NEAR-EDGE X-RAY ABSORPTION FINE-STRUCTURE STUDIES OF RING MOLECULES ADSORBED ON SINGLE CRYSTAL SURFACES

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NEAR-EDGE X-RAY ABSORPTION FINE-STRUCTURE STUDIES OF RING

MOLECULES ADSORBED ON SINGLE CRYSTAL SURFACES

M. Bader, J. Haase, K.-H. Frank, C. Ocal, A. Puschmann

To cite this version:

M. Bader, J. Haase, K.-H. Frank, C. Ocal, A. Puschmann. NEAR-EDGE X-RAY ABSORPTION FINE-STRUCTURE STUDIES OF RING MOLECULES ADSORBED ON SINGLE CRYSTAL SUR- FACES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-491-C8-496. �10.1051/jphyscol:1986892�.

�jpa-00226223�

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JOURNAL DE PHYSIQUE

Colloque C8, supplkment au n o 12, Tome 47, dbcembre 1986

NEAR-EDGE X-RAY ABSORPTION FINE-STRUCTURE STUDIES OF RING MOLECULES ADSORBED ON SINGLE CRYSTAL SURFACES

M. BADER, J. HAASE, K. -H. FRANK, C. OCAL* and A. PUSCHMANN

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, F.R.G

' ~ n i v e r s i d a d Autonoma de Madrid

A b s t r a c t

Near edge X-ray absorption f i n e s t r u c t u r e (NEXAFS) has been used t o e s t a b l i s h the o r i e n t a t i o n o f t h e six-membered r i n g molecules benzene, p y r i d i n e , pyrazine and s - t r i a z i n e adsorbed on Cu and Ag s i n g l e c r y s t a l surfaces. For submonolayer coverages o f p y r i d i n e on A g ( l l 1 ) an o r i e n t a t i o n a l phase t r a n s i t i o n has been observed w i t h NEXAFS i n r e a l time.

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

The determination o f the s t r u c t u r e of adsorbed species on s w f a c e s i s a key problem i n surface science and d u r i n g t h e l a s t few years considerable progress has been made. With 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) spectroscopy

/1/

a technique now i s a v a i l a b l e which y i e l d s i n f o r m a t i o n on t h e o r i e n t a t i o n o f molecules on surfaces i n an easy and s t r a i g h t f o r w a r d way. I n t h e present study we have i n - v e s t i g a t e d t h e molecular o r i e n t a t i o n o f t h e six-membered r i n g molecules benzene

(C6H6), p y r i d i n e (C5H5N), pyrazine (C4H4N2) and s - t r i a z i n e (C3H3N3) on Cu(llO), C u ( l l 1 ) and A g ( l l 1 ) s i n g l e c r y s t a l surfaces by a n a l y s i ng t h e p o l a r i z a t i o n dependence of t h e a resonance i n t e n s i t i e s i n t h e i r NEXAFS spectra. Our s p e c i a l i n t e r e s t was focused on p y r i d i n e a d s o r p t i o n which c o u l d have model character f o r determining t h e r e l a t i v e c o n t r i b u t i o n s of n i t r o g e n l o n e - p a i r and a bonding t o t h e chemisorp- t i o n bond. I n p a r t i c u l a r we were i n t e r e s t e d i n showing t h a t w i t h high-brightness storage r i n g s and advanced grazing-incidence monochromators s t r u c t u r a l hase t r a n -

5 F - a - s i t i o n s

-

as they were found i n t h e p y r i d i n e / A g ( l l l ) system /2/

-

^{can be}^o^serve

byr?iFXAFS i n r e a l time.

2. Experimental

The experiments were performed a t t h e B e r l i n storage r i n g f o r synchrotron r a d i a t i o n , BESSY, u s i n g t h e p l a n e - g r a t i n g grazing-incidence monochromator SX-700 / 3 / w i t h a 1200 l/mm-grating. The data were taken i n the p a r t i a l e l e c t r o n - y i e l d mode /4/ run- rting t h e monochromator w i t h a sweep r a t e o f 30 eV/min. Spectra shown a r e background c o r r e c t e d (covered/clean). The NEXAFS was s t u d i e d a t t h e C and N K-edges. The X-ray incidence angle on t h e sampae c o u l d be v a r i e d from near g r a z i n g incidence (6 = 20 ) t o normal incidence ( 8 = 90

1.

The Cu and Ag c r y s t a l s , mechanically and chemically polished, were cleaned by Ar bombardment and subsequent annealing a t

-

600 K. Good

~ ( 1 x 1 ) LEED p a t t e r n s were obtained. The s u r f a c e cleanness was checked by AES. Du- r i n g exposure and measurement t h e c r y s t a l s were h e l d a t a temperature o f

-

^{100 K.}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986892

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JOURNAL DE PHYSIQUE

3 . Results and Discussion 3.1 Peak assignment

C h a r a c t e r i s t i c K-edge NEXAFS spectra o f six-membered r i n g molecules condensed on C u ( l l 0 ) a r e shown i n F i g . 1. The benzene ( l e f t ) and p y r i d i n e ( r i g h t ) spectra were taken a t t h e C and N K-edge, r e s p e c t i v e l y . Both spectra have t h e same main s t r u c - t u r e s l a b e l e d A

-

D.

F i g . 1: C and

N

K-edge NEXAFS spectra o f condensed m u l t i l a y e r s o f benzene ( l e f t ) and p y r i d i n e ( r i g h t ) , r e s p e c t i v e l y , on C u ( l l 0 ) a t 100.K.

For peak n o t a t i o n see t e x t .

I n benzene, peaks A and B correspond t o t r a n s i t i o n s o f 1s e l e c t r o n s i n t o two un- f i l l e d a* o r b i t a l s w i t h ezu and b2n symmetry, r e s p e c t i v e l y , separated by 3.7 eV /5/.

I n p y r i d i n e t h e degenerate e2, o r b i t a l o f benzene i s s p l i t by 0.6 eV i n t o two l e v e l s o f bl and a2 symmetry /5/, which i n t h i s experiment were n o t resolved because t h e monochromator r e s o l u t i o n was s e t a t 1.2 eV (N K-edge). The measured energy separa- t i o n between peaks A and

B

i s (3.7 i. 0.2) eV f o r benzene and ( 3 . 8 i. 0.2) eV f o r py- r i d i n e . I n p y r a z i n e spectra ( n o t shown here) peak A c l e a r l y has a shoulder i n agreement w i t h t h e much l a r g e r s p l i t t i n g o f t h e e2, o r b i t a l i n t o two l e v e l s of b3U and au symmetry /5/. I n general, peaks A and B show t h e same p o l a r i s a t i o n dependence. I n t h e remainder o f the paper we w i l l analyse o n l y peak A and r e f e r t o i t as t h e "n resonance". There i s no disagreement i n t h e l i t e r a t u r e about t h e assignment o f peak D as a a -resonance corresponding t o t r a n s i t i o n s o f 1s e l e c t r o n s i n t o a* o r b i t a l s . I n i n n e r - s h e l l EELS measurements o f benzene /6/ peak C, however, was assumed t o be a shakeup s t r u c t u r e , whereas r e c e n t m u l t i p l e s c a t t e r i n g c a l c u l a t i o n s f o r benzene /7/ suggest t h a t peak C i s a l s o a a resonance. F i g . 2 shows' NEXAFS spec- t r a f o r a submonolayer goverage o f benzene adsorbed on C u ( l l 0 ) . The spectra were taken a t normal ( 9 = 90 ) and near g r a z i n g ( 9 = 20 ) incidence and a r e i d e n t i c a l i n both azimuths LOOlJand ~-1i0]. Whereas t h e i n t e n s i t i e s o f peaks A, B and D show t h e expected l a r g e p o l a r i s a t i o n dependence o f a f l a t l y i n g molecule t h e i n t e n s i t y o f peak C stays n e a r l y constant. Hence, t h e dominant c o n t r i b u t i o n t o neak C cannot have a resonance character.

3.2 Benzene, p y r i d i n e and pyrazine on C u ( l l 0 )

NEXAFS spectra f o r benzene have a l r e a d y been shown i n F i g . 2 . As t h e a resonance i n t e n s i t y v a r i e s w i t h t h e angle y between t h e E v e c t o r and t h e normal t o t h e mo- l e c u l a r plane,

ii

(see i n s e t i n F i g . 2), according t o / 8 /

In ^acos L y

t h e a n a l y s i s y i e l d s an angle a between t h e r i n g plane and t h e surface o f a = 0 2 10'.

The benzene molecule i s thus l y i n g f l a t on t h e C u ( l l 0 ) surface and i s t h e r e f o r e pre- dominantly a-bonded t o i t . This i s i n agreement w i t h NEXAFS r e s u l t s f o r benzene on P t ( . l l l ) /9/ and i n agreement w i t h t h e r e s u l t s o f numerous o t h e r surface science techniques.

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I

1L Benzene on CulllOt - IOOK

I

F i g . 2: C K-edge NEXAFS spectra o f submono- l a y e r coverage o f Benzene on C u ( l l 0 ) f o r normal g0

7

90 ) and near graz- i n g (0 = 20 ) incidence. The spectra are i d e n t i c a l i n the A O l J a n d (110J i z i m u t h s . I n s e t : Measuring geometry.

M denotes t h e normal t o t h e r i n g - plane, n t h e surface normal and E t h e e l e c t r i c f i e l d v e c t o r .

PhOton energy l e v

For p y r i d i n e t h e ^Tresonance i n t e n s i t i e s a r e p l o t t e d as a f u n c t i o n o f t h e p o l a r angle 0 i n F i g . 3. &he a n a l y s i s o f t h e angular dependence i n t h e ,CllO]azimuth y i e l d s CY = 90

+

10 whereas t h e constant i n t e n s i t y i n [OOl/indicates t h a t t h e r i n g

~ ---

I L Pyridine on Cu (110) - 100 K

F i g . 3: N K-edge a resonance (peak A) i n t e n - s i t i e s f o r a submonolayer coverage o f p y r j d i n e on C u ( l l 0 ) i n t h e [OOl]

and [ l l O j azimuths as f u n c t i o n o f t h e p o l a r angle 0.

plane 1 ie s para1 l e l t o [OOl]. This f i n i t e i n t e n s i t y i n [OOlj'is due t o i m p e r f e c t ( ~ 8 5 % ) source p o l a r i s a t i o n as expected

/ l o / .

Our data a r e i n agreement w i t h angle- r e s o l v e d photoemission r e s u l t s on t h e same system /11/ which i n d i c a t e d t h a t p y r i d i n e adsorbs w i t h i t s aromatic r i n g perpendicular t o t h e surface. T h i s l a t t e r study a l s o proved t h a t t h e adsorbate/substrate i n t e r a c t i o n occurs through t h e l o n e - p a i r o f t h e n i t r o g e n atom.

The a n a l y s i s o f o u r NEXAFS data f o r pyrazine y i e l d s CY = 90' k 10' w i t h t h e r i n g plane of p y r a z i n e a l i g n e d along t h e [OOl] d i r e c t i o n . This r e s u l t i s i d e n t i c a l t o t h a t f o r p y r i d i n e .

Also, measurements a t t h e C and ?I K-edge f o r t h e n i t r o g e n c o n t a i n i n g molecules py- r i d i n e and p y r a z i n e y i e l d e x a c t l y t h e same a-values.

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C8-494 JOURNAL DE PHYSIQUE

3.3 Benzene, pyridine, pyrazine and s - t r i a z i n e on Cu(ll1)

On f c c ( l l 1 ) surfaces the n resonance i n t e n s i t y f o r molecules with the ring plane t i l t e d by an angle a with respect t o the surface i s given by /12/

where P i s the degree of l i n e a r polarisation of the synchrotron radiation. Our data f o r submonolayer coverages of the ring molecules adsorbed on Cu(ll1) have been ana- lysed according t o equ. ( 2 ) . The determined t i l t angles a a r e summarised in Table l together with the measured a-values f o r adsorption on the Cu(ll0) and Ag(ll1) surfaces.

Table 1: Measured t i l t angles (see Fig. 2 )

Molecule Cu(ll0) Cu(ll1) Ag(ll1)

Benzene 0 _-

+ lo0

₀

+

- l o 0

Pyridine 90' _-

+

l o 0 70'

- +

^5' ^45'_70'

5 +

5' 5' (mono1 (low coverage)

.

coverage) Pyrazine 90' _-

+

l o 0 58'

- +

lo0

Whereas benzene l i e s f l a t on Cu(ll1) as on Cu(llO), the measured t i l t angles f o r the other six-membered ring mglecuJes d i f f e r from those measured on C u ( l l 0 ) . The measured t i l t angle of a = 70 ? 5 f o r pyridine i s in agreement with nearly a l l r e s u l t s of NEXAFS /9/ o r of other surface science techniques /13

-

15/ applied t o pyridine adsorbed on d i f f e r e n t (111) surfaces, which indicated more or l e s s upright standing pyridine molecules. There i s even q u a n t i t a t i v e agreement with NEXAFS data on P t ( l l 1 ) (high-temperature s t a t e ) /9/ and with ESDIAD data on I r ( l l 1 ) /13/. The only known r e s u l t s /2/ a t varaance ~ i t h our data a r e discussed in chapter 3.4. The measured t i l t angle of a = 58

+

10 f o r pyrazine, however, i s in c o n f l i c t with EELS data /16, 171 taken on A o f l l l l which indicated f l a t lvina molecules. The aareement w i t h a = 55' measured by'ell;psometry on pyrazine condetised on Ni(ll1). /1G/ might only be accidental.

We should s t r e s s here t h a t in no case has a phase t r a n s i t i o n been observed f o r submonolayer coverages of the investigated ring molecules on Cu(ll1).

3.4. Phase t r a n s i t i o n in the system pyridine/Ag(lll) /12/

The normalized nitrogen K-edge r resonance i n t e n s i t y f o r normal incidence ( 0 = go0), I g O , and the i n t e n s i t y r a t i o 120/190 as a function of dose during permanent adsorption of pyridine (c 0.1 L/min) on Ag(ll1) i s shown in Fig. 4. The r a t i o 1 2 ~ / I g 0 i s constant a t dosgs l o t e r than ~4 L . According to equ. ( 2 ) i t corresponds t o a t i l t angle of a = 45

+

⁵

.

Between 4 and 5.5 L the r a t i o 120/190 decreases d r a s t i c a l l y and s t g a d i l x increases a t higher doses. The minimum value of 120/190 corresponds t o a = 70

+

⁵

.

Finally 120/190 = 1 f o r a randomly oriented condensed multilayer i s approached. We therefore ass-~gn a monolayer t o an exposure of approximately 5.5 L in agreement with inverse photoemission, UPS, and work-function r e s u l t s /14/. Hence, we observed an orientational phase t r a n s i t i o n f o r a submonolayer coverage in agreement with the vibrational EELS (VEELS) data /2/. The measured t i l t angles compare 3

well w i t h those found by NEXAFS f o r pyridine on P t ( 1 l l ) /9/ where a temperature de- pendent phase t r a n s i t i o n was observed. The t i l t angle of 45' measured f o r the low- coverage, 1 ow-temperature phase of pyridine on Ag(ll1)

,

however, i s a t variance with

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t h a t i n d i c a t e d by VEELS f o r t h e same system. F u r t h e r c a r e f u l VEELS and NEXAFS s t u - d i e s on i d e n t i c a l systems should h e l p t o s o l v e t h e obvious discrepancies /12/. Our NEXAFS r e s u l t s suggest t h a t even a t low coverages

N

l o n e - p a i r bonding occurs.

F i g . 4: N K-edge a resonance (peak A) i n t e n s i t y f o r normal incidence ( 8 = 90'1, IgO ( f u l l c i r c l e s ) , and t h e i n t e n s i t y r a t i o 120/190 (crosses) as a f u n c t i o n o f dose d g r i n g permanent a d s o r p t i o n o f p y r i d i n e (?.0.1 L/min.) on A g ( l l 1 ) a t 100 K. The 1 2 0 / 1 9 0 data p o i n t s w i t h e r r o r bars were i n - dependently measured a t f i x e d p y r i d i n e doses. There i s a phase t r a n s i - t i o n near 4.5 L,

4. Conclusions

With NEXAFS t h e o r i e n t a t i o n o f molecules on s i n g l e c r y s t a l surfaces can be determined i n an easy and s t r a i g h t f o r w a r d way. Our NEXAFS r e s u l t s on six-membered r i n g molecules adsorbed on Cu and A? surfaces show: While benzene l i e s f l a t on b o t h C u ( l l 0 ) and C u ( l l 1 ) surfaces, and p y r i d i n e and pyrazine stand u p r i g h t on C u ( l l 0 ) w i t h t h e r i n g plane a l i g n e d along & h e C O O l J ~ i r e c t i o n , p y r i d i n e , p y r a z i n e and s - t r i a z i n e a r e more o r l e s s t i l t e d (45 5 a I 70 ) on C u ( l l 1 ) and A g ( l l 1 ) surfaces. I t has been shown t h a t NEXAFS can d e t e c t o r i e n t a t i o n a l phase t r a n s i t i o n s i n r e a l time. Such a phase t r a n s i t i o n was observed a t a submonolayer coverage i n t h e system p y r i d i n e / A g ( l l l ) .

Acknowledgements

The authors a r e pleased t o acknowledge t h e support o f t h e Bundesministerium fir Forschung und Technologie under Grant No. 05 238 JH.

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