A NEW TECHNIQUE FOR SUBMONOLAYER NEXAFS : FLUORESCENCE YIELD AT THE CARBON K EDGE

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A NEW TECHNIQUE FOR SUBMONOLAYER NEXAFS : FLUORESCENCE YIELD AT THE

CARBON K EDGE

D. N. Arvanitis, U. Dobler, L. Wenzel, K. Baberschke, J. Stöhr

To cite this version:

D. N. Arvanitis, U. Dobler, L. Wenzel, K. Baberschke, J. Stöhr. A NEW TECHNIQUE FOR

SUBMONOLAYER NEXAFS : FLUORESCENCE YIELD AT THE CARBON K EDGE. Journal

de Physique Colloques, 1986, 47 (C8), pp.C8-173-C8-178. �10.1051/jphyscol:1986832�. �jpa-00226154�

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Colloque C8, supplkment au

n o

1 2 , Tome 47, dkcembre 1986

A

NEW TECHNIQUE FOR SUBMONOLAYER NEXAFS : FLUORESCENCE YIELD AT THE CARBON K EDGE

D. ARVANITIS, U. DOBLER", L . WENZEL,

K.

BABERSCHKE and J . STOHR*

*

Institut fiir Atom und Festkb'rperphysik, Freie Universitdt Berlin, Arnimallee 14, 0-100'0 Berlin 33, F.R.G.

'Siemens Systemtechnik, Nonnendammallee 101, 0-1000 Berlin 13, F.R.G.

'"IBM, Almaden Research Center, San Jose, C A 95120-6099, U.S.A.

Resume: La s t r u c t u r e f i n e au v o i s i n a g e du s e u i l d ' a b s o r p t i o n X (NEXAFS) K du c a r b o n e e s t mesuree pour de m o l e c u l e s d ' h y d r o c a r b u r e s l i n k a i r e s s u r du Cu(100)

d

6 0 K . O u a t r e modes p o s s i b l e s de d e t e c t i o n s o n t

compares: y i e l d en f l u o r e s c e n c e ( F Y ) , y i e l d e l e c t r o n i q u e en mode Auger (AEY)

,

p a r t i e l ( P E Y ) e t t o t a l (TEY) s o u s l e s memes c o n d i t i o n s e x p e r i - m e n t a l e s . On prouve que l a f l u o r e s c e n c e e s t l a t e c h n i q u e l a p l u s f i a b l e ,

d

c a u s e du r a p p o r t a c c r u s i u n a l s u r fond JR:JR(FY) = 10, ,JR(FEY) = @ , 6 , 7 ~ ( P E Y ) = 0 , 1 7 e t JR(TEY) = 0 , 0 5 . Des s p e c t r e s e n sous-monocouche de C2H2, C2H4, C2H6 s u r du Cu(100) s o n t p r 6 s e n t e s . A b s t r a c t : Near Edge X-ray A b s o r p t i o n F i n e S t r u c t u r e (NEXAFS) measure- ments a t t h e C K edge a r e u s e d t o i n v e s t i a a t e l i n e a r hydrocarbon mo- l e c u l e s on Cu(100) a t 60 K . Four d i f f e r e n t d e t e c t i o n modes a r e compared u n d e r t h e same e x p e r i m e n t a l c o n d i t i o n s : f l u o r e s c e n c e y i e l d (FY)

,

^Auger

e l e c t r o n y i e l d , (AEY)

,

p a r t i a l e l e c t r o n y i e l d (PEY) and t o t a l e l e c t r o n y i e l d (TEY). Because of an i n c r e a s e of t h e edge jump r a t i o JR due t o r e d u c t i o n i n background i n t e n s i t y t h e f l u o r e s c e n c e y i e l d method i s shown t o b e t h e most r e l i a b l e . The v a l u e s o b t a i n e d f o r JR a r e : J R ( F Y ) = 10, J R ( A E Y ) = 0 , 6 , JR(PEY) = 0,17 and J R ( T E Y ) = 0 , 0 5 . Submonolayer s p e c t r a of C2H2, C 2 H 4 , C2H6 on Cu(100) a r e p r e s e n t e d .

I . I n t r o d u c t i o n

The Near Edae X-ray A b s o r p t i o n F i n e S t r u c t u r e (NEXAFS) of

a

molecule c o n t a i n s u s e f u l i n f o r m a t i o n a b o u t t h e o r i e n t a t i o n w i t h r e s p e c t t o t h e s u b s t r a t e and i n t r a - m o l e c u l a r bond l e n g t h s

/ I / .

Many s t u d i e s t h a t mea- s u r e t h e e l e c t r o n y i e l d a s s o c i a t e d t o t h e Auger d e e x c i t a t i o n of t h e c r e a t e d Is c o r e h o l e have been c a r r i e d o u t i n t h e p a s t f o r monolayer c o v e r a q e s on m e t a l s u r f a c e s / 1 , 2 / above t h e oxygen and n i t r o g e n K e d g e s . F o r many chemical s t u d i e s , t h e c a r b o n K edge i s t h e most impor- t a n t one. But around t h e c a r b o n

K

e d a e , t h e s t r o n g l y enerqy dependent t r a n s m i s s i o n f u n c t i o n of a l l monochromators, c a u s e d by c o n t a m i n a t i o n of t h e o p t i c a l e l e m e n t s ,

l i m i t s

t h e s u r f a c e s e n s i t i v i t y which can b e r e a c h e d by means of t h e e l e c t r o n y i e l d t e c h n i q u e s .

A s t a n d a r d f i g u r e of m e r i t f o r any d e t e c t i o n scheme

a t

a n a d s o r b a t e edge i s t h e edge jump r a t i o JR, which

i s

d e f i n e d a s t h e c o u n t r a t e d i f f e r e n c e above and below t h e edge, n o r m a l i z e d t o t h e pre-edge back-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986832

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JOURNAL DE PHYSIQUE

ground / 3 / . A t y p i c a l v a l u e of JR f o r one l a y e r of hydrocarbons ( s u c h a s C2H4) on a m e t a l s u r f a c e

i s

on t h e o r d e r of o n l y 1 % f o r t h e p a r t i a l e l e c t r o n y i e l d mode / 4 / . I t i s t h e r e f o r e worthwhile t o d e v e l o p a l t e r - n a t i v e d e t e c t i o n schemes t h a t o f f e r a l a r g e r JR and an u l t i m a t e l y h i g h e r s u r f a c e s e n s i t i v i t y .

R e c e n t l y , f l u o r e s c e n c e y i e l d d e t e c t i o n a t t h e C K edge / 5 / , h a s demon- s t r a t e d t h e a d v a n t a n e of a l a r g e r JR, compared t o e l e c t r o n d e t e c t i o n . F e r e we i l l u s t r a t e t h e a c t u a l s t a t u s of t h i s t e c h n i q u e t h r o u g h measure- ments on l i n e a r hydrocarbons on submonolayer c o v e r a g e s on a Cu(100) s u r f a c e a t 60 K . These e x p e r i m e n t s i l l u s t r a t e t h e p o s s i b i l i t i e s of t h e t e c h n i q u e a t t h e C K e d a e , which

i s

i m p o r t a n t b e c a u s e of t h e importance of c a r b o n i n s u r f a c e p h y s i c s and c h e m i s t r y .

T h i s t e c h n i q u e opens new f i e l d s of i n v e s t i q a t i o n , due t o t h e b i g e s - c a p e d e p t h of p h o t o n s ( f i g u r e

I ) ,

a s i n t e r l a y e r s t u d i e s , measures a t h i g h e r p r e s s u r e s under r e a c t i o n c o n d i t i o n s . I t i s a l s o s u i t a b l e f o r t h e s t u d y of i n s u l a t i n q m a t e r i a l s , a s it

i s

n o t d i r e c t l y s e n s i t i v e t o c h a r p i n q e f f e c t s .

11. Experimental

The e x p e r i m e n t s were performed a t t h e s t o r a g e r i n g BESSY i n B e r l i n , u s i n g t h e SX-700 p l a n e p r a t i n g monochromator. The f l u x around t h e C K edae was a b o u t 2 -1 01° p h o t o n s / s e c (400

m

A c u r r e n t )

,

w i t h an energy r e s o l u t i o n of 1 eV. The sample was c l e a n e d by

~ r +

bombardement, a n n e a l e d t o 300°C and checked u s i n u LEED and Auqer s p e c t r o s c o p i e s . S p e c t r a were r e c o r d e d on t h e Auaer, p a r t i a l , t o t a l e l e c t r o n y i e l d modes a s b e f o r e / 6 / , a s w e l l a s t h e f l u o r e s c e n c e y i e l d mode. The ex- p e r i m e n t a l d e t a i l s of t h e UEV c o m p a t i b l e p r o p o r t i o n a l c o u n t e r a r e q i v e n e l s e w h e r e / 5 / . The s o l i d a n g l e was 5% of

4

T

sr

w i t h an e n e r g y r e s o l u - - t i o n of 280 eV FFXM a t t h e C K, l i n e (277 eV). By u s i n g a m u l t i c h a n n e l a n a l y s e r , a window was s e t around t h e C K, peak t o

l i m i t

t h e e f f e c t s of e l e c t r o n i c n o i s e on t h e low energy s i d e and t h e p o s s i b l y second o r d e r e x c i t e d Cu L edges a t t h e h i g h energy s i d e / 5 / . I n an e l e c t r o n y i e l d e x p e r i m e n t , t h e p o s s i b l y h i g h e r harmonics e x c i t e d c h a n n e l s would a i v e a background c o n t r i b u t i o n and, f u r t h e r d e c r e a s e Jp..

A l l d e t e c t o r s were p o s i t i o n e d a t a r a z i n u a n q l e s w i t h r e s p e c t t o t h e sample s u r f a c e ( f i q u r e 1 ,

B

= 0O)

.

The s p e c u l a r l y r e f l e c t e d X-ray

beam from t h e sample and t h e f l u o r e s c e n c e d e t e c t o r a r e a t normal a n g l e s t o e a c h o t h e r a t normal X-ray i n c i d e n c e ( f i q u r e 1, a =

oO) .

I n t h a t geometry t h e background due t o t h e s u b s t r a t e

i s

r e d u c e d and JR i s maximized.

F i g . 1 : F l u o r e s c e n t photons and Auaer e l e c - t r o n s a r e c r e a t e d by f i l l i n g a deep c o r e h o l e c r e a t e d by a pho-, t o n . The e s c a p e d e p t h of t h e p h o t o n s

( L &I000

8) i s

much l a r - ae!i? t h a n t h e one of t h e e l e c t r o n s ( L 210

2) .

We have chosen

&

⁼

o0

and

a

= C0 f o r n o r r ~ a l X - r a y i n c i d e n c e

.

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*Estimated u s i n g TDS s p e c t r o s c o p y

**CRC Handbook o f S p e c t r o s c o p y , Vol.

1 , J . W .

Robinson Edt. 1974

Photon f l u x (ph/sec)

~ t o m s / c m ²

K edge c r o s s - s e c t i o n (cm /atom) 2

**

K edge f l u o r e s c . y i e l d

**

S o l i d a n g l e f r a c t i o n Window t r a n s m i s s i o n D e t e c t o r e f f i c i e n c y E s t i m a t e d c o u n t s / s Measured c o u n t s / s

T a b l e 1: E s t i m a t e d and measured f l u o r e s c e n t y i e l d c o u n t r a t e s a t t h e

S / 3 / and C K

e d g e s .

A

s m a l l c o u n t r a t e i s e x p e c t e d i n o u r c a s e due t o t h e s m a l l f l u o r e s c e n c e d e e x c i t a t i o n p r o b a b i l i t y w i t h r e s p e c t t o t h e Auger p r o c e s s

( % I

o - ~ ) . The e x p e r i m e n t a l c o n d i t i o n s a r e comparable.

Here t h e sample

i s

l o o k i n g t o t h e d e t e c t o r ( a

=

70°) a t g r a z i n g X-ray i n c i d e n c e . The c o u n t r a t e i s d e c r e a s e d a t normal X-ray i n c i d e n c e w i t h a

=

0O (see t e x t ) .

U

0 2 -

z C ) PEY

z

^{1 -}

a

3 0

0 I I I I

280 290 300 310

2 . 1 0 ~ ~ 8 . 1 0 ' ~ 6 8

011 Ot4 7s 1 3070 3000

F i q .

2: Raw, unnormalized,

s p e c t r a of f l u o r e s c e n c e ( a ) , Auger ( b ) and p a r t i a l e l e c t r o n ( c ) , y i e l d s , of 10

L C 2 ~ 4 _(QJ

2ML) , a t normal X-ray I n c i d e n c e on

C u (

100) a t 6 0

K .

2

1 . 1 0 ~ 0

< 3 .1015

*

%

8 , 4 . 1 0 - ~ ~ 1 , 5 . 1 0 - ~

0 , 0 5 0 , 2 5 Z 1

4 70

400

I

PHOTON ENERGY lev)

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JOURNAL DE PHYSIQUE

111.

R e s u l t s and d i s c u s s i o n

The raw, unnormalized, s p e c t r a f o r 10 L C2H4 a t normal X-ray i n c i d e n c e on Cu (1 0 0 ) a t 60

K

a r e shown f o r t h r e e d e t e c t i o n modes i n f i g u r e 1 . The p a r t i a l e l e c t r o n y i e l d spectrum

( f i g u r e I c , r e a a r d i n g v o l t a g e : -220

V)

a i v e s t h e t r a n s m i s s i o n f u n c t i o n of t h e monochromator. No s p e c i f i c f e a t u r e s due t o t h e

C F4

m o l e c u l e s on t h e s u r f a c e

( % ~ M L ?

c a n b e de- t e c t e d , a s t h e Auger

X W

peak i s s i t t i n g on an i n e l a s t i c e l e c t r o n background / 7 / . T h i s spectrum h a s t o b e n o r m a l i z e d i n o r d e r t o s e p a r a t e t h e a d s o r b a t e c o n t r i b u t i o n , and y i e l d s

JR =

0 , 1 7 ( f i g u r e

3 ) .

I f o n l y t h e Auger e l e c t r o n s a r e d e t e c t e d , a s we d i d u s i n g a

CMA,

t h e c o n t r i - b u t i o n of t h e background e l e c t r o n s

i s

reduced. Then a peak can b e i d e n - t i f i e d on t h e spectrum ( f i g u r e 2b a t a285 eV) t h a t c o r r e s p o n d s t o t h e t r a n s i t i o n from t h e

C

( I o) i n i - t i a l s t a t e t o a f i n a l n * m o l e c u l a r s t a t e . The v a l e n c e bands of Cu, a r e swept t h r o u g h t h e window of t h e a n a l y s e r below t h e

C K

edge.

U)

Total electron yield

1 ^d;jy F i g u r e 2a shows t h e f l u o r e s c e n c e y i e l d mode. The pre-edge backqround

i s r e d u c e d t o %I 0 c o u n t s / s e c , which i s c o n s t a n t o v e r t h e pre-edge r e a i o n c a u s e d by e l e c t r o n i c n o i s e 10 /5/. Once t h e Auger c h a n n e l

i s

280

290 300 310 320 opened, one c a n a l s o f o l l o w t h e

PHOTON ENERGY (eV)

t r a n s m i s s i o n c u r v e of t h e monochro-

mator. Then, t h e C(1o) t o n* t r a n s i - F i g . 3 : Comparison of f l u o r e s c c n c

y i e l d w i t h t h e t h r e e e l e c t r o n y i e l d d e t e c t i o n modes

/ 5 / .

a , b , c ) Normal X-ray i n c i d e n c e . d) 20° Grazing X-ray i n c i d e n c e .

A l l

s p e c t r a a r e normalized:

c o v e r e d / c l e a n .

e t i o n

i s

much more v i s i b l e .

JR =

10 f o r t h a t mode, which c o n f i r m s t h a t f l u o r e s c e n c e i s t h e more r e l i a b l e and s e n s i t i v e d e t e c t i o n scheme.

The l a r g e v a l u e f o r

JR c a n b e under-.

s t o o d i n t e r m s of a s m a l l background.

A t

20° q r a z i n q X-ray i n c i d e n c e ( f i q u r e 1 , a

=

70°) ,

Ja

d e c r e a s e s t o 0 , 2 i n s p i t e of an x n c r e a s e by a f a c t o r 4 i n t h e a d s o r b a t e due c o u n t r a t e . T h i s d e c r e a s e c a n b e under- s t o o d i n t e r m s of s c a t t e r e d r a d i a t i o n from t h e s u b s t r a t e .

Because of t h e low c o u n t i n g r a t e , t h e s t a t i s t i c a l n o i s e i s much b i g g e r i n t h e c a s e of t h e f l u o r e s c e n c e and t h e Auger y i e l d modes. But a s one d e c r e a s e s t h e c o n c e n t r a t i o n of t h e a d s o r b a t e atoms, what d e t e r m i n e s t h e s u r f a c e s e n s i t i v i t y and r e l i a b i l i t y of a d e t e c t i o n scheme i s JR. For J ~ < 0 , @ 1 one h a s o f t e n o b s e r v e d f e a t u r e s r e l a t e d t o i n c o m p l e t e n o r m a l i - z a t i o n of t h e s p e c t r a .

F l u o r e s c e n c e y i e l d submonolayer s p e c t r a f o r

C Z H ~ , C ~ H ~

and C2H6are shown

i n F i g .

4 .

The c o v e r a g e s a r e e s t i m a t e d by

TDS

s p e c t r o y c o p y . The s t a t i s -

t i c s do n o t a l l o w t h e d e t e r m i n a t i o n of t h e p o s i t i o n of t h e cr-shape

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i s g i v e n e l s e w h e r e I n t h i s c a s e , t h e s h a r p r e s o n a n c e of

C2H

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

i s

r a i s e d .

A t

% I 1 0

K

( f i q u r e

5 ) ,

t h e h e i g h t of t h e

^IT

r e s o n a n c e d e c r e a s e s .

A

check u s i n g

TDS

showed t h a t

C H 2

t r a n s f o r m s t o C2F6. T h i s d e c r e a s e i n

IT

i n t e n s i t y i s e a s i l y u n d e r s t o o 2 a s t h e

C2H6

molecule p r e s e n t s no

^T

bonding.

F i g . 4

:

Submonolayerspec- t r a a t normal X-ray i n c i - dence f o r l i n e a r hydro- c a r b o n on Cu

( 1

00) a t 6 0

K

u s i n g t h e f l u o r e s - c e n c e y i e l d mode.

F i q .

5: C H

s p e c t r a a t submonolayer c o v e r a g e s and normal X-ray

i n c i d e n c e ? 2) ^{0 , 5}

^L

f l u o r e s c e n c e y i e l d a t 60

K ,

b ) 0 , 2 5

L

p a r -

t i a l e l e c t r o n y i e l d a t 60

K

c ) same a s a ) wormed up t o a l l 0

K .

d) same a s b ) , warmed up t o a130

K .

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C8-178 JOURNAL

DE

PHYSIQUE

These t e s t e x p e r i m e n t s d e m o n s t r a t e t h e u t i l i t y of f l u o r e s c e n c e y i e l d d e t e c t i o n , even when t h e a b s o r b a t e e d g e

i s

f o r an e l e m e n t w i t h a low a t o m i c number. With s y n c h r o t r o n f a c i l i t i e s of t h e second g e n e r a t i o n h a v i n g one t o t h r e e o r d e r s of magnitude h i g h e r photon f l u x , f l u o r e s - c e n c e s h o u l d b e t h e method of c h o i s e f o r s u r f a c e s c i e n c e i n v e s t i g a t i o n s . Acknowledgements

We acknowledge t h e h e l p of D.A. F i s c h e r d u r i n g t h e e a r l y s t a g e of t h e e x p e r i m e n t . The work was s u p p o r t e d by BMFT u n d e r g r a n t no. 0233 BB.

R e f e r e n c e s

/ I / S e e f o r example J . S t b h r , 2. Phys. B, 6 1 , 439, (1985) / 2 / See f o r example J . Haase, Appl. Phys.

38,

181, (1985) / 3 / J . S t o h r , E.B. K o l l i n , D . A . F i s c h e r , J.B. P a s t i n g s , F. Z a e r a ,

a n d F. S e t t e , Phys. Rev. L e t t . 55, 1468, (1985)

/ 4 / J. S t b h r , F. S e t t e a n d A.L. ~ o h n s o n , Phys. Rev. L e t t

53,

1684, (1984)

/ 5 / D.A. F i s c h e r , U . D b b l e r , D. A r v a n i t i s , L. Wenzel, K . B a b e r s c h k e , a n d J . S t B h r , S u r f . S c i i n p r e s s

/ 6 / J . S t b h r , R. J a e g e r a n d S. Brennan, S u r f . S c i

117,

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