EXTENDED ENERGY LOSS FINE STRUCTURE (EELFS) ABOVE THE CARBON K-EDGE IN CF4 STUDIED AT DIFFERENT MOMENTUM TRANSFERS

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EXTENDED ENERGY LOSS FINE STRUCTURE (EELFS) ABOVE THE CARBON K-EDGE IN CF4

STUDIED AT DIFFERENT MOMENTUM TRANSFERS

P. Letardi, R. Camilloni, G. Stefani

To cite this version:

P. Letardi, R. Camilloni, G. Stefani. EXTENDED ENERGY LOSS FINE STRUCTURE (EELFS) ABOVE THE CARBON K-EDGE IN CF4 STUDIED AT DIFFERENT MOMEN- TUM TRANSFERS. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-1125-C9-1128.

�10.1051/jphyscol:19879205�. �jpa-00227324�

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Colloque C9, supplement au n012, Tome 48, decembre 1987

EXTENDED ENERGY LOSS FINE STRUCTURE (EELFS) ABOVE THE CARBON K-EDGE IN CF, STUDIED AT DIFFERENT MOMENTUM TRANSFERS

P. L E T A R D I ~ ' ) , R. CAMILLONI and G. STEFAN1

Istituto Metodologie Avanzate Inorganiche-CNR, Area della Ricerca di Roma, CP 10, I-00016 Monterotondo, Italy

ABSTRACT-Angle resolved EELFS on gas-phase CFq i s analyzed a t s c a t t e r i n g angle 8 r a n g i n g from 5' t o 50', The extended f i n e s t r u c t u r e a t small and large 8 a r e shown to be different, In t h e dipolar regime t h e steep behavlour of t h e background produces uncertainties i n t h e d a t a analysis which does not affect large angle spectra.

I N T R O D U C T I O N

I t has been recently recognized t h a t t h e interference effect leading t o EXAFS also af f eCtS experimental phenomena other than X-Ray absorption. The different EXAFS- liKe techniques /l/ utilize electron beam as excitation source, t h u s avoiding t h e requirement of a syncrotron radiation facility.

Extended Energy Loss Fine S t r u c t u r e (EELFS) i s t h e technique more similar to EXAFS. When t h e ionization source i s a photon beam t h e cross'section i s given by:

o(E) a E XI < f l E y .rl i > l 2

where Ey i s t h e photon p o l a r i z a t i o n u n i t v e c t o r .

For electron energy loss t h e cross section f o r angle resolved measurements i s : a(E) a q-4 K Z l i f leiq'rli>Ip

where q i s t h e momentum transfer, whose value i s determined, f o r a given energy loss E and f o r p r i m a r y energy Eg, by t h e s c a t t e r i n g angle 8. Thus d i f f e r e n t values of t h e cross section can be expected f o r t h e same enengy loss E depending on t h e experimental conditions. In t h e l i m i t q->O it i s

tf leiq r l i > :: i q < f l E , . r l i >

and t h e same matrix element c h a r a c t e r i z e s both t h e photon and t h e electron induced ionization. For larger values of t h e momentum t r a n s f e r t h e multipole contributions t o t h e extended f i n e s t r u c t u r e have t o be considered. To o u r knowledge n e i t h e r experimental analysis n o r t h e o r e t i c a l calculation on t h l s subject have been published. Still t h e interest on electron induced extended f i n e s t r u c t u r e is increasing due to t h e appealing f e a t u r e of s u r f a c e sensitivity. The i n t e r e s t on adsorbed atoms have led t o use these tecniques on low Z elements.

Questions about the technique sensititivity and o t h e r experimental problems f o r such systems a r e addressed i n t h e following sections.

E X P E R I M E N T A L

The inelastic electron yield N(E) h a s been collected with a hemispherical analizer a t constant f i n a l energy Es=1500 eV and 2.5 eV FWHM resolution on t h e e l a s t i c

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879205

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C9-1126 JOURNAL DE PHYSIQUE

peak The angular resolution was 3 mrad. We measured angle resolved EXTENDED ENERGY LOSS FINE STRUCTURE (EELFS) above t h e Carbon K edge i n t h e CF4 molecule (I.P.=301.8eV 2 a t s c a t t e r i n g a n g l e s 8 = 5 ' , 1 5 * , 4 O ' a n d SO',

w i t h 5yA c u r r e n t a t low 0's and 1OOvA a t large ones. Our 8 = 5 * spectrum shows similar f e a t u r e s to t h e photoabsorption and EELFS measurements a t e=2*,E0=2keV r e p o r t e d i n l i t e r a t u r e / 3 , 4 / . Our s p e c t r a a r e characterized by a momentum t r a n s f e r q ranging from - 2 ~ - I to z l 9 ~ - ~ ; as the r aspectation value for t h e Carbon-is orbital i s aO.iA, both dipolar and not dipolar regime a r e investigated.

The key quantity i n t h e extended fine s t r u c t u r e spectroscopies i s t h e function

x , defined a s t h e per cent difference between t h e actual cross section o and

t h e value a' t h e cross section shou'ld assume i n absence of d i f f r a c t i o n by t h e neighbouring atoms, i.e. i n absence of interference effects (a' i s sometimes called "atomic cross section")

0

-

⁰^'

x :

---

0'

To determine x from t h e N(E) spectrum one needs to remove t h e background and to determine a'.

The background i s essentially due t o t h e excitation and ionization of outer electrons.

I t has been removed, a s usual, w i t h a n extrapolation of t h e pre-edge behaviour. In dipolar conditions it has roughly an inverse t h i r d power dependence on t h e energy loss. In t h e case of t h e few hundreds eV of t h e core

edges of light atoms t h i s dependence produces _a_steep decrease of t h e signal. Because .

-

of t h e s h o r t bond-lenghis r J s typical of

3

molecules w i t h light atoms t h e modulations of

t h e cross section a r e observed over a wide

'

_!3

range of k. Hence a wide energy loss spectrum u

needs t o be recorded and analyzed. As a

consequence t h e extrapolation procedure can ^f'5W lead t o uncertainties i n t h e region f a r from

t h e edge.

Furthermore, f o r electron impact ionization the presence of t h e Bethe-ridge must be taken into account a t scattering angles f a r from 0.

A t large scattering angles t h e bacKgroundJs

slope is noticeable reduced w i t h respect t o _W _J _W _# _I _W ₆ _n ₁ t h e dipolar regime (fig. I) and less problems

are found i n the extrapolation procedure. E

[evl

In our analysis we found useful and physically

meaningful to describe the background w i t h t h e FIG.i-Experimental data a t 5. and f u n c t i o n 50'. The l a t t e r a r e renormalized to

f (E) : c2E-2 + C3E-3 + Ae-(E-Ea)' /B into aax,unt the different current and integrabon time The a r r o w indicates the where t h e gaussian term accounts f o r t h e onset for C-1s excitation

Bethe ridge. For fixed energy of t h e scattered

electron Es t h e position Ea and t h e amplitude a r e linlted to t h e o u t e r electron binding energy En and t o t h e i r average o r b i t a l momentum ^<q>by t h e r e l a t i o n s :

The o' component of t h e ionization spectra ["atomic absorption"] i s i t s smooth, structureless p a r t left once t h e outer snells contribution (background) has been removed. I t may be determined by smoothing t h e data between two fixed points

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of t h e r a p i d decrease of t h e oscillation i n t e n s i t y with increasing k, and a greater number of iterations was needed i n t h e low k region. Furthermore t h e small angles spectra a r e r a t h e r s e n s i t i v e t o t h e choice of t h e kmax value.At large scattering angles t h e problems a r e d u e t o t h e decrease of t h e signal-to-nolse r a t i o .

The x o b t e i n e d a t 8=5*,50* along w i t h t h e i r f o u r i e r t r a n s f o r m s

a r e shown i n figg.2-3.

The contribution to t h e x function due t o a single shell of neighbours can be obtained backtrasforming t h e related peaK i n t h e F(r). This procedure i s useful even In t h e case of a single-shell system, a s t h e CF4 molecule, because it allows to remove any spurious component due t o noise or t o uncertainties i n t h e d e r i v a t i o n of x.

FIG.2-The Kx(k) spectra from t h e C-1s

~ Fof CF4 a t small and large scattering S ^~ ^~ ^~ ^< ^- ^~ ^~ ^,

mrresmndig

^~ ^~ ^f to ^~ ^~ ⁱ ^O ⁿ angle.

R E S U L T S

O u r analysis shows t h a t for each of t h e scattering angles considered t h e Fourier f i l t e r e d x f u n c t i o n can be described by t h e formula :

a s i n photoabsorption

.

Since t h e Carbon-Fluorine bond-lenght i n CFy i s well Known (r-1.32 A), we can use t h e p h a s e - s h i f t f u n c t i o n s P(lc;B) I n o r d e r to characterize t h e extended fine s t r u c t u r e a t t h e different scattering angles. These functions a r e found t o be systematically d i f f e r e n t a t small and large scattering angles. Nevertheless t h e differences a r e comparable w i t h t h e uncertainty due to t h e data reduction procedure. In t h e f i g . 4 t h e experimental

vs

a r e compared wlth t h e ones computed by Teo and Lee /5/ f o r t h e photoabsorption case (dipole selection rule), where plane waves were used f o r t h e ejected electron. The large

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C9-1128 JOURNAL DE PHYSIQUE

discrepancy between t h e experimental data and t h e Teo and Lee predictions i s more likely to be due to t h e inadequacy of t h e plane wave approximation t h a n to non dipolar c o n t r i b u t i o n s t o t h e ionization process. This i s because similar discrepancies between theory and experiments were reported by Yang and Kirz /6/

f o r t h e photoabsorption extended f i n e s t r u c t u r e i n C02. In t h i s l a t t e r case only dipolar contributions exist and we a r e l e f t with t h e f a i l u r e of t h e plane waves approximation.

FIG.4-Comparison of the experimental phase s h i f t f a r a t c a n i c ~ G F at different scattering angles w i t h t h e one ampiuted by Teo and Lee

rn

E "

0-MT -6

-

Teo a n d Lee

U I

-8

-10

-12 " " ~ ~ ~ ~ ' ~ ~ ~ ~ ' ~ ~ ~ ~ ' ~ ' ~ ~

2 4 8 10

C O N C L U S I O N S

-

Extended f i n e s t r u c t u r e s above t h e i n n e r shell ionization edge have been observed also i n t h e energy loss s p e c t r a of low Z compounds.

-

Light atoms compounds have s h o r t bond-lengths, t h u s t h e analysis is t o be extended a t k values a s l a r g e a s 10 A - ~ . The analysis over a large K i n t e r v a l implies a s e n s i t i v i t y t o t h e bacKground s u b t r a c t i o n u n c e r t a i n t i e s t h a t is much larger t h a n i n t h e case of heavy atoms. The influence of t h i s problem i s greatly reduced a t large s c a t t e r i n g angle.

-

The x f u n c t i o n measured i n energy loss experiments shows remarKable similarities with t h e one determined i n photo-absorption.

-

The phase s h i f t function 0 shows some dependence on t h e momentum t r a n s f e r , though t h e magnitude of t h e v a r i a t i o n s is comparable t o t h e c u r r e n t experimental u n c e r t a i n t i e s .

-

The 0 function a s calculated by Teo and Lee i n plane wave approximation and dipolar i n t e r a c t i o n largely disagrees w i t h t h e experimental findings. The plane wave description of t h e ejected electron i s liKely t o be responsible f o r t h e major p a r t of t h e disagreement.

-

A more quantitative understanding of these effects will be t h e goal f o r f u t u r e s t u d i e s .

R E F E R E N C E S

/l/ f o r a b r i e f review see : E.A.Stern, J. de Phys. C8-47 3 (1986)

/2/ K.Siegbahn, C.Nordling, G.Johansonn, J.Hedman, P.F. Heden, K. Hamrin, U. Gelius, T. BergmarK, L.O. Werme, R. Manne and Y. Baer, ESCA Applied To' Free Molecules (North-Holland, Amsterdam, 1971)

/3/ F.C. Brown, R.Z. Bachrach and A.Bianconi, Chem. Phys. L e t t e r s 54, 425 (1978)

/ 4 / A.P. HitchcocK and I. I s h i i , J. de Phys. C8-47 199 (1986) /5/ B.K. Teo a n d P.A. Lee, J. Am. Chem. Soc. 101, 2815 (1979) . .

/6/ B.X. Yang and J. K i r z , J. Phys. B 35, 6100 (1987)