PROBING HIGHER ORDER CORRELATION FUNCTIONS IN LIQUIDS BY XANES (X-RAY ABSORPTION NEAR EDGE STRUCTURE)

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PROBING HIGHER ORDER CORRELATION

FUNCTIONS IN LIQUIDS BY XANES (X-RAY

ABSORPTION NEAR EDGE STRUCTURE)

A. Bianconi, J. Garcia, A. Marcelli, M. Benfatto, C. Natoli, I. Davoli

To cite this version:

+ I.N.F.J. Laboratori NazionaZi d i Frascati, 00044 Frascati, ItaZy

++ Dipartimento d i Matematica e Fisica, Università d i Camerino, 62032

Camerino, I t a Zy

Abçtract- The higher order mrrelation functions of local atomic distribution at metal ion sites in acquews solutions have been probed by XANES (X ray absorption near edge structure) experiments using synchrotron radiation. Multiple scattering thmry bas been spplied to interpret the deta and we show that in these systems the expansion of the total absorption cross section in terms of contributions of higher order scattering processss 1s possible wer a 18rge energy range. Thwefore higher order distribution functions, beyond the pair distribution function probed by EXAFS, are probed by XANES, which is shown to be a powerful method for site geometry determination in liquids and amorphws systems.

lnterest i n local structure determination beyond the pair distribution function has recently stimulated the growing of X-ray absorption near edge structure (XANES) ( I + ) . The low energy part of x-ray absorption spectra i s sensitive to the geometrical arrangement of the environment surrounding the absorbing atom because the strong scattering power, for the low energy photoelectrons, favors multiple scattering (MS) processes. At high energies such that the atomic scattering power becomes substantially small a single scattering (SS) regime take place, where the modulation i n the absorption coefficent (EXAFS) i s substantially due to the interference effect of the outgoing photoelectron wave from the absorbing atom and the backscattered wave from each surrounding atom (6~7). Hence this latter part of the spectrum provides information about the pair correlation function. By decreasing the photoelectron kinetic energy a gradua1 t u m over occurs from the EXAFS SS regime to the XANES f u l l multiple scattering (FMS) regime where al1 the MS pathways which begin and end at the absorbing atom contribute to the total absorption cross section. A transition region where only low order multiple scattering (MS) paths are relevant i s expected. By making the expans ton of the total cross section i n the energy region, where i t i s possible, i t i s has been shown (6) that the contributions of successive scattering orders t o the total absorption cross section can be ident i f ied.

Here we report a study of the site geometry of transition metal ion i n solution by XANES

.

"permanent address : Department of Thermology, Zaragoza University, Spain

JOURNAL DE PHYSIQUE

1

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.

.

.

,

i

.

.

.

i

j

8

-.

..

-50 O 50 100 150 200

E-EO

_-

(eV)

fi(LL X-ray absorption spectrum and itsderivative (lower panel) of fIn2+ion i n

solution. The zero of the energy =le is fixed at the week 1 s-3d excitation at threshold.

O 2 4 6 8 1 O

ENERGY (Ryd)

spectrum shown i n fig.2 for an octahedral cluster formed by the central manganese and six neighbouring oxygens i n good agreement with the experiment. The expansion of the total cross section A t =

%(

l

+EXn)

i s demonstrated to be feasible above threshold.

oC,

the atomic absorption and

Xn

the contributions due to succesive orders of scattering pathways are shown i n fig. 2. The multiple scattering pathways are classified according with the number of scattering events n, therefore the corresponding number of neighbourg atoms partecipating t o the scattering process are n- 1. In the octahedral geometry we observe that the multiple scattering contributions extend up t o 150 eV, but a destructive interference effect between the n=3 and n=5 contributions. Because of his destructive interference effect the EXAFS (n=2) becomes the main contribution above about 60 eV. At lower energies the contributions of higher orders mu1 tiple scatering events become dominant.

In fig.3 and 4 we report the experimental and theoretical spectra of the Mn04 cluster respect ively.

The expansion of the total absorption i n the tetrahedral geometry shows that the successive oders of scattering become negligible w i t h increasing the energy. What is important to remark i n fig.4 i s that higher order

JOURNAL DE PHYSIQUE

u 3 XANES spectrum of Hn04 in acqmus solution, and its derivative (lower panel) W

g .

O n Z

2

5

S

Li&4

Theoreticel absorption spectrum showing, 8s in fig. 2 , the total, atornic

(deshed line) wd the n=2,3,4,5 partiel hi*

or-

contributions.

C9-106 JOURNAL DE PHYSIQUE

In fig. 5 we show the good agreement between the experimental and theoretical XANES spectrum of Crû4 tetrahedral cluster. In fig. 6 the contribution of higher order scattering pathways to the theoretical total absorption coefficient,as i n fig.2 and fig.4, i s shown for the Crûq cluster. These spectra confirm the results obtained for the manganese solutions.

In conclusion we have shown that XANES data analysis has reached the level of quantitative agreement with the experimental spectra of ion i n solution and the way as higher order correlation functions contribute t o

XANES has been clarified. This open the way t o quantitative application of

XANES t o more complex structural problems i n liquids, proteins and surfaces.( l )

1 A . B i m i ,Appl. of Surfm Science 6,392 ( 1980)

2 M.Belli, ASmfati, A . B i m i , S.Mobilio, L.Palladino. A.Rmle, and E.Burattini .%/id SMe Lbmmun. 35.392 ( 1980)

3 ) ABiarmmi,S.Doniach, snd D.Lublin Chm. Pm. Lefi. 59, 121 ( 1978)

4 1 A . B i a m i , L.lncoccia, S.Stipchich eds.

" F M

8nd Ne8r

Fm

SIructufe" "ringer

Series in Chem. Phys. vol. 27 ( 1983)

5 ) ABianmi, M.DelllAriccia, P.J.Durham and J.B.Pendry P M . Rev. 826,6502, ( 1982) 6) M.Benfatto, C.R.Natoli, A . B i a m i , J.üarci8, A.Marœlli and I.Davoli, P M . Rev. B, to be

published and Frascati L.N.F. Report ( 1985)

7 ) D.E.Sayers, EA.Stern and F.W.Lytle Pm. Rev. Letf. 2 7 , 1204,( 1971 1