IR SPECTROSCOPY WITH SURFACE ELECTROMAGNETIC WAVES

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IR SPECTROSCOPY WITH SURFACE ELECTROMAGNETIC WAVES

A. Sievers, Z. Schlesinger, Y. Chabal

To cite this version:

A. Sievers, Z. Schlesinger, Y. Chabal. IR SPECTROSCOPY WITH SURFACE ELEC- TROMAGNETIC WAVES. Journal de Physique Colloques, 1984, 45 (C5), pp.C5-167-C5-178.

�10.1051/jphyscol:1984525�. �jpa-00224143�

JOURNAL DE PHYSIQUE

Colloque C5, supplément a u n04, Tome 45, a v r i l 1984 page C5-167

I R SPECTROSCOPY WITH SURFACE ELECTROMAGNETIC WAVES A . J . Sievers, 2. ~ c h l e s i n ~ e r * and Y . J . chabal*

h b o r a t o r y o f Atomic and S o l i d S t a t e P h y s i c s and M a t e r i a l s S c i e n c e Center, Cornez2 U n i v e r s i t y , I t h a c a , NY 19853, U.S.A.

Resume

-

On u t i l i s e des ondes é l e c t r o m a g n e t i q u e s de s u r f a c e pour t e s t e r l e s modes de v i b r a t i o n de molécules s u r des s u r f a c e s m e t a l l i q u e s .

A b s t r a c t

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S u r f a c e e l e c t r o m a g n e t i c waves a r e used t o probe m o l e c u l a r v i b r a t i o n a l modes on m e t a l s u r f a c e s .

1

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INTRODUCTION

Because o f i t s i n t r i n s i c h i g h s p e c t r a l r e s o l u t i o n capabi l i t y and p r e c i s i o n a t a l 1 p r e s s u r e ranges, i n f r a r e d spectroscopy was one o f t h e f i r s t t e c h n i q u e s used f o r t h e s t u d y o f v i b r a t i o n a l modes o f adsorbates on m e t a l s u r f a c e s . So f a r t h i s l o w s e n s i t i v i t y t e c h n i q u e has o n l y been used t o s t u d y i n r e f l e c t i o n t h e s t r o n g modes o f CO on v a r i o u s s i n g l e c r y s t a l m e t a l s u b s t r a t e s o r i n t r a n s m i s s i o n t h e modes o f o t h e r absorbed gases on f i n e metal p a r t i c l e s

/1/.

S u r f a c e e l e c t r o m a g n e t i c waves (SEW's), which i n t h e i n f r a r e d can propagate f o r a few cm on metal s u r f a c e s /2,3/ s h o u l d p r o v i d e enhanced s u r f a c e s e n s i t i v i t y o v e r t h e r e f l e c t i o n - a b s o r p t i o n s p e c t r o s c o p i c t e c h n i q u e s . I n t h i s paper, which i s t h e second i n a t w o - p a r t s e r i e s on our e x p e r i m e n t a l i n v e s t i g a t i o n s w i t h SEW's /4/, we h i g h l i g h t our s t u d i e s o f t h e v i b r a t i o n a l modes o f molecules on m e t a l s u r f a c e s .

The f e a s i b i l i t y o f u s i n g b o t h s i n g l e f r e q u e n c y l a s e r and broad band SEW t r a n s m i s s i o n spectroscopy i n t h e IR t o s t u d y t h e v i b r a t i o n a l modes o f molecules on m e t a l s u r f a c e s has been demonstrated b o t h w i t h a d i s p e r s i o n compensating edge c o u p l e r

/5/,

and a l s o w i t h a g r a t i n g c o u p l e r /6/. Measurements have been made o f t h e t r a n s m i s s i o n o f SEW's across a v a r i e t y o f d i e l e c t r i c - c o a t e d m e t a l s u r f a c e s /7/. F o r s p e c i f i c c o a t i ng geometri es t h e i n t e r f e r e n c e between SEN and s u r f a c e skimmi ng e l e c t r o m a g n e t i c r a d i a t i o n can be used t o probe t h e p r o p e r t i e s o f t h e SEW's.

For metal s u r f a c e s w i t h monolayer c o a t i n g s t h e s e n s i t i v i t y o f t h e SEW and t h e s u r f a c e r e f l e c t i o n - a b s o r p t i o n s p e c t r o s c o p i c t e c h n i q u e are compared e x p e r i m e n t a l l y . Given equal p r o b i n g i n t e n s i t y a t a met al-absorbate i n t e r f a c e , SEW spectroscopy p r o v i d e s about an o r d e r o f magnitude improvement i n c o n s t r a s t o v e r r e f l e c t i o n spectroscopy /5,6,8/.

II

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BACKGROUND

E x p e r i m e n t a l l y , t h e SEN on a b a r e m e t a l s u r f a c e i s c h a r a c t e r i z e d b y two q u a n t i t i e s : t h e h e i g h t o f t h e f i e l d above t h e m e t a l , c a l l e d Y i n Eq (1-14), and t h e

a t t e n u a t i o n c o e f f i c i e n t f o r p r o p a g a t i o n along t h e s u r f a c e , c a l l e d a i n Eq (1-15) / 4 / . The c a l c u l a t e d Y and a f o r Drude Ag as a f u n c t i o n - p f f r e q u e n c y a r e shown i n Fig.1. The r e l a x a t i o n f r e q u e n c y o f t h e Ag e l e c t r o n s , T

,

i s t a k e n t o be t h e room t e m p e r a t u r e value.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984525

C5-168 J O U R N A L D E PHYSIQUE

Wovelenqth (microns)

io3 ioZ IO I F i g u r e 1. C a l c u l a t e d SEW h e i g h t

'

1 '

and a b s o r p t i o n c o e f f i c i e n t versus

102

- 1

f r e q u e n c y f o r a b a r e Ag l i k e Drude

s u r f a c e . The c a l c u l a t i o n i s f o r T=300°K and t h e r e l a x a t i o n t i m e of

-

t h e m e t a l T i s i d e n t i f i e d i n t h e

-

f i g u r e . The v e r t i c a l dashed l i n e

5

i n d i c a t e s t h e f r e q u e n c y where t h e

O -

- ,

evanescent wave h e i g h t i s equal t o

2 t h e wavelength o f t h e mode. At

-

lower f r e q u e n c i e s t h e wave i s 5 d e l o c a l i z e d s i n c e t h e h e i g h t - exceeds t h e wavelength w h i l e a t

l a r g e r f r e q u e n c i e s t h e converse i s

- 3 t r u e . Note t h a t n e i t h e r t h e h e i g h t nor t h e a b s o r p t i o n c o e f f i c i e n t s c a l e wi t h t h e wavelength.

L 1 , l

IO 102 io3

Frequency ~cm"1 ¹⁰⁴

Since t h e SEW wavevector q, i s g r e a t e r t h a n t h e f r e e space wavevector qo (see Eq 1-11) p l a n e waves cannot be used d i r e c t l y t o e x c i t e SEW's on metal s u r f a c e s . By u s i n g p r i sms, edges o r g r a t i n g s as i n t e r m e d i a r i es, t h i s wavevector d i f f e r e n c e between p l a n e waves and t h e bound mode can be e l i m i n a t e d . Three d i f f e r e n t t y p e s o f evanescent f i e l d c o u p l e r s a r e shown i n F i g . 2.

The i d e a o f u s i n g t o t a l i n t e r n a l r e f l e c t i o n t o i n c r e a s e t h e p a r a l l e l component o f t h e i n c i d e n t wavevector was p i o n e e r e d i n t h e v i s i b l e by O t t o /9/. When l i g h t i s i n c i d e n t f r o m t h e h i g h i n d e x s i d e o f a d i e l e c t r i c i n t e r f a c e a t an a n g l e g r e a t e r t h a n t h e c r i t i c a l angle t h e wavevector o f t h e r e f r a c t e d evanescent wave i s g r e a t e r t h a n qo and can be matched t o t h e SEW wavevector. T h i s geometry i s shown i n F i g . 2a.

The two p r i s m t e c h n i q u e shown i n F i g . 2b was d e v i s e d b y Schoenwald, e t a l

/IO/,

i n 1973. They were t h e f i r s t t o show t h a t , i n t h e IR, SEW's propagate macroscopic d i s t a n c e s . F i g . 2c shows t h e edge c o u p l e r geometry which was developed i n o r d e r t o e l i m i n a t e t h e d i f f r a c t i o n o f b u l k r a d i a t i o n which o c c u r s a t t h e c o r n e r o f t h e i n p u t p r i s m i n F i g . 2b, and which produces an annoying background s i g n a l o f r a d i a t i o n a t t h e o u t p u t prism. A l 1 o f t h e e x p e r i m e n t a l measurements d e s c r i b e d h e r e were made w i t h edge c o u p l e r s .

C O ) F i g u r e 2. Some SEW c o u p l i n g g e o m e t r i e s

which use t h e p r i n c i p l e o f t o t a l i n t e r n a l r e f l e c t i o n .

( a ) The p r i s m - a i r - m e t a l c o u p l e r o f O t t o /9/. The c o u p l i n g o f e l e c t r o m a g n e t i c r a d i a t i o n t o t h e SEW mode o c c u r s most s t r o n g l y when t h e component o f t h e wave- v e c t o r p a r a l l e l t o t h e m e t a l s u r f a c e matches t h e SEW wavevector.

( b ) The two p r i s m IR c o u p l e r o f Schoenwald, e t a l / I O / . SEW's a r e generated a t t h e f i r s t prism, t r a n s m i t t e d t o t h e second p r i s m and t h e n c o n v e r t e d back t o e l e c t r o m a g n e t i c r a d i a t i o n a t t h e second orism.

(C 1

( c ) The I R edge c o u p l e r o f Chabal and S i e v e r s

/ I l / .

SEW's a r e generated a t t h e edge o f an o p t i c a l l y - t h i c k m e t a l f i l m which has been evaporated on t h e prism. T h i s geometry a u t o m a t i c a l l y separates t h e SEN o u t p u t f r o m t h e " s u r f a c e skimming" b u l k r a d i a t i o n generated a t t h e f i r s t edge.

III

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SEW INTERFERENCE EFFECTS ON PARTIALLY COATED METAL SURFACES

I n t h i s section, we describe some o f t h e experimental measurements which lead t o t h e i d e n t i f i c a t i o n o f i n t e r f e r e n c e between SEW's and b u l k r a d i a t i o n /4/. CO and CO, l a s e r s were used t o e x c i t e SEW's. The two types o f f i l m geometries t h a t were used are shown i n Fig. 3. With both geometries t h e SEW i s launched on and recovered from t h e edge o f t h e o p t i c a l l y - t h i c k bare metal f i l m , b u t propagates across a Ge coated r e g i o n i n between. The syrnbol Rc i s used t o represent t h e l e n g t h o f t h e SEW path through t h e coated region. With t h e geometry shown i n Fig. 3a t h e transmission across t h e surface i s measured as a f u n c t i o n o f frequency w i t h a, f i x e d whereas w i t h t h e geometry shown i n Fig. 3c t h e transmission i s measured as a f u n c t i o n o f Rc a t a f i x e d frequency.

Figure 3. Sample geometries f o r studying SEW's on p a r t i a l ly-coated metal surfaces.

(a) The l a r g e r e c t a n g l e represents an o p t i c a l l y t h i c k metal f i l m on an even l a r g e r coupler sub- s t r a t e . A t h i n Ge c o a t i n g has been evaporated ont0 t h e metal i n t h e r e g i o n shown. A narrow beam o f SEW's, which have been generated by a l a s e r source, propagates from l e f t t o r i g h t across t h e bare and Ge coated surface. I n t h i s manner, several d i s c r e t e values o f t h e Ge l e n g t h a, can be sequenti a l l y probed w i t h SEW's.

( c ) The wedge shaped Ge c o a t i n g on t h e o p t i c a l l y t h i c k metal f i l m allows ac t o be v a r i e d

c o n t i n u o u s l y by t r a n s l a t i n g t h e sample i n t h e plane o f t h e F i g u r e perpendicular t o t h e SEW beam

d i r e c t i o n . (SEN i n t e r f e r o m e t e r geometry).

The e a r l i e s t measurements were made on f i l m s w i t h f i x e d Ge pathlengths.

Transmission f o r such d i e l e c t r i c o v e r c o a t i n g taken w i t h a tuneable CO, l a s e r a r e shown i n Fig. 4. I n Fig. 4c f o r t h e l a r g e s t Ge overcoating two minima are

observed. This experimental r e s u l t i s reminiscent o f t h e channel spectrum obtained f o r a d i e l e c t i v e w i t h p a r a l l e l sides. I n t h a t case t h e frequency d i f f e r e n c e between t h e two minima i s i n v e r s e l y proportioned t o t h e round t r i p o p t i c a l path. Upon f i t t i n g t h e date i n Fig. 4 t o such an expression one discovers t h a t t h e e f f e c t i v e index o f r e f r a c t i o n must be much smaller than one.

The phenomenon u n d e r l y i n g t h i s i n t e r f e r e n c e behavior can be more e a s i l y understood by studying samples w i t h v a r i a b l e SEW pathlengths across t h e Ge c o a t i n g (Fig. 3c).

Fig. 5 shows p l o t s o f t h e transmission as a f u n c t i o n o f ac a t a frequency 1043 cm-' f o r t h r e e d i f f e r e n t arrangements. To t h e l e f t o f t h e p o i n t Rc = O t h e SEW propagates across t h e bare metal surface. The f l u c t u a t i o n s i n s i g n a l l e v e l i n t h i s r e g i o n are due t o f i l m i m p e r f e c t i o n s . The s t r o n g q u a s i p e r i o d i c o s c i l l a t i o n s observed f o r a

>

^O i n t h e t o p t r a c e occur f o r a Ge c o a t i n g t h i c k n e s s o f 0.24 p.

When t h e c o a t i n g thickness i s increased t o 0.75 p t h e SEW i n t h e coated r e g i o n i s completely attenuated b e f o r e i t reaches t h e o t h e r s i d e o f t h e d i e l e c t r i c f i l m . The data shown i n t h e middle t r a c e o f F i g . 5 must be caused by surface skimming b u l k waves which are produced a t t h e f i r s t d i e l e c t r i c edge and which then generate SEW's a t t h e second d i e l e c t r i c edge. This conclusion was v e r i f i e d by making one geometry which was coated w i t h metal except f o r a wedge-shaped area where t h e Ge c o a t i n g would normally be, but which was now l e f t uncoated so t h e d i e l e c t r i c coupler was exposed. When SEW's rneet t h e ernpty edge they are coupled o u t and o n l y

C5-170 JOURNAL

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PHYSIQUE

t h e s u r f a c e skimming b u l k r a d i a t i o n remains t o generate a new SEM a t t h e f a r s i d e o f t h e wedge. The lower t r a c e i n F i g . 5 shows t h e measured SEM o u t p u t f o r t h i s l a s t case.

wove*rqïitn I+d

F i g u r e 4. SEM t r a n s m i s s i o n vs. f r e q u e n c y

01 lc = 2 m m

f o r a p a r t i a l l y coated Au s u r f a c e i n t h e 10 p wavelength r e g i o n . The Ge c o a t i n g t h i c k n e s s i s 0.12 p and t h e w i d t h s are shown. The t r a n s - m i s s i o n i s measured as a f u n c t i o n o f frequency

O O w i t h a l i n e t u n e a b l e CO2 l a s e r .

F i g u r e 5. SEW i n t e r f e r o m e t e r t r a n s m i s s i o n measurements across t h r e e d i f f e r e n t k i n d s o f d i e l e c t r i c wedges. The observed t r a n s m i s s i o n s i g n a l as a f u n c t i o n o f SEM p a t h l e n g t h t h r o u g h t h e Ge coated l e n g t h ( l c ) i s shown f o r a l a s e r frequency = 1043 cm-'.

(a) Ge on Au. The Ge t h i c k n e s s i s 0.25 p.

The SEW component i s s m a l l e r t h a n t h e b u l k wave component.

( b ) Ge on Au. Now t h e Ge t h i c k n e s s i s 0.7 ^p and t h e SEW component i s e x t i n g u i s h e d . Only t h e b u l k component remains i n t h e Ge r e g i o n . (c! The Au f i l m c o n t a i n s a wedge-shaped h o l e which i s t h e same s i z e as i n ( a ) and ( b ) above. At t h e wedge edge t h e SEM i s coupled back i n t o t h e c o u p l e r and l o s t . Only a b u l k wave remains i n t h e wedge r e g i o n ( j u s t as f o r case ( b ) ) .

A more complex f r e q u e n c y dependence i s observed when an Au f i l m i s f i r s t covered wi t h 2008 thick-wedge of KReO, and t h e n coated again wi t h a second wedge o f Ge about 0.15 t h i c k . Because t h e Re04- molecule has a d i p o l e a c t i v e resonance a t 950 cm-', t h e d i e l e c t r i c f u n c t i o n o f t h i s composite c o a t i n g i s f r e q u e n c y dependent. The e x p e r i m e n t a l t r a c e s f o r t h i s sample a r e shown i n Fig. 6. Note t h a t t h e i n t e r f e r e n c e p e r i o d i s v e r y l o n g j u s t above t h e Re04- resonance frequency, b u t decreases r a p i d l y w i t h i n c r e a s i n g frequency. For a l 1 o f t h e s e d a t a t h e o s c i l l a t i o n s e x h i b i t e d i n t h e f i g u r e s have p e r i o d s much 1 arger t h a n t h e wavelength o f t h e i n c i d e n t r a d i a t i o n . The o r i g i n , frequency dependence and c o a t i n g t h i c k n e s s dependence o f a l 1 o f t h e s e o s c i l l a t i n g and monotonic t r a n s m i s s i o n s i g n a l s can be understood i n terms of a SEN--bulk wave i n t e r f e r e n c e phenomenon 141.

F i g u r e 6. Frequency dependence o f t h e SEW i n t e r f e r o g r a m f o r an Au f i l m c o a t e d w i t h a resonant absorber. An o p t i c a l l y - t h i c k Au f i lm f i r s t coated w i t h 200A o f KReO, and t h e n w i t h 0.15 p o f Ge b o t h i n a wedge shape. The d i s - p e r s i v e n a t u r e o f t h e d i e l e c t r i c f u n c t i o n o f Re04 near i t s v 3 v i b r a t i o n a l mode at 950 cm-' causes t h e measured i n t e r f e r e n c e p e r i o d t o s h i f t s u b s t a n t i a l l y f o r r e l a t i v e l y small changes i n l a s e r f r e q u e n c y near t h e resonance.

The i n t e r f e r e n c e p e r i o d i s v e r y l o n g j u s t above t h e resonance (965 cm-' ) , b u t decreases r a p i d l y w i t h i n c r e a s i ng 1 aser frequency.

The b a s i c i d e a i s q u i t e s i m p l e . An SEW i s launched on a b a r e r e g i o n o f t h e m e t a l surface and propagates towards a Ge c o a t e d r e g i o n as i l l u s t r a t e d i n F i g . 7a.

When t h e SEW i s i n c i d e n t a t t h e Ge s t e p (z=0) b o t h an SEM and a b u l k wave packet are t r a n s m i t t e d i n t o t h e Ge c o a t e d r e g i o n ( F i g . 7b) s i n c e b o t h El[ and HB must be

c o n t i n u o u s across t h e (z=0) boundary. The SEW and b u l k wave p a c k e t s propagate across t h e Ge coated m e t a l s u r f a c e w i t h d i f f e r e n t phase v e l o c i t i e s . At t h e second Ge s t e p (z=R,) b o t h components c o n t r i b u t e t o t h e a m p l i t u d e o f an SEW t r a n s m i t t e d i n t o t h e b a r e m e t a l r e g i o n , F i g . 7c. The edges o f t h e Ge c o a t i n g t h u s act as b e a m s p l i t t e r s , f i r s t s e p a r a t i n g t h e i n c i d e n t SEW i n t o two beams a t z=O and t h e n r e c o m b i n i n g t h e s e two beams a t z=Rc. Each beam c a r r i e s a m p l i t u d e and phase i n f o r m a t i o n .

F i g u r e 7. P i c t o r i a l d e s c r i p t i o n o f t h e SEW i n t e r - f e r o m e t e r . For t h e TM p o l a r i z e d SEW t h e t r a n s v e r s e magnetic f i e l d i s p e r p e n d i c u l a r t o t h e p l a n e o f t h e F i g u r e . The magnetic f i e l d p r o f i l e s o f t h e SEN and b u l k wave packet a r e superimposed on a drawing o f a p a r t i a l l y c o a t e d m e t a l .

Z = O Z= lC ( a ) The b a r e m e t a l SEW i s i n c i d e n t on t h e c o a t i n g edge.

( b ) 5 0 t h a c o a t e d m e t a l SEW and a b u l k wave packet

( b ) p r o p a g a t i n g i n t h e c o a t i n g r e g i o n are launched.

( c ) At t h e second d i e l e c t r i c edge t h e SEW and b u l k wave o a c k e t s combine t o launch a b a r e metal SEW and ,,, ,, , ,,,,,,. ^{, ,,/} a b u l k wave packet ( t h e l a t t e r i s n o t shown).

Because t h e SEW and b u l k waves move a t d i f f e r e n t v e l o c i t i e s i n p a r t ( b ) c o n s t r u c t i v e and d e s t r u c t i v e

I I

i n t e r f e r e n c e o c c u r s .

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I V

-

COMPARISON OF SEW AND REFLECTION SPECTROSCOPY

A d e m o n s t r a t i o n t h a t SEW s p e c t r o s c o p y has t h e s e n s i t i v i t y t o measure v i b r a t i o n a l modes f o r monolayer coverages on metal can be o b t a i n e d f r o m a s i m p l e m o d i f i c a t i o n t o t h e Thomas-Kuhn o p t i c a l sum r u l e . The i n t e g r a t e d a b s o r p t i o n s t r e n g t h o f a

v i b r a t i o n a l mode i n a t h r e e - d i m e n s i o n a l system i s d i r e c t l y r e l a t e d t o t h e number o f absorbing c e n t e r s p e r u n i t volume by t h e o p t i c a l sum r u l e ; narnely,

where n i s t h e number o f c e n t e r s p e r cm3, e* and m* t h e e f f e c t i v e charge and e f f e c t i v e mass o f t h e normal mode, and t h e l a s t f a c t o r c o r r e c t s f o r t h e d i f f e r - ence between t h e a p p l i e d f i e l d and t h e l o c a l f i e l d . For molecules adsorbed o n t 0 a m e t a l s u r f a c e w i t h t h e I R a c t i v e v i b r a t i o n a l mode p e r p e n d i c u l a r t o t h e s u r f a c e t h e e l e c t r i c d i p o l e moment i s enhanced b y t h e image d i p o l e moment. We s h a l l i n c l u d e t h i s f a c t o r i n e*.

For monolayer coverages i t i s ns, t h e number p e r cm2, which i s u s u a l l y quoted.

The n i n Eq. 1 can be r e w r i t t e n i n terms o f ns b y n o t i n g t h a t t h e number o f molecules w i t h i n t h e h e i g h t o f t h e SEW wave i s n S / y which i s t h e number per t h e a p p r o p r i a t e v o l urne so

The assumption i s t h a t t h e a b s o r p t i o n i s t h e same independent o f how t h e ns m o l e c u l e s a r e d i s t r i b u t e d w i t h i n t h e SEW h e i g h t Y.

The g e n e r a l f o r m o f t h e sum r u l e as i t a p p l i e s t o SEW's i s

where t h e l o c a l f i e l d c o r r e c t i o n i s i g n o r e d f o r t h e weak s u r f a c e d i p o l e l i m i t . The coverages o f SF6 l i s t e d i n F i g . 8 a r e c a l c u l a t e d f r o m Eq. 3 assuming no image depth enhancement.

F i g u r e 8. A b s o r p t i o n c o e f f i c i e n t

0 4 - vs. SEW f r e q u e n c y f o r p h y s i s o r b e d

SF6 on AU (T=10O0K).

Curve A: ns=l.6x1015 ( 4 mono1 a y e r s ) ;

6 0 3 -

z Curve

B:

n s = 1 . 2 x 1 ~ 1 5 cm-'

%! (2.8 monolayers);

Curve C : ns=6.4x1014 (1.6 monolayers).

Curve G = t h e a b s o r p t i o n f o r 0.07 T o r r o f SF6 gas a t T=300°K ( f o r r e f e r e n c e ) . The SEW's were generated w i t h a l i n e t u n e a b l e CO, l a s e r . A f t e r Ref. 12.

9M 94 5 960 975 990

Y (cm-')

With t h i s sum r u l e one can show t h a t t h e minimum coverage which can be seen by SEW's i n t h e weak source l i m i t i s r e l a t e d t o t h e v i b r a t i o n a l mode parameters and t h e Drude m e t a l parameters. The SEW a t t e n u a t i o n c o e f f i c i e n t f o r a m e t a l w i t h a monolayer coverage can be w r i t t e n

where am i s due t o t h e b a r e m e t a l and a a i s due t o t h e adsorbate. The maximum s i g n a l t o n o i s e i s o b t a i n e d i n t h e weak source l i m i t where t h e adsorbate induced change i n t r a n s m i t t e d i n t e n s i t y across t h e s u r f a c e A I due t o t h e presence o f a a s a t i s f i e s t h e e q u a t i o n :

where aa

<

am.

For a sharp v i b r a i i o n a l a b s o r p t i o n l i n e , t h e l i n e w i d t h i s much l e s s t h a n t h e c e n t e r f r e q u e n c y v o and Eq. 3 becomes

A m*cZ A 1

n l =

- -

(",,,Y )

(II Imi

min

x

(e*12

I n t h e l i m i t ^UT

>>

1 f o r a Drude m e t a l Eq. 6 reduces t o

Note t h a t Eq. 7 i s a p r o d u c t o f t h r e e t.errm: The f i r s t i s r e l a t e d t o t h e p r o p e r t i e s o f t h e v i b r a t i o n a l mode; t h e second, t o t h e p r o p e r t i e s o f t h e m e t a l s u b s t r a t e ; and t h e t h i r d , t o t h e s e n s i t i v i t y o f t h e e x p e r i i n e n t a l arrrangement.

An i m p o r t a n t q u e s t i o n i s : How do t h e s e n s i t i v i t i e s o f r e f l e c t i o n (RA) and SEW s p e c t r o s c o p y compare? To answer t h i s q u e s t i o n we model an absorbate by a l a y e r o f o p t i c a l t h i c k n e s s d and a b s o r p t i v i t y A. Next t h e model a d s o r b a t e i s p l a c e d on a Drude m e t a l and t h e responses t o RA and SEW experiments a r e e s t i m a t e d .

For RA spectroscopy w i t h a weak source so t h a t t h e measurement i s d e t e c t o r n o i s e l i m i t e d , Greenler 1131 showed t h a t t h e s i g n a l - t o - n o i s e r a t i o i s o p t i m i z e d f o r t h e TM p o l a r i z a t i o n when AR ( t h e depth o f an RA band) i s maximized w i t h r e s p e c t t o t h e number o f r e f l e c t i o n s No. T h i s maximum v a l u e occurs when

C5-174 JOURNAL DE PHYSIQUE

where R i s t h e r e f l e c t i o n c o e f f i c i e n t a s s o c i a t e d w i t h one r e f l e c t i o n f o r a p a r t i c u y a r a n g l e o f i n c i d e n c e ( t y p i c a l l y = 8 5 " ) .

For l a r g e angles o f i n c i d e n c e (45"

<

8

<

8 6 " ) , t h e f r a c t i o n a l change i n r e f l e c t i v i t y f o r No r e f l e c t i o n s i s

The s i n 2 8 f a c t o r o c c u r s because o n l y t h e component o f t h e E f i e l d p e r p e n d i c u l a r t o t h e s u r f a c e i s i m p o r t a n t . The angular dependence o f t h e o p t i c a l p a t h g i v e s t h e cos8 term.

For SEW s p e c t r o s c o p y we have a l r e a d y seen from Eq. 4 t h a t

AI a a

- z - = A

1 am SEW

We must now r e l a t e t h e ASEW f o r inhomogeneous waves t o t h e A o b t a i n e d f o r p l a n e waves. Because much o f t h e SEW wave i s s p a t i a l l y removed f r o m t h e adsorbate l a y e r , t h e SEW c o n f i g u r a t i o n has a sample f i l l f r a c t i o n which i s s m a l l e r t h a n t h e p l a n e wave f i l l f r a c t i o n by d/Y where Y i s t h e h e i g h t o f t h e SEW above t h e m e t a l so ASEW i s s m a l l e r t h a n A by t h i s f a c t o r . On t h e o t h e r hand, s i n c e t h e SEW i n t e r a c t s over t h e l e n g t h Io i n s t e a d o f t h e t h i c k n e s s d, A(d/Y) i s i n c r e a s e d by t h e

a d d i t i o n a l f a c t o r (Ro/d). The r e s u l t a n t expression, which i s independent o f d, i s

By combining Equations 9, 4, and 10, an e q u a t i o n which r e l a t e s t h e two s p e c t r o s c o p i e s i s o b t a i ned; namely,

A I no cos 8 AR

y "

^Nosinie

T h i s e q u a t i o n can be s i m p l i f i e d f u r t h e r s i n c e a a

<<

am so t h a t i n t h e energy 1 i m i t e d r e g i o n

1 0

-

⁼ (amy)-' = 2wp7 Y

( 1 2 )

f o r a Drude metal i n t h e UT

>>

1 l i m i t . I n s e r t i n g E q u a t i o n 12 i n t o Eq. 11 g i v e s

cos 6 AR

RA experiments are t y p i c a l l y mqde a t 0

- ^85"

w i t h No 2 so t h a t cos 0 / s i n 2 0

-

0.1. For m s t m e t a l s wpz

-

¹⁰

.

P u t t i n g t h e s e parameters i n t o Eq. 13 we f i n d

Given equal p r o b i n g i n t e n s i t y a t t h e m e t a l - a b s o r b a t e i n t e r f a c e , SEW spectroscopy would p r o v i d e more s e n s i t i v i t y i n t h e d e t e c t o r n o i s e - l i m i t e d regime. However, because o f t h e l o w - c o u p l i n g e f f i c i e n c y , l i t t l e improvement can be expected at t h e p r e s e n t t i m e over RA s p e c t r o s c o p y i f a weak source i s used.

A d i f f e r e n t comparison s h o u l d be made i f one makes b o t h measurements w i t h a s t r o n g IR l a s e r source. I n t h i s l i m i t i t i s t h e source n o i s e n o t t h e d e t e c t o r n o i s e which dominates /Il/. Because source n o i s e i s p r o p o r t i o n a l t o t h e i n t e n s i t y , t h e

s i g n a l - t o - n o i se r a t i o i s o p t i m i zed b y maximi z i n g t h e c o n t r a s t , namely, AR/R o r AI/I. Although b o t h Equation 9 ( f o r AR/R) and E q u a t i o n 2 ( f o r A I / I ) i n d i c a t e t h a t t h e sample should be made as l o n g as p o s s i b l e i n t h e s t r o n g source l i m i t , one can compare t h e two t e c h n i q u e s b y r e q u i r i n g t h a t t h e two c o n t r a s t s be equal, t h e r e f o r e ,

The s u b s c r i p t N r e f e r s t o t h e number o f r e f l e c t i o n s needed i n RA spectroscopy t o s a t i s f y Eq. 11, namely,

a0 cos 0 N = -

y

sin'B'

Again f o r 0

-

^{85' and Y = 60 p t h e n N = 17 a,.} For t h e RA experiment w i t h t h e sample c o n s i s t i n g o f two p a r a l l e l m e t a l p l a t e s separated b y a 1 nun gap t h e RA sample would have t o be 38 t i m e s t h e l e n g t h o f t h e SEM sample i n o r d e r t o o b t a i n t h e same c o n t r a s t . Since t h e sample l e n g t h s f o r t h e SEW measurements i n t h e 10 p m r e g i o n are on t h e o r d e r o f a few cm, t h e c o r r e s p o n d i n g RA samples are i m p r a c t i c a l . For t h e s t r o n g IR source l i m i t SEM spectroscopy i s t o be p r e f e r r e d .

The f i r s t d i r e c t comparison o f SEM and RA s p e c t r o s c o p y was made u s i n g an edge c o u p l e r i n t h e weak source l i n i t

151.

The p o l y c h r o m a t i c edge c o u p l e r used i n t h i s experiment i s c o n t r a s t e d w i t h t h e monochromatic two-prism c o u p l e r i n Fig. ( 9 ) .

F i g u r e 9. Monochromatic and p o l y c h r o m a t i c

o ' ~ J A

g

SEM c o u p l i n g geometries. Because d i e l e c t r i c

m a t e r i a l s a r e d i s p e r s i v e each frequency has

-

i t s own t o t a l i n t e r n a 1 r e f l e c t i o n o r SEW

/',,'/ , / , / ' / i , / I 7 c o u p l i n g angle.

( a ) The two p r i s m evanescent wave c o u p l i n g t e c h n i q u e f a v o r s t h e l a u n c h i n g and r e c o v e r i n g

b) o f m n o c h r o m a t i c SEW's a t a f i x e d angle. The

i n t e r n a l a n g l e o f i n c i d e n c e a t t h e p r i s m base

wL?-?'-

^{8,; i s} ( b ) independent The edge c o u p l i n g of frequency. t e c h n i q u e i n a geometry g,

5;

which f a v o r s t h e l a u n c h i n g and r e c o v e r i n g o f

19 p o l y c h r o m a t i c SEW's. The d i s p e r s i v e r e f r a c t i o n

O o f t h e i n c i d e n t beam p a r t i a l l y compensates f o r t h e f r e q u e n c y dependence o f t h e c o u p l i n g angle f o r SEM g e n e r a t i o n .

C5-176 J O U R N A L DE PHYSIQUE

The e x p e r i m e n t a l arrangement f o r broadband SEW g e n e r a t i o n i s shown i n F i g . ( 1 0 ) . Broadband SEW's were generated on Au and Ag f i l m s w i t h a Nernst glower thermal I R source. A Michelson i n t e r f e r o m e t e r coupled t o a As:Si p h o t o d e t e c t o r was used t o measure s p e c t r o s c o p i c a l l y t h e t r a n s m i t t e d SEW1s i n t h e 600 and 1800 cm-' frequency r e g i o n .

rnirror

rnirror

F i g u r e 10. Top view o f t h e e x p e r i m e n t a l o p t i c a l s e t - u p f o r a broadband SEW t r a n s - m i s s i o n experiment

.

I n c o h e r e n t r a d i a t i o n f r o m a Nernst glower i s focused on a d i s - p e r s i o n compensated edge c o u p l e r . The k n i f e edge r n i r r o r r e d i r e c t s t h e SEM beam i n t o a M i c h e l s o n i n t e r f e r o m e t e r . A S i :As d e t e c t o r measures t h e i n t e r f e r o m e t e r o u t p u t s i g n a l . For i n i t i a l alignment t h e glower i s removed and He-Ne and CO, l a s e r beams a r e d i r e c t e d t h r o u g h t h e o p t i c a l path.

interferorneter

/W

\ f

detector

For a m e t a l samplf c o a t e d w i t h one monolayer of KReO, a sharp v i b r a t i o n a l a b s o r p t i o n l i n e near 930 cm- was measured as shown i n F i g u r e l i a . The r e s o l u t i o n o f t h i s spectrum i s about 50 cm-'. The SEW spectrum o f a b a r e r e g i o n o f t h e same Au s u r f a c e i s a l s o shown f o r r e f e r e n c e . Next, t h e same sample was measured by s u r f a c e

r e f l e c t i o n spectroscppy.. The presence o f t h e KRe0, c o a t i n g produces a 25 cm-' wide m u l t i p l e t a t 930 cm which i s about 1% deep f o r an a n g l e of i n c i d e n c e o f 70". T h i s spectrum shows a t r i p l e t s t r u c t u r e which t h e SEW measurement d i d n o t r e s o l v e . T h i s a b s o r p t i o n was used t o compare t h e SEW and RA s p e c t r o s c o p i c t e c h n i q u e s . The i n t e g r a t e d o p t i c a l d e n s i t y o f t h e a b s o r p t i o n l i n e i s at l e a s t an o r d e r o f magnitude l a r g e r wi t h SEW spectroscopy t h a n wi t h r e f l e c t i o n s p e c t r o s c o p y no m a t t e r what angle o f i n c i d e n c e i s chosen.

F i g u r e 11. SEW f o u r i e r t r a n s f o r m s p e c t r a and b u l k wave r e f l e c t a n c e s p e c t r a i n a KRe0, c o a t e d Au sub- s t r a t e . The c o a t i n g t h i c k n e s s i s about 1 monolayer.

( a ) The upper c u r v e i s a SEW t r a n s m i s s i o n spectrum o f a KRe0, c o a t e d Au s u r f a c e . The lower c u r v e i s a spectrum o f an uncoated r e g i o n o f t h e sarne Au f i l m . I n bothlcases t h e t r a n s m i s s i o n goes t o zero a t 500 cm- because t h e d i e l e c t r i c s u b s t r a t e i s opaque.

1000 2000 The c u t o f f a t about 1500 cm- i s m a i n l y due t o t h e

Frequency (cm-') f r e q u e n c y dependence o f t h e a t t e n u a t i o n c o e f f i c i e n t . The v 3 mode of t h e Re04- m7nolayer produces t h e

( b ) a b s o r p t i o n l i n e a t 930 cm-

.

.;; 099 ( b ) R e f l e c t a n c e s p e c t r a from t h e same sample as i n ( a ) a r e shown f o r a TM p o l a r i z e d beam o f r a d i a t i o n i n c i d e n t a t any a n g l e o f 70" w i t h r e s p e c t t o t h e normal. The upper t r a c e i s a spectrum o f t h e KReO,

097 c o a t e d r e g i o n w h i l e t h e lower t r a c e i s f r o m t h e b a r e m e t a l s u b s t r a t e .

L96ou;sJ

Frequency (cm-')

Another d e m o n s t r a t i o n o f t h e s e n s i t i v i t y o f t h e SEM t e c h n i q u e was o b t a i n e d b y u s i n g t h e broadband set-up w i t h Ag f i l m s . The KReO, was evaporated o n t 0 one h a l f o f t h e Ag f i l m w i t h i n one m i n u t e of t h e Ag e v a p o r a t i o n . The SEM t r a n s m i s s i o n s p e c t r a o f b o t h t h e b a r e and t h e two monolayer t h i c k coated p o r t i o n s o f t h e f i l m were observed s p e c t r o s c o p i c a l l y t o change as a f u n c t i o n o f t i m e i n a i r . The s p e c t r a a r e shown i n F i g u r e 12. The o r i g i n o f t h e new Ag l i n e i s unknown.

F i g u r e 12. SEW t r a n s m i s s i o n s p e c t r a o f f r e s h l y evaporated and contaminated Ag s u b s t r a t e s .

( a ) and ( b ) Transmission s p e c t r a o f b a r e and KReO, coated r e g i o n s o f an Ag f i l m t a k e n one day a f t e r t h e f i lm e v a p o r a t i o n .

( c ) and ( d ) S p e c t r a o f t h e same sample t a k e n two weeks l a t e r . A f t e r Ref. 14.

O I 2 r IO'

Frequency (cm-')

V

-

CONCLUSIONS

So f a r 0 n l y one UHV experiment has been attempted w i t h SEM spectroscopy

/a/.

^{The vl}

v i b r a t i o n a l mode o f H chemisorbed on W(100) a t s a t u r a t i o n coverage was measured w i t h 2 cm-' r e s o l u t i o n . A UHV c o m p a t i b l e b u t e x c e e d i n g l y i n t r i c a t e p a r a l l e l p l a t e geometry was devised so t h a t SEW's c o u l d be e x c i t e d on t h e W(100) s u r f a c e w i t h quasi - t u n e a b l e CO2 1 aser r a d i a t i o n . At room t e m p e r a t u r e t h e measured c e n t e r

f r e q u e n c y o f t h e v l mode, as measured w i t h SEW's, was i n good agreement w i t h e a r l i e r e l e c t r o n energy l o s s measurements. Because t h e sample geometry was not c o m p a t i b l e w i t h temperature dependent s t u d i e s o r w i t h o t h e r s u r f a c e s e n s i t i v e probes, work w i t h t h i s c o u p l i n g c o n f i g u r a t i o n has been d i s c o n t i n u e d .

R e c e n t l y Z h i z h i n , e t a l /6/, showed t h a t g r a t i n g c o u p l e r s t o g e t h e r w i t h broadband f o u r i e r t r a n s f o r m s p e c t r o s c o p i c t e c h n i q u e s c o u l d be used t o p e r f o r m SEN

s p e c t r o s c o p i c measurements on b u l k rnetal s u b s t r a t e s . We, i n t u r n , have found t h a t g r a t i n g c o u p l e r s can be r e a d i l y etched i n t o s i n g l e c r y s t a l W and t h a t t h e c o u p l i n g e f f i c i e n c y i s l a r g e r t h a n f o r t h e edge c o u p l e r . The e l i m i n a t i o n o f t h e d i e l e c t r i c c o u p l e r i s p a r t i c u l a r l y i m p o r t a n t f o r SEN s t u d i e s o f s i n g l e c r y s t a l samples i n UHV and i t i s now c l e a r t h a t t h e n e x t g e n e r a t i o n o f SEM c o u p l e r s wi 11 make use of p e r i o d i c s t r u c t u r e s .

T h i s work was supported b y t h e N a t i o n a l Science Foundation under Grant No.#

DMR-81-06097 and b y t h e A i r Force under Grant No.# AFOSR-81-0121B. M a t e r i a l s Science Center Report No.# 5125.

JOURNAL

DE

PHYSIQUE

V I

-

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*

C u r r e n t address: Bel 1 Telephone Labs, 600 Mountain Avenue, Murray Hi 11, New Jersey, 079741.

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