adapted from Ref. 29) regions. T h i s i d e a is i l l u s t r a t e d i n F i g u r e 3 i n w h i c h t h e m e a n e n e r g y loss is p l o t t e d against r a n g e f o r 1 1 9

S n L - c o n v e r s i o n electrons.

5 7

F e K - a n d L - c o n v e r s i o n electrons a n d

I t s h o u l d b e n o t e d t h a t s u c h a figure c a n

o n l y b e u s e d as a v e r y first a p p r o x i m a t i o n i n the i n t e r p r e t a t i o n of C E M s p e c t r a , as n o a l l o w a n c e f o r e l e c t r o n s c a t t e r i n g o r a b s o r p t i o n is m a d e . H o w e v e r , t h e figure does g i v e a n i n d i c a t i o n of t h e d e p t h s p r o b e d i n and

1 1 9

5 7

Fe

S n w o r k i f electrons of a p a r t i c u l a r e n e r g y are d e t e c t e d .

A degree of d e p t h p r o f i l i n g m a y b e a c h i e v e d e i t h e r w i t h detectors ( 6 )

He/CH

o r b y e v a p o r a t i n g i n e r t overlayers o n t o t h e s a m p l e

4

(28),

b u t m o r e a c c u r a t e w o r k r e q u i r e s t h e use of m o r e s o p h i s t i c a t e d e q u i p m e n t . I n a pioneering paper, Bonchev, Jordanov, a n d M i n k o v a (29)

described

t h e d e s i g n a n d use of a m a g n e t i c i r o n - f r e e b e t a - r a y s p e c t r o m e t e r i n t e r m e d i a t e i m a g e f o c u s i n g f o r use i n

1 1 9

eter h a d a n e n e r g y r e s o l u t i o n of a b o u t 5%

a n d a luminosity of about

T h e s e w o r k e r s w e r e a b l e to d e m o n s t r a t e t h a t t h e

1 1 9

2

2

8%.

S n C E M s p e c t r a of a

b r o m i n a t e d t i n m e t a l f o i l c o n s i s t e d of superpositions of peaks f r o m tt-Sn, S n 0 , S n B r , a n d S n B r .

with

S n experiments. T h e spectrom

arising

M o r e s i g n i f i c a n t l y , t h e area ratios of

4

t h e s p e c t r a l c o m p o n e n t s c h a n g e d w i t h spectrometer settings (i.e., c h a n g i n g e l e c t r o n e n e r g y ) i n a m a n n e r t h a t suggested consisted o f S n B r

4

overlaying SnBr .

a n d c o - w o r k e r s (30-33)

2

that the

overlayer

O t h e r groups, notably Baverstam

at S t o c k h o l m , c o n s t r u c t e d s i m i l a r spectrometers

a n d d e m o n s t r a t e d t h e f e a s i b i l i t y of m a k i n g d e p t h - r e s o l v e d measurements with

5 7

F e . D e v e l o p m e n t s i n t h i s area w i l l b e d e s c r i b e d i n m o r e d e t a i l i n

l a t e r sections.

Advances in Instrumentation Integral Detectors.

T h e m a j o r i t y of i n t e g r a l C E M S

experiments

p e r f o r m e d to d a t e h a v e b e e n c a r r i e d o u t u s i n g t h e u b i q u i t o u s H e / C H


4

3.

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flow p r o p o r t i o n a l counters. S u c h detectors a r e s i m p l e to c o n s t r u c t a n d a n u m b e r o f designs h a v e a p p e a r e d i n t h e l i t e r a t u r e (5,6,34-37). d e s i g n o f a t y p i c a l d e t e c t o r is i l l u s t r a t e d i n F i g u r e 4 (36).

The

A particular

f e a t u r e o f this d e t e c t o r is t h e s m a l l sensitive v o l u m e o f 2 5 0 m m

2

X 3mm,

thus e n s u r i n g t h a t t h e detector h a s v i r t u a l l y zero s e n s i t i v i t y f o r x - r a y a n d gamma-ray photons.

I n fact, f o r t h e case of

5 7

F e , a 3 - m m layer of

h e l i u m gas at o n e a t m o s p h e r e has o n l y a 0 . 0 1 % efficiency f o r t h e 6 . 3 - k e V x-rays a n d less t h a n 0 . 0 0 1 % f o r t h e 1 4 . 4 - k e V g a m m a r a y s . I n contrast, t h e b a c k s c a t t e r e d electrons a r e d e t e c t e d w i t h v i r t u a l l y 1 0 0 % efficiency i n a 27r-geometry.


unenriched

W i t h s u c h a d e v i c e , 2 0 % effects are o b t a i n a b l e w i t h

stainless-steel f o i l s .

T h e b a c k g r o u n d l a r g e l y arises

from

p h o t o - a n d A u g e r electrons ejected f r o m t h e s a m p l e a n d detector w a l l s b y 14.4-keV g a m m a rays a n d from the « 1 0 0 - k e V

photo- a n d C o m p t o n

electrons p r o d u c e d b y t h e 1 2 2 - k e V g a m m a r a y s . S i n c e these latter h i g h e n e r g y electrons d e p o s i t l i t t l e e n e r g y i n t h e gas, t h e i r c o n t r i b u t i o n t o t h e b a c k g r o u n d m a y be suppressed b y a correct choice

of discriminator

s e t t i n g (36).

Bulletin of the Institute for Chemical Research

Figure 4. The FLe\CFL flow proportional counter for CEMS studies (36). (A) Counter frame (Lucite); (B) anode wire (tungsten); (C) Teflon; (D) stainless-steel pipe; (E) Teflon pipe; (F) steel spring; (G) sample foil; (H) gas inlet; (I) aluminum foil; (J) aluminum-evaporated Mylar foil; (K) rubber sheet; (L) brass plate. k


70


A serious d r a w b a c k of the simplest H e / C H

detectors just r e f e r r e d

4

to is t h a t it is not possible to v a r y the s a m p l e t e m p e r a t u r e a n d thus e x p l o r e t h e M o s s b a u e r p a r a m e t e r s of the s a m p l e as a f u n c t i o n of t e m p e r a t u r e . Suitably modified H e / C H

4

detectors h a v e b e e n d e s c r i b e d t h a t operate

satisfactorily at 80 K (38,39).

A t 4.2 K the C H

q u e n c h gas has to b e

t

o m i t t e d , a n d this c a n l e a d to u n d e s i r a b l e c o u n t i n g characteristics I s o z u m i (41)

r e c e n t l y has d e s c r i b e d a H e / C H

t e m p e r a t u r e s u p to 2 9 0 ° C .

4

(40).

detector that operates at

T h e detector is of c o n v e n t i o n a l d e s i g n b u t

the b o d y is of T e f l o n r a t h e r t h a n the m o r e c o m m o n l y u s e d L u c i t e . T h e entire detector is l o c a t e d i n a f u r n a c e a n d is filled w i t h H e / C H


use.

before

4

T h e u p p e r w o r k i n g t e m p e r a t u r e is l i m i t e d b y the p r o d u c t i o n

of

s p u r i o u s signals at 290° C c a u s e d b y e l e c t r i c d i s c h a r g e at a T e f l o n p i p e s u r r o u n d i n g the a n o d e l e a d . T h i s a u t h o r also discusses the h i g h - t e m p e r a t u r e e l e c t r i c a l characteristics of other m a t e r i a l s i n r e l a t i o n to t h e i r use i n detector c o n s t r u c t i o n . W e y e r a n d others (42,43) counter.

have described a parallel-plate avalanche

T h e c o n s t r u c t i o n is s i m i l a r to a H e / C H

4

detector except t h a t

t h e t h i n w i r e anodes are r e p l a c e d b y p a r a l l e l plates b e t w e e n w h i c h t h e gas m u l t i p l i c a t i o n takes p l a c e . 1 1 9

Sn,

1 8 1

T a , and

1 6 1

C E M spectra w e r e o b t a i n e d w i t h

Fe,

Dy.

A n a l t e r n a t i v e a p p r o a c h to gas detectors detectors

5 7

s u c h as c h a n n e l t r o n s or o p e n - e n d e d

is to use other

electron

p h o t o n - m u l t i p l i e r tubes.

If this is done, the s a m p l e a n d detector m u s t necessarily b e m o u n t e d i n a v a c u u m c h a m b e r , b u t u n d e r these c o n d i t i o n s no difficulties are e n countered i n v a r y i n g the sample temperature. O s w a l d a n d O h r i n g

(44)

h a v e d e s c r i b e d a s i m p l e a p p a r a t u s i n w h i c h t h e electrons scattered f r o m the s a m p l e surface are c o l l e c t e d i n t h e c o n e of a h o m e m a d e

channeltron

that is c a r e f u l l y s h i e l d e d f r o m the i n c i d e n t g a m m a photons.

A

40%

effect ( o n L i n e s 1 a n d 6 ) w a s o b t a i n e d u s i n g a 5 0 0 0 - A t h i c k e n r i c h e d i r o n f o i l at a c o u n t rate of several h u n d r e d s p e r s e c o n d w i t h a 5 - m C i source. based

Jones a n d c o - w o r k e r s

(45)

have described a similar apparatus

on a commercially available channeltron.

i n c l u d e d f a c i l i t i e s to c o o l the absorber to 80 K .

T h i s apparatus O n e disadvantage

also of

arrangements of this t y p e is that because of the s m a l l size of t h e c h a n n e l t r o n cone ( « l c m ) , angle are detected. scattered electrons

o n l y electrons scattered i n t o a r a t h e r s m a l l s o l i d

This situation can be i m p r o v e d b y focusing

the

into the cone of t h e c h a n n e l t r o n u s i n g a u n i f o r m

l o n g i t u d i n a l m a g n e t i c field (46)

(Figure 5).

F o r t h e case of

5 7

F e , the

7 . 3 - k e V K - c o n v e r s i o n electrons are b r o u g h t to focus i n the c h a n n e l t r o n c o n e w h i c h is l o c a t e d 245 m m f r o m t h e a b s o r b e r i n a field of «=*60 G . I t w a s f u r t h e r o b s e r v e d t h a t a p p l i c a t i o n of a s m a l l p o s i t i v e p o t e n t i a l , « 2 0 0 V , to the c h a n n e l t r o n cone i n c r e a s e d t h e u s e f u l c o u n t rate b y T h i s effect arises f r o m t h e c o l l e c t i o n of l o w - e n e r g y s e c o n d a r y

100%.

electrons



3.

Conversion

TRICKER

Electron

HIGH

Mossbauer

71

Spectroscopy

VOLTAGE Journal of Physics

Figure 5.

Schematic

of low-temperature CEMS neltron detector (46)

apparatus using a chan-

f r o m t h e w a l l s of the detector p r o d u c e d b y c o n v e r s i o n electrons t h a t miss t h e detector. a 100-mCi

5 7

Co

C o u n t rates o f 20,000 c o u n t s / m i n w e r e o b t a i n e d w i t h source

with 10%

effects f o r n a t u r a l i r o n f o i l s .

An

a d d i t i o n a l a n d significant feature of the d e v i c e is t h a t the s a m p l e m a y b e c o o l e d to 4.2 K . A s i m i l a r a p p a r a t u s has b e e n d e s c r i b e d b y T i b y

(47).

A n e x t r e m e l y v e r s a t i l e d e v i c e f o r C E M S w o r k has b e e n d e s c r i b e d Carbuccichio (48).

by

T h e electrons are d e t e c t e d u s i n g a n E M I 9 6 4 3 / 2 B

o p e n - e n d e d p h o t o n - m u l t i p l i e r t u b e . T h e d e v i c e i n c o r p o r a t e s f a c i l i t i e s for e l e c t r o n d e t e c t i o n as w e l l as f o r i n - s i t u t r e a t m e n t of the s a m p l e , c o n t r o l l i n g t h e s a m p l e t e m p e r a t u r e i n t h e r a n g e 80 to 800 K a n d h a v i n g a n e x t e r n a l l y m o u n t e d detector to c o u n t b a c k s c a t t e r e d p h o t o n s . T h e c a p a b i l i t i e s of t h e a p p a r a t u s are i l l u s t r a t e d i n F i g u r e 6, w h e r e the C E M S a n d b a c k s c a t t e r e d x - r a y s p e c t r a of a l i g h t l y o x i d i z e d u n e n r i c h e d i r o n f o i l are s h o w n . CEM

s p e c t r u m reveals t h e presence of a t h i n ( « 1 0 0

overlayer.

nm)

iron

The oxide

T h e c o u n t i n g times w e r e 5 a n d 3 d a y s f o r t h e C E M a n d

x-ray spectra, respectively, u s i n g a 5 - m C i Beta-Ray Spectrometers

5 7

C o source.

for D C E M S Studies.

selective m e a s u r e m e n t s , a n u m b e r of

groups

T o perform depth-

(30-33,45,49-52)

have

c o n s t r u c t e d m a g n e t i c b e t a - r a y spectrometers s i m i l a r to t h a t d e s c r i b e d b y Bonchev

(29)

( r e f e r r e d to e a r l i e r ) .

resolutions o f « 5 %

S u c h spectrometers

a n d transmission of « 8 % .

have

energy

B e c a u s e the e l e c t r o n

s p e c t r u m is n o w e n e r g y - r e s o l v e d , t h e c o u n t rates are l o w e r t h a n those o b t a i n e d w i t h i n t e g r a l detectors, a n d t h e use of e n r i c h e d samples is



72


-6

-4

-2

0

2

4

velocity (mm I sec) Nuclear Instruments and Methods

Figure 6. Room-temperature Mossbauer spectra of 6.3-keV x-rays (a) and electrons (b) for a natural iron sample previously heated for 10 min in air at 350° C, obtained using the apparatus of Carbucicchio (48). (I) Sextet due to metallic iron; (II) sextet due to Fe O ; (III and IV) sextets due to Fe O . 2

s

s

h

u n a v o i d a b l e i f d a t a a c q u i s i t i o n t i m e s are to

be

realistic.

A

typical

m a g n e t i c b e t a - r a y spectrometer is s h o w n s c h e m a t i c a l l y i n F i g u r e 7.

In

the d e v i c e , t h e electrons of different energies e m i t t e d f r o m the s a m p l e surface are b r o u g h t to focus o n the detector b y c h a n g i n g t h e c u r r e n t flowing

i n t h e coils.

T h e e l e c t r o n s p e c t r u m of a n i r o n f o i l i n t h e " i n

r e s o n a n c e " c o n d i t i o n is s h o w n i n F i g u r e 8 w h e r e i t c a n b e seen t h a t t h e 1 3 . 6 - k e V L - a n d K - c o n v e r s i o n electrons are c l e a r l y r e s o l v e d f r o m another. C E M s p e c t r a of a eter settings ^7

fluorinated

one

iron foil obtained w i t h spectrom

k e V a n d ^ 1 3 . 6 k e V are s h o w n i n F i g u r e 9. I t c a n b e

seen t h a t t h e d o u b l e t a r i s i n g f r o m the surface p h a s e is r e l a t i v e l y m o r e


3.

Conversion

THICKER

Electron

Mossbauer

Spectroscopy

73


MOVING SAMPLE

VACUUM

['* ton) *

'

A • LEAD APERTURES Applications of Surface Science

Figure 7. Essential features of a magnetic beta-ray spectrometer (45). The radius of the magnetic coils is 10 cm and the sample detector distance is 26 cm.

Applications of Surface Science

Figure 8. Electron energy spectrum obtained from a 90%-enriched Fe foil illuminated by a moving Co source using a magnetic beta-ray spectrometer (45) 57

57



74

MOSSBAUER SPECTROSCOPY A N D ITS C H E M I C A L

—I •8

l 0

1

*4

,

VELOCITY

1 -4 mm s

APPLICATIONS

L 8 1

Figure 9. The Fe DCEM spectra of a fluorinated iron foil using mainly (a) K-conversion electrons (lower) and (b) L-conversion electrons (upper) 57

(45). The surface phase is manifest as a doublet with one resolved peak at « 0 mms' and another obscured by Line 3 of the iron substrate spectrum. Note the enhanced intensity doublet in the spectrum obtained with the K-conversion electrons compared to the spectrum obtained with the L-conversion electrons. 1

intense i n t h e K - c o n v e r s i o n e l e c t r o n s p e c t r u m t h a n i n t h e L - c o n v e r s i o n electron spectrum.

I t is of interest to note t h a t t h e ratios of t h e a r e a

of t h e substrate-to-surface s i g n a l i n t h e L - c o n v e r s i o n e l e c t r o n s p e c t r u m is c o m p a r a b l e to t h a t o b t a i n e d u s i n g a H e / C H

4

detector, thus e m p h a

s i z i n g t h e c o n t r i b u t i o n of t h e L - c o n v e r s i o n electrons, t h e X P E s , a n d t h e G P E s i n t h e latter s p e c t r u m .

F u r t h e r degrees of d e p t h r e s o l u t i o n m a y


3.

TRiCKER

Conversion

Electron

Mossbauer

Spectroscopy

b e o b t a i n e d b y r e c o r d i n g s p e c t r a at spectrometer K-edge.

75

settings a l o n g t h e

A s a n e x a m p l e of t h i s m o d e of o p e r a t i o n , F i g u r e 10 shows t h e

spectra of a stainless-steel f o i l c o v e r e d w i t h 36 n m of i r o n r e c o r d e d at v a r i o u s spectrometer settings b y B a v e r s t a m et a l . ( 3 2 ) .

T h e change i n

surface-to-substrate s i g n a l is c l e a r l y seen to b e a f u n c t i o n of the spec t r o m e t e r setting. T h e d e t a i l e d analysis of s p e c t r a of this t y p e w i l l

be

d e s c r i b e d later. O v e r the p a s t f e w years, significant a d v a n c e s h a v e b e e n m a d e i n t h e d e s i g n a n d c o n s t r u c t i o n of electrostatic b e t a - r a y spectrometers, n o t a b l y T h e y have described

(53)

the design a n d


b y the Stockholm group.

(c) Nuclear Instruments and Methods

Figure 10. The Fe DCEM spectra of a 360-A thick layer of iron on a stainless-steel substrate obtained at various electron energies along the K-edge (32). Note the relative enhancement of the iron signal relative to the stainless-steel signal as the electron energy is increased (bottom to top). 57



76


Nuclear Instruments and Methods

Figure 11. Drawing of the main parts of the electrostatic spectrometer (53): (C ) inner cylinder (radius = 54 mm); (C ) outer cylinder (radius = 158 mm); (G) thin grids; (A) absorber (i.e., electron source) (B) detector baffle; (D) detector t

2

c o n s t r u c t i o n of a n electrostatic c y l i n d r i c a l m i r r o r spectrometer 11) o p e r a t i n g at a 2 . 5 % e n e r g y r e s o l u t i o n , a 6 % 5 - m m d i a m e t e r e l e c t r o n source.

(Figure

luminosity, and w i t h a

T h e d e s i g n of t h e spectrometer

was

o p t i m i z e d u s i n g a c o m p u t e r p r o g r a m , a n d i t is p r i m a r i l y i n t e n d e d for depth-resolution w o r k w i t h

5 7

Fe.

T h e e l e c t r o n s p e c t r u m of a n i r o n f o i l

i n t h e " i n r e s o n a n c e " c o n d i t i o n is s h o w n i n F i g u r e 12, w h e r e i t c a n b e seen t h a t t h e 5.6-eV A u g e r p e a k is c l e a r l y r e s o l v e d f r o m t h e K-conversion electron peak.

7.3-keV

E x p e r i m e n t s w i t h this a p p a r a t u s w i l l

be

d i s c u s s e d i n a l a t e r section. B e n c z e r - K o l l e r a n d K o l k (54)

have built a h i g h transmission spheri

c a l electrostatic spectrometer w i t h a t r a n s m i s s i o n of 7 %

a n d an energy


3.

TRiCKER

Conversion

r e s o l u t i o n of 1 . 5 %

Electron

Mossbauer

77

Spectroscopy

at 14.4 k e V f o r a 1-cm d i a m e t e r source. T h i s spec

t r o m e t e r is p r i m a r i l y i n t e n d e d f o r t h e m e a s u r e m e n t of

5 7

Fe

L - and

M - i n t e r n a l c o n v e r s i o n coefficients, w h e r e g o o d r e s o l u t i o n o f t h e M - a n d L - c o n v e r s i o n e l e c t r o n p e a k s is necessary. T o r i y a m a a n d c o - w o r k e r s

(55)

have described a retarding-field electron spectrometer w i t h a n energy range 0-20 k e V .

U s i n g a h o t f i l a m e n t as a source of

electrons, t h e

r e s o l u t i o n w a s 0 . 1 % at 1 % t r a n s m i s s i o n . H o w e v e r , f o r c o n v e r s i o n elec trons t h e t r a n s m i s s i o n w a s f o u n d to b e a n o r d e r of m a g n i t u d e w o r s e . T h e system i n c o r p o r a t e d a v a c u u m e v a p o r a t o r for s a m p l e p r e p a r a t i o n .

Advances in Theoretical Aspects and Data Reduction Downloaded by UNIV OF MONTANA on April 4, 2015 | http://pubs.acs.org Publication Date: July 1, 1981 | doi: 10.1021/ba-1981-0194.ch003

A m a j o r o b j e c t i v e i n t h e field of C E M S is t h e e x t r a c t i o n of i n f o r m a t i o n f r o m t h e r a w d a t a c o n c e r n i n g t h e w a y i n w h i c h the M o s s b a u e r p a r a m K conv

5000

5500

6000

(Volts) Nuclear Instruments and Methods

Figure 12. Experimental (- • -) and computer-simulated ( ) line profiles from a thin Co source with radius = 5 mm, obtained using the spectrometer shown in Figure 11 (53). 57

The inserted figure shows the spectrometer profile used for convolution with the simulated K-conversion-electron energy-loss distribution; it is constructed on the basis of the computed spectrometer profile. A source thickness of 47 fig/cm (corresponding to 600 A of iron) was used in the electron scattering simulation. 2


78


eters v a r y as a f u n c t i o n of d e p t h . T h e s e p a r a m e t e r s m a y t h e n b e r e l a t e d to t h e v a r i a t i o n s of the p r o p e r t i e s of t h e s a m p l e w i t h d e p t h a n d / o r the n a t u r e a n d d i s t r i b u t i o n of surface phases. w i t n e s s e d significant advances

T h e past f e w

i n this area a n d these

years

have

advances

have

u n d e r p i n n e d the essentially p r a g m a t i c a n d e m p i r i c a l a p p r o a c h to d a t a r e d u c t i o n a d o p t e d b y m a n y w o r k e r s . T h i s section is a g a i n d i v i d e d i n t o t w o parts d e a l i n g w i t h d a t a o b t a i n e d w i t h i n t e g r a l d e t e c t i o n a n d e n e r g y r e s o l v e d studies.


I n t e g r a l C E M S Studies.

M a n y of t h e p r a c t i c a l p r o b l e m s

amenable

to s t u d y b y C E M S i n v o l v e the c h a r a c t e r i z a t i o n of r a t h e r discrete

over-

layers of, for example, a corrosion p r o d u c t , o n a t h i c k substrate.

It is

c l e a r l y i m p o r t a n t to d e v e l o p m e t h o d s

whereby

the thickness of

substrates m a y be extracted f r o m i n t e g r a l C E M S measurements.

such Much

of the w o r k i n this area so f a r has a s s u m e d a s i m p l e e x p o n e n t i a l l a w f o r the a t t e n u a t i o n of electrons i n m a t t e r .

Bainbridge (56)

has

extended

t h e early w o r k of K r a k o w s k i a n d M i l l e r ( 5 7 ) a n d has d e r i v e d expressions f r o m w h i c h t h e thickness of i n d i v i d u a l layers w i t h i n a m u l t i p l e x film m a y be extracted f r o m e x p e r i m e n t a l s p e c t r a , p r o v i d i n g the c o m p o s i t i o n o r d e r of these layers is k n o w n . H e discusses t h e case of

5 7

and

F e C E M S and

o n l y i n c l u d e s the K - c o n v e r s i o n electrons i n his analysis. H u f f m a n n (58, 5 9 ) has d e r i v e d s i m i l a r expressions to extract q u a n t i t i e s of interest f r o m CEM

spectra.

Both

5 7

F e and

1 1 9

Sn C E M S

were considered,

a n d the

effects of c o n v e r s i o n a n d A u g e r electrons w e r e i n c l u d e d i n t h e t h e o r y . To

use theories

absorption

of

this t y p e , a k n o w l e d g e

coefficients

is necessary.

(fi)

of t h e

appropriate

Mass absorption

mass

coefficients

s h o u l d t h e o r e t i c a l l y b e d e r i v a b l e f r o m first p r i n c i p l e s , b u t i n p r a c t i c e are m o r e often c a l c u l a t e d f r o m e m p i r i c a l l a w s s u c h as those of C o s s l e t t a n d T h o m a s (25).

A n a l t e r n a t i v e a p p r o a c h is to d e r i v e t h e m f r o m c a l i b r a t i o n

experiments i n v o l v i n g C E M S

studies of substrates c o a t e d w i t h k n o w n

thicknesses of w e l l - c h a r a c t e r i z e d overlayers. S u c h experiments h a v e b e e n p e r f o r m e d b y T h o m a s et a l . (19)

for t h e case of

5 7

F e C E M S b y t h e use

of stainless-steel substrates c o a t e d w i t h k n o w n thicknesses of i r o n u s i n g a He/CH (substrate

4

detector.

T h e a r e a r a t i o of the o v e r l a y e r to t o t a l s i g n a l

and overlayer)

was

thickness, a n d a v a l u e of fi =

measured

1.3 X

as a f u n c t i o n of

10 c m g 4

2

_ 1

overlayer

was derived. T h i s

is r o u g h l y h a l f t h a t e x p e c t e d f o r t h e K - c o n v e r s i o n electrons f r o m the f o r m u l a of C o s s l e t t a n d T h o m a s ( 2 5 ) .

figure

calculated

U s i n g this v a l u e , a c u r v e

r e l a t i n g the o x i d e o v e r l a y e r thickness o n a n i r o n substrate to the s p e c t r a l area ratios c a n b e c a l c u l a t e d p r o v i d e d t h a t p is insensitive to c h a n g e i n a t o m i c n u m b e r (19,60). performed similar

5 7

G r a h a m , M i t c h e l l , a n d C h a n n i n g (61)

have

F e C E M S c a l i b r a t i o n experiments b y g r o w i n g m a g

netite films i n the r a n g e 2 6 5 - 4 2 5 0 A o n n a t u r a l i r o n substrates. T h e o x i d e thicknesses w e r e m o n i t o r e d d u r i n g g r o w t h b y

measuring the


oxygen

3.

Conversion

TRICKER

Electron

Mossbauer

79

Spectroscopy

u p t a k e of t h e foils a n d c a l c u l a t e d b y a s s u m i n g s m o o t h surfaces. percentage

(P)

o x i d e w a s d e t e r m i n e d as a f u n c t i o n of thickness (d) a l a w of the f o r m d(k) data were

The

of the t o t a l C E M s p e c t r u m a r e a c o n t r i b u t e d b y

compared

Bainbridge (56).

=

the

a n d f o u n d to f o l l o w

- 1 . 9 5 X 1 0 In ( 1 - 0 . 0 1 P ) . T h e e x p e r i m e n t a l 3

w i t h the predictions

of

Huffmann

C a l c u l a t i o n s of P b a s e d o n p-values

(58,59)

and

calculated from

the expression of C o s s l e t t a n d T h o m a s ( 1 9 ) a n d t h e H u f f m a n n t r e a t m e n t ( 5 8 , 5 9 ) y i e l d e d v a l u e s of P 3 0 to 4 0 % data. 1.10 X

10

c m V

4

a n d 1.73 X

electron, respectively.


higher than the experimental

B e t t e r fits to t h e d a t a w e r e o b t a i n e d w i t h v a l u e s of fi e q u a l to 10

4

c m V

for the 7.3-keV a n d 5.4-keV

H u f f m a n n a n d P o d g u r s k i (62)

have

performed

s i m i l a r experiments b u t i n v o k e d changes i n surface roughness to e x p l a i n t h e n o n l i n e a r i t y of plots of o v e r l a y e r thickness d e r i v e d f r o m the

CEMS

d a t a vs. t h e o x y g e n u p t a k e m e a s u r e m e n t s . S u b s e q u e n t to these studies, a c o m p r e h e n s i v e s t u d y of the i n t e n s i t y a n d e n e r g y d i s t r i b u t i o n of i n i t i a l l y m o n o c h r o m a t i c from an absorber

surface

L i l j e q u i s t et a l . ( 2 6 ) .

was made

5 7

emanating by

T h e t o t a l t r a n s m i s s i o n as w e l l as t h e t r a n s m i s s i o n

into various angular and energy depths i n a

electrons

using M o n t e C a r l o methods

i n t e r v a l s of

electrons

F e absorber w e r e c a l c u l a t e d . T h e

from

different

L - , Auger, G P E , and

X P E electrons w e r e a l l i n c l u d e d i n t h e c a l c u l a t i o n s . T h e s c a t t e r i n g a n d energy loss of the electrons w e r e c o m p u t e d for i r o n , F e 0 , a n d a l u m i n u m . 2

3

It w a s f o u n d that t h e difference i n the results w a s n e g l i g i b l e a n d t h a t t h e d e p t h m a y b e m e a s u r e d as m a s s / u n i t area.

T h e results of t h e i r

c a l c u l a t i o n s w e r e c o m p a r e d w i t h the e x p e r i m e n t a l l y m e a s u r e d area ratios of the stainless-steel i r o n s a n d w i c h d e s c r i b e d

earlier.

T h e results are

i l l u s t r a t e d i n F i g u r e s 13 a n d 14, w h e r e i t c a n b e seen t h a t a n excellent fit to t h e

data was

obtained.

a c c o u n t f o r a l l t h e electrons designed

to

These

figures

emphasize

the

need

c o n t r i b u t i n g to t h e s i g n a l i n a n y

extract i n f o r m a t i o n f r o m

integral

5 7

Fe

CEMS

to

theory

data.

In

a d d i t i o n , these figures s h o w t h a t t h e use of a n y t h e o r y that neglects t h e c o n t r i b u t i o n s f r o m L - c o n v e r s i o n , G P E , a n d X P E electrons to t h e t o t a l flux, w h e n c o m b i n e d w i t h the p v a l u e s of C o s s l e t t a n d T h o m a s ( 2 5 )

will

l e a d to p o o r estimations of o v e r l a y e r t h i c k n e s s , regardless of e i t h e r t h e correctness of the //.-values or t h e a p p r o p r i a t e n e s s of t h e e x p o n e n t i a l l a w . The

apparent

discrepancies

between

the /i-values

derived

from

the

c a l i b r a t i o n experiments just d e s c r i b e d arise f r o m t h e f a c t t h a t the v a l u e measured by Thomas

et a l . (19)

is s i m p l y a n effective

value for a l l

electrons d e t e c t e d , w h e r e a s t h e v a l u e s d e r i v e d b y G r a h a m et a l .

(61)

are effective values a r i s i n g f r o m t h e expressions u s e d . H o w e v e r , there is a r e a l d i s c r e p a n c y b e t w e e n the o x i d e thicknesses d e r i v e d b y t h e m e t h o d of T h o m a s et a l . ( 2 5 ) a n d those d e r i v e d b y G r a h a m et a l . ( 6 1 ) .

The origin

of this d i s c r e p a n c y is not u n d e r s t o o d a n d w a r r a n t s f u r t h e r i n v e s t i g a t i o n .



80

MOSSBAUER SPECTROSCOPY AND ITS CHEMICAL APPLICATIONS


Figure 13. Relative contribution from various electrons in the integral CEM spectrum of an iron overlayer on a stainless-steel substrate as a function of overlayer thickness (d is the thickness of the iron overlayer) (26). Fe

I n r e l a t i o n s h i p to F i g u r e 13, i t is of interest to note t h e e n h a n c e d d e p t h s e l e c t i v i t y g a i n e d b y d e t e c t i n g the K - c o n v e r s i o n electrons alone r a t h e r t h a n w i t h t h e i n t e g r a t e d signals, as m e n t i o n e d e a r l i e r i n c o n n e c t i o n w i t h the s p e c t r a of

fluorinated

iron foils.

T h e size of t h e resonant M o s s b a u e r effects o n /?-Sn, C a S n 0 , a n d 3

Sn0

2

have been measured using

retical values ( 2 7 ) .

1 1 9

S n C E M S a n d compared w i t h theo

T h e m e a s u r e d p e r c e n t a g e effects w e r e 4 6 % , 5 2 0 % ,

a n d 5 1 0 % f o r £-Sn, S n 0 , a n d C a S n 0 , r e s p e c t i v e l y . 2

3

Good

agreement

w a s o b t a i n e d b e t w e e n e x p e r i m e n t a n d t h e o r y , i f the effects of n o n i s o tropic scattering of

photoelectrons

anomalously large percentage

are i n c l u d e d i n the theory.

effect of 9 5 0 %

for S n 0

2

measured

The by

Y a g n i k et a l . ( 6 3 )

w a s s h o w n t o arise f r o m a n i n a d e q u a t e c u r v e - f i t t i n g

procedure

by

caused

neglect

of

quadrupole

l i n e b r o a d e n i n g of

the

resonance. Depth-Resolved C E M S Studies. ratios of t h e c o m p o n e n t s

A s pointed out earlier, the area

of C E M spectra of i n h o m o g e n e o u s

absorbers

r e c o r d e d at different spectrometer settings w i l l v a r y a n d c o n t a i n i n f o r m a -



3.

THICKER

Conversion

Electron

Mossbauer

81

Spectroscopy


Figure 14.

Relative stainless-steel signal in an iron-on-steel CEMS gral) measurement (26).

(inte-

Dots show experimental results from Thomas et al. Full curve (a) shows the result predicted by the present theory. Dashed curve (b) shows the predicted result if the XPE and GPE electrons are neglectd. Full curve (c) shows the predicted result if the APK interval is selected in the detector (all angles). Dashed curve (d) shows the same result if the GPE and L-conversion electrons are neglected. d*> is the thickness of the iron layer.

t i o n r e l a t i n g to the d e p t h d i s t r i b u t i o n of t h e M o s s b a u e r p a r a m e t e r s . B a v e r s t a m et a l . (32)

h a v e s h o w n t h a t t h e n u m b e r of counts r e c o r d e d i n

t h e n t h c h a n n e l i n a M o s s b a u e r s p e c t r u m b y means of scattered electrons at a spectrometer s e t t i n g c o r r e s p o n d i n g to a n energy E m a y b e w r i t t e n as w(E,x)P(x) dx

T(E) w h e r e w(E,x)

n

is a ( w e i g h t ) f u n c t i o n g i v i n g the p r o b a b i l i t y of a n e l e c t r o n

o r i g i n a t i n g at d e p t h x to b e d e t e c t e d i f the spectrometer s e t t i n g is £ . P(x)

n

is t h e e m i s s i o n p r o b a b i l i t y f o r electrons at d e p t h x w h e n

g a m m a source v e l o c i t y corresponds not

w(E,x) electrons

only

describes

to the n t h c h a n n e l .

the intensity a n d energy

the

The function d i s t r i b u t i o n of

e m a n a t i n g f r o m t h e surface, b u t also c o n t a i n s i n f o r m a t i o n

r e l a t i n g t o t h e response

of

the detection

device.

Using

a

magnetic


82


b e t a - r a y spectrometer,

B a v e r s t a m et a l . (33)

c a r r i e d o u t a series

of

experiments i n v o l v i n g the s c a t t e r i n g of electrons w i t h energies close to 7.3 k e V t h r o u g h t h i n i r o n G i v e n w(E,x),

w(E,x).

films,

P(x)

n

i n o r d e r to d e t e r m i n e t h e

function

can now, i n principle, be extracted f r o m

a series of C E M spectra a c c u m u l a t e d at different s p e c t r o m e t e r settings Ej as

£

T(Ej) =


n

where P

i n

[ J^

w{E x)dx^

1

h

P

j —1,2, 3,...

i n

is u n k n o w n , / the n u m b e r o f s p e c t r a r e c o r d e d , I t h e n u m b e r of

layers separated, a n d U t h e l i m i t s of these layers. A p r a c t i c a l e x a m p l e of this t e c h n i q u e is s h o w n i n F i g u r e 15. H e r e the separate signals a r i s i n g f r o m the substrate a n d o v e r l a y e r h a v e b e e n e x t r a c t e d f r o m t h e series of spectra s h o w n i n F i g u r e 10.

T h e d e p t h r e s o l u t i o n of the m e t h o d w a s

e s t i m a t e d to be a b o u t 50 A f o r t h e i r m a g n e t i c b e t a - r a y

spectrometer

w h i c h h a d a 5 % energy resolution. M o r e r e c e n t l y these w o r k e r s (26)

have calculated weight functions

( F i g u r e 16) u s i n g the M o n t e C a r l o - t y p e c a l c u l a t i o n s d e s c r i b e d p r e v i o u s l y , i n the energy i n t e r v a l 6.3-7.3 k e V c o r r e s p o n d i n g to a l m o s t p u r e K - c o n v e r s i o n electrons. T h e s o l i d l i n e i n F i g u r e 13 is i n fact c o m p u t e d o n the basis of these f u n c t i o n s c o n v o l u t e d w i t h a s u i t a b l e s p e c t r o m e t e r

line

shape. T h e i n t e r p r e t a t i o n a n d p r a c t i c a l analysis of d e p t h - s e l e c t i v e C E M spectra also h a v e b e e n d i s c u s s e d u s i n g a s i m p l i f i e d t h e o r y (64).

Expedi

ent analysis i n terms of a b s o r b e r s t r u c t u r e has d e m o n s t r a t e d t h a t D C E M S reveals m o r e f u n d a m e n t a l i n f o r m a t i o n t h a n i n t e g r a l C E M S . B o n c h e v a n d c o - w o r k e r s (64) to the i n t e r p r e t a t i o n of

1 1 9

have developed an empirical approach

S n data. T h e method was based on the experi

m e n t a l d e t e r m i n a t i o n of the c h a n g e i n energy d i s t r i b u t i o n of L - c o n v e r s i o n electrons e m i t t e d f r o m a source t h a t w a s p r o g r e s s i v e l y c o v e r e d w i t h t h i n a b s o r b i n g layers of c o p p e r i n t h e r a n g e 0.02-0.25 m g c m " . U s i n g these 2

d a t a the d i s t r i b u t i o n of

1 1 9

S n i n u n k n o w n samples m a y b e d e t e r m i n e d

either b y s u i t a b l y c o n s t r u c t e d n o m o g r a m s o r b y s o l v i n g a series of l i n e a r equations w i t h e x p e r i m e n t a l l y d e t e r m i n e d coefficients. I n a l a t e r extension of this w o r k , B o n c h e v a n d c o - w o r k e r s (66,67)

i n v e s t i g a t e d the i n f l u e n c e

of a t o m i c n u m b e r , c r y s t a l s t r u c t u r e , a n d a p p l i e d e l e c t r i c field o n the d i s t r i b u t i o n of layers.

It

1 1 9

was

S n c o n v e r s i o n electrons after passage t h r o u g h s u i t a b l e demonstrated

that e l e c t r o n

energy

distribution was

d e p e n d e n t o n a t o m i c n u m b e r b y p e r f o r m i n g experiments w i t h overlayers of b e r y l l i u m , c o p p e r , s i l v e r , a n d g o l d . 67)

Furthermore, it was noted

(66,

that t h e results c o u l d not b e e x p l a i n e d i n terms of present theories

of the i n t e r a c t i o n s of l o w - e n e r g y electrons w i t h m a t t e r .



Figure

15.


The "depth-selected" spectra corresponding to the layer (a) 0-375 A and (b) from 350 A and inward in the absorber extracted from the spectra shown in Figure 10 (32)

(b)


84



INTENSITY

(FeA) Journal of Vacuum Science and Technology

Figure 16. "Weight functions" corresponding to the settings, 5400, 5600, and 5800 V in the Stockholm group's electron spectrometer (104). These functions give the relative probabilities to detect electrons of initial energy 7.3 keV, ejected at different depths in iron and at different electron spectrometer settings.

Applications of Fe CEMS 57

T h e p o t e n t i a l of

5 7

Fe CEMS

a p p l i c a t i o n s to t h e s o l u t i o n of

real

p r o b l e m s c o n n e c t e d w i t h t h e p r o p e r t i e s of surfaces has b e e n r e a l i z e d o v e r t h e p a s t f e w years. T h e a b i l i t y of the t e c h n i q u e to p r o b e t h e surface regions of l o w - a r e a solids i n s i t u a n d i n a n o n d e s t r u c t i v e m a n n e r has m a d e t h e m e t h o d p a r t i c u l a r l y s u i t a b l e f o r the e x a m i n a t i o n of t e c h n i c a l p r o b l e m s s u c h as those e n c o u n t e r e d i n m e t a l l u r g i c a l studies

(68,69).

T h e m a j o r i t y of a p p l i c a t i o n s to date h a v e u s e d i n t e g r a l C E M S t e c h n i q u e s , often c o m b i n e d w i t h the x - r a y s c a t t e r i n g M o s s b a u e r m e t h o d , thus a l l o w i n g t h e o u t e r f e w m i c r o n s of t h e surface to b e p r o b e d a n d t h e d i s t r i b u t i o n of the surface phases d e t e r m i n e d . V a r i o u s a p p l i c a t i o n s of t h e t e c h n i q u e s are n o w d e s c r i b e d i n m o r e d e t a i l . Aqueous Oxidation Corrosion of Iron and Steels.

Early work fully

e s t a b l i s h e d t h e p o t e n t i a l of C E M S as a p o w e r f u l t e c h n i q u e f o r t h e s t u d y of t h e i n i t i a l stages of t h e c o r r o s i o n of i r o n a n d steels. co-workers

(18)

Simmons a n d

h a v e s t u d i e d t h e o x i d a t i o n of e n r i c h e d i r o n foils at

2 2 5 ° C , 3 5 0 ° C , a n d 4 5 0 ° C , a n d f u l l y d e m o n s t r a t e d the a b i l i t y of

the

t e c h n i q u e to i d e n t i f y n e w surface phases s u c h as F e 0

2

3

f o r m e d at surfaces i n t h e t h i c k n e s s r a n g e of

five

4

and

Fe 0

to s e v e r a l tens


3

of

3.

Conversion

TRICKER

nanometers.

Electron

Mossbauer

S i m i l a r studies (19,20)

85

Spectroscopy

have been performed w i t h natural

i r o n substrates, a n d F e 0 , a - F e 0 , a n d w u s t i t e w e r e a l l i d e n t i f i e d as 3

4

2

3

oxidation products. I n a d d i t i o n to this phase i d e n t i f i c a t i o n aspect of C E M S , the d a t a r e d u c t i o n m e t h o d s d e s c r i b e d e a r l i e r h a v e b e e n u s e d to d e r i v e k i n e t i c p a r a m e t e r s for o x i d a t i o n processes f r o m a k n o w l e d g e

of

total

thicknesses d e r i v e d f r o m s p e c t r a l area m e a s u r e m e n t s (18,20,60). m a t i o n c o n c e r n i n g the d e p t h d i s t r i b u t i o n ( z o n i n g ) can be gained.

oxide Infor

of o x i d e layers also

I n cases w h e r e t h e o x i d e thickness is t h e o r d e r of t h e


p r o b i n g d e p t h of C E M S , this m a y b e d o n e b y either m a k i n g use of the l i m i t e d e n e r g y r e s o l u t i o n of H e / C H

4

beta-ray spectrometer

F o r t h i c k e r layers i n f o r m a t i o n m a y

(50,52,70).

detectors

(6)

or b y t h e use of a

be d r a w n f r o m a c o m b i n a t i o n of C E M S a n d b a c k s c a t t e r e d x-ray M o s s b a u e r spectra. A n i n t e r e s t i n g e x a m p l e of this l a t t e r a p p r o a c h is s h o w n i n F i g u r e 17, w h i c h relates to a s t u d y (71)

of the o x i d a t i o n of a

9%

c h r o m e - s t e e l i n C 0 . S p e c t r u m a is the i n t e g r a l C E M s p e c t r u m of t h e 2

o r i g i n a l steel. fields

H e r e there are at least five different i n t e r n a l h y p e r f i n e

at the i r o n sites a r i s i n g f r o m d i f f e r i n g n u m b e r s of i r o n - c h r o m i u m

n e i g h b o r s i n the d i s o r d e r e d alloys. S p e c t r a b a n d c a r e C E M spectra of the c o r r o s i o n p r o d u c t .

T h i s p r o d u c t consists of a n o u t e r l a y e r of m a g n e

t i t e a n d a n i n n e r l a y e r of F e i + ^ C r ^ a A i (1.4 < x < 1.8).

a n i r o n - c h r o m i u m s p i n e l of

composition

D u r i n g the o x i d a t i o n , i r o n diffuses o u t to

t h e o x i d e / g a s i n t e r f a c e to g i v e F e 0 3

4

a n d c h r o m i u m r e m a i n s b e h i n d to

g i v e t h e m i x e d s p i n e l . B e n e a t h t h e l o w e r o x i d e l a y e r is a r e g i o n t h a t is c h r o m i u m depleted.

T h i s is c l e a r l y r e v e a l e d i n the b a c k s c a t t e r e d x - r a y

s p e c t r u m d w h e r e t h e outmost 2 0 pm or so of the s a m p l e are p r o b e d . T h i s s p e c t r u m is c o n s i d e r a b l y s h a r p e r t h a n t h a t of the o r i g i n a l steel, i n d i c a t i n g a r e d u c t i o n i n the v a r i e t y of i r o n e n v i r o n m e n t s c a u s e d b y the c h r o m i u m d e p l e t i o n . T h e f e a s i b i l i t y of d e p t h - p r o b i n g t h e o u t e r 100 n m of o x i d e o r o x y h y d r o x i d e o v e r l a y e r s o n i r o n b y D C E M S at a r e s o l u t i o n o n t h e o r d e r of 5 n m also has b e e n d e m o n s t r a t e d (50, 52,

70).

I n t h e area of aqueous c o r r o s i o n , G u t l i c h a n d c o - w o r k e r s (72)

have

s t u d i e d the f o r m a t i o n of p r o t e c t i v e o x i d e coatings o n steam generator tubes i n the presence o f w a t e r at h i g h t e m p e r a t u r e s a n d pressures.

Magnetite

w a s the o n l y o x i d e phase d e t e c t e d at t h e surface of a m a r t e n s i t i c i r o n c h r o m i u m steel a n d o n a n a u s t e n i t i c i r o n - c h r o m i u m - n i c k e l steel, w h e r e a s a n i r o n - n i c k e l f e r r i t e w a s f o r m e d o n a n I n c o l o y 800 steel after t r e a t m e n t . T h e t i m e d e p e n d e n c e of t h e o x i d e g r o w t h w a s m o n i t o r e d a n d

oxide

thicknesses w e r e d e r i v e d u s i n g t h e m e t h o d of T h o m a s et a l . (19).

An

analysis of these d a t a r e v e a l e d that the o x i d a t i o n process is most p r o b a b l y c o n t r o l l e d b y s h o r t - c i r c u i t d i f f u s i o n . I n a later p a p e r (73)

these w o r k e r s

e x t e n d e d this r e s e a r c h to s t u d y the i n f l u e n c e of the presence of P 0 2

Si0

2

i n the w a t e r u p o n c o r r o s i o n .

T i b y (47)

5

r e c e n t l y has m a d e


and an


86


I

1 -6

-8

1 -4

i -2

i i i 0 2 4 Velocity (mm/s)

i 6

i 8

1

1 10

Non-Destructive Testing

Figure 17. Mossbauer scattering spectra of 9% chromium-steel after oxidation in CO (68): (a) original steel (CEM); (b) outer oxide layer (CEM); (c) inner oxide layer (CEM); (d) first 20 pm below inner oxide layer using the scattered x-rays t

a p p l i c a t i o n of i n t e g r a l C E M S to the s t u d y of c o r r o s i o n p r o d u c t s f o r m e d d u r i n g the aqueous c o r r o s i o n of i r o n , at v a r i o u s p H a n d i n t h e presence of d i s s o l v e d salts, u s i n g t h e v a r i a b l e t e m p e r a t u r e d e v i c e d e s c r i b e d i n a p r e v i o u s section. T h e aqueous c o r r o s i o n of i r o n m a y l e a d to a v a r i e t y of products that include 0 - F e O O H , y - F e O O H , a - F e O O H , y - F e 0 , « - F e 0 , 2

and F e 0 . 3

4

3

2

I f o n l y r o o m - t e m p e r a t u r e C E M spectra are r e c o r d e d ,

3

diffi

culties m a y b e e n c o u n t e r e d i n the a s s i g n m e n t of spectra t h a t c o n t a i n only quadrupole y-FeOOH

or

doublets,

even

s i n c e these

superparamagnetic

may

arise f r o m

p a r t i c l e s of

e i t h e r /?-

either

or

a-FeOOH,

y - F e 0 ,

Journal of Inorganic and Nuclear Chemistry

Figure 20. (a) Transmission and (b) CEM spectra of a vivianite single crystal before heating (100). The spectrum after heating for 1 h at 120° C indicates conversion of the bulk ((c) is the transmission spectrum) to a mainly ferric-containing species, whereas the CEM spectrum (d) suggests that the surface is mainly ferrous in nature.


3.

Conversion

TRICKER

Electron

Mossbauer

95

Spectroscopy

context i t is k n o w n t h a t c e r t a i n l o w e r i r o n ( I I ) p h o s p h a t e h y d r a t e s a r e resistant to o x i d a t i o n Iron-57 C E M S

(101). also has b e e n u s e d i n c o m b i n a t i o n w i t h

scanning

e l e c t r o n m i c r o s c o p y a n d x - r a y s c a t t e r i n g M o s s b a u e r s p e c t r o s c o p y to s t u d y t h e n a t u r e of r e d a n d b l a c k glazes o n G r e e k - a n d I n d i a n - p a i n t e d w a r e C E M S w a s s h o w n to b e p a r t i c u l a r l y u s e f u l i n t h e s t u d y o f I n d i a n

(102).

p o t t e r y w h e r e t h e glazes a r e e x t r e m e l y t h i n . Applications of D C E M S . y-FeOOH

( 4 9 ) , iron oxide

i m p l a n t e d a l u m i n u m (90)


measure

the magnetic

A p a r t f r o m its a p p l i c a t i o n t o studies of a n d oxyhydroxide

just d e s c r i b e d ,

field

n e a r t h e surface

and iron-

(50,52,70),

DCEMS

has been used

of i r o n

to

It was

(103).

d e m o n s t r a t e d t h a t t h e i n t e r n a l m a g n e t i c h y p e r f i n e field i n t h e o u t e r m o s t 50 A of t h e f o i l w a s 5 % s m a l l e r t h a n the b u l k v a l u e .

T h e Stockholm

g r o u p has u s e d t h e electrostatic b e t a - r a y s p e c t r o m e t e r d e s c r i b e d e a r l i e r to e x a m i n e t h e surface c o n d i t i o n of i r o n samples i n a n o i l - d i f f u s i o n p u m p e d v a c u u m system (104). of F e C 0

3

A f t e r a n n e a l i n g at 7 5 0 ° C , t h e p r e s e n c e

w a s d e t e c t e d i n the surface r e g i o n s o f t h e s a m p l e .

were analyzed using the w e i g h t functions computed

T h e data

according to the

m e t h o d d e s c r i b e d i n a n e a r l i e r section. I t w a s f o u n d t h a t t h e c o n c e n t r a t i o n of F e C 0 d e c r e a s e d l i n e a r l y f r o m t h e surface d o w n to 1100 A , w h e r e 3

as a M o s s b a u e r i n e r t p h a s e i n c r e a s e d i n c o n c e n t r a t i o n f r o m t h e surface d o w n to at least 1200 A . I n a c l e a n e r t u r b o - p u m p e d v a c u u m s y s t e m , austenite w a s p r o d u c e d

at t h e surface o f a n i r o n f o i l after

repeated

o x i d a t i o n / r e d u c t i o n cycles o v e r a p e r i o d of o n e m o n t h . A n a l y s i s of these d a t a s h o w e d t h a t t h e austenite is c o n f i n e d to w i t h i n 500 A of t h e surface. I t w o u l d a p p e a r t h a t e v e n i n t h e c l e a n v a c u u m system c a r b o n c o n t a m i n a t i o n of samples c a n o c c u r after l o n g p e r i o d s of t i m e . A D C E M S s t u d y ( 5 2 ) of t h e n a t u r e of t h e c o r r o s i o n p r o d u c t f o r m e d o n i r o n after exposure to a h u m i d a t m o s p h e r e also has b e e n d e s c r i b e d . I t w a s d e m o n s t r a t e d t h a t after 4 8 h a n o n u n i f o r m l a y e r o f y - F e O O H a b o u t 300 n m t h i c k w a s f o r m e d .

I t also w a s s h o w n t h a t a l a y e r of F e ( P 0 ) 2 * 3

4

8 H 0 , « 4 0 n m t h i c k f o r m e d at a n i r o n s u r f a c e after i m m e r s i o n i n 0 . 1 M 2

H P0 3

4

f o r 20 s.

Applications of Sn CEMS 119

I n a d d i t i o n t o t h e a p p l i c a t i o n of

1 1 9

Sn DCEMS

to the study of

b r o m i n a t e d t i n foils d e s c r i b e d e a r l i e r , o t h e r a p p l i c a t i o n s o f h a v e b e e n m a d e . Y a g n i k a n d c o - w o r k e r s (63) for unenriched S n 0 range of the

1 1 9

2

absorbers.

1 1 9

Sn CEMS

o b t a i n e d v e r y l a r g e effects

Sano a n d c o - w o r k e r s (105)

estimated the

S n L - c o n v e r s i o n electrons t o b e 1.17 rfc 0.20 m g cm* a n d

also u s e d t h e t e c h n i q u e to s t u d y t h e a q u e o u s c o r r o s i o n of t i n (106).

In

this latter study the corrosion products f o r m e d o n t i n m e t a l i m m e r s e d i n


96


20 r

Sn0 —

2

Metallic Sn FeSn

2

I— z


LU O

I

I

I

-12

-

I 8

-

I

I

I

4

I

0

I

I 4

V (MM/S) Journal of the Electrochemical Society

Figure 21. The Sn CEM spectra of tinplate samples with coatings of 0.15 Ib/bb (top) and 0.22 Ib/bb (bottom,) (106). The approximate contributions of the oxide, metal, and alloy (FeSn ) layers are indicated separately in the top spectrum. 119

2

6.7M H N 0 , 5.7M HC1, and 9.0M H S 0 3

2

4

w e r e d e t e r m i n e d to b e S n 0 • 2

n H 0 ; S n ( O H ) C l , and S n S 0 , respectively. H u f f m a n n a n d D u n m y r e 2

4

2

6

4

have made detailed

(106)

1 1 9

S n C E M S studies of t i n p l a t e . T y p i c a l s p e c t r a

of t i n p l a t e o n i r o n are s h o w n i n F i g u r e 21 w h e r e t h e c o n t r i b u t i o n s f r o m metallic Sn, S n 0 , and F e S n 2

2

c a n b e c l e a r l y seen. T h e o v e r l a y e r t h i c k

nesses w e r e d e t e r m i n e d a n d s h o w n to b e i n g o o d a g r e e m e n t w i t h t h e results o b t a i n e d f r o m s t a n d a r d s t r i p p i n g t e c h n i q u e s . A c o m b i n e d and

1 1 9

S n C E M S s t u d y (108)

ESCA

w a s m a d e o f t h e o x i d a t i o n of t i n . T i n m e t a l

w a s exposed to d r y o x y g e n at 1000°C, a n d i t w a s d e m o n s t r a t e d t h a t u n d e r these c o n d i t i o n s r e d S n O w a s f o r m e d at t h e t i n surface. a n d L l a b a d o r (52) DCEM SnF

4

made a D C E M S

study of a

Schunk, Friedt,

fluorinated

tin foil;

spectra r e c o r d e d at v a r i o u s e n e r g y settings d e m o n s t r a t e d t h a t

and S n F

2

w e r e f o r m e d at the surface.


3.

TRICKER

Conversion

Electron

Mossbauer

Spectroscopy

97

Conclusions It is c l e a r f r o m t h e d i v e r s i t y of a p p l i c a t i o n s d e s c r i b e d h e r e t h a t C E M S , e v e n i n its s i m p l e s t m o d e of o p e r a t i o n , has b e g u n t o m a k e a significant i m p a c t i n m a n y areas of p u r e a n d a p p l i e d r e s e a r c h .

I t is

e x p e c t e d that t h e recent a d v a n c e s i n d a t a r e d u c t i o n , t h e o r y , a n d i n s t r u m e n t a t i o n , e s p e c i a l l y w i t h r e g a r d to t h e c o n t r o l of s a m p l e t e m p e r a t u r e and

d e p t h selection, w i l l b e u s e d a d v a n t a g e o u s l y i n f u t u r e studies o f

academic a n d technological problems.


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;



3.

Conversion

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Electron

Mossbauer

Spectroscopy

99

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;


100


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R E C E I V E D June 2 7 , 1980.


Conversion Electron MÃ¶ssbauer Spectroscopy and Its Recent

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