HAL Id: jpa-00226005
https://hal.archives-ouvertes.fr/jpa-00226005
Submitted on 1 Jan 1986
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
OXYGEN K-EDGE ABSORPTION SPECTRA OF SMALL MOLECULES IN THE GAS PHASE
Bin Yang, J. Kirz, T. Sham
To cite this version:
Bin Yang, J. Kirz, T. Sham. OXYGEN K-EDGE ABSORPTION SPECTRA OF SMALL
MOLECULES IN THE GAS PHASE. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-585-
C8-588. �10.1051/jphyscol:19868109�. �jpa-00226005�
JOURNAL DE PHYSIQUE
Colloque C8, suppl6ment a u n o 12, Tome 47, dkcembre 1986
OXYGEN K-EDGE ABSORPTION SPECTRA OF SMALL MOLECULES IN THE GAS PHASE
B.X. Y A N G , J. KIRZ and T.K. SHAM*
Department of Physics, State University of New York at Stony Brook, Stony Brook, NY 11794, U.S.A.
" ~ e p a r t m e n t of Chemistry, Brookhaven National Laboratory, Upton, NY 11973, U.S.A.
Abstract
The absorption spectra of 02, CO, C02 and OCS have been recorded in a transmission mode in the energy region from
500to
950eV. Kecent observation of EXAFS in these molecules is confirmed in this study.
Introduction
We report absorption spectra of CO, 02, C02 and OCS in the gas phase above the oxygen K-edge. Observation of EXAFS in three of these molecules was recently reported , l although it has been suggested that it should be very weak and
perhaps not ~bservable.~,~ The objective of this study is to further
investigate the EXAFS of these molecules. Using additional data and an improved data analysis procedure, we have confirmed our recent observation of EXAFS in these molecules.
Results and Mscussion
The schematics of the experimental arrangement are given in Fig. 1.
X - r a y
- ,
+ V SIG
Q
Fig. 1 Experimental setup: A) . I monitor, B) Si3N,, window (1300 A ) , C) Ion chamber, D) I monitor.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19868109
C8-586 JOURNAL DE PHYSIQUE
The incoming and t r a n s m i t t e d X-rays were monitored w i t h P31 phosphor
d e t e c t o r s
A andD. T h e i r response t o d i f f e r e n t photon e n e r g i e s and f l u x l e v e l had been c a l i b r a t e d e a r l i e r . 4 The a b s o r p t i o n spectrum was o b t a i n e d a s t h e r a t i o of t h e t r a n s m i t t e d f l u x w i t h and w i t h o u t g a s i n t h e c e l l .
The measurements were made a t t h e U15 beamline of t h e NSLS a t a gas p r e s s u r e of about 0.1 t o 0.3 t o r r . T h i s beamline has a t o r o i d a l g r a t i n g monochromator which p r o v i d e s a f l u x i n t h e range of 10" photons/second with second o r d e r c o n t a m i n a t i o n up t o about 15% n e a r t h e oxygen ~ - e d ~ e . ~ A procedure has been d e s i g n e d t o e f f e c t i v e l y c o r r e c t t h e second o r d e r c o n t r i b u t i o n . The c o r r e c t e d s p e c t r a taken a t 0.3 t o r r a r e shown i n Fig. 2 t o g e t h e r w i t h t h e sum of t h e atomic c r o s s s e c t i o n s of t h e elements compiled by Henke e t a ~ . ~
7
-
7 ,6 O2 6 .
co2
-
t h i s work-
t h i s work5 .
---
Henke e t . 01.---
Henke e t . 01.---.
'goo
600 700 800 900 1000 O 500 600 700 800 goo 10004 . 7
to 6
o c s
3 .
-
t h i s work-
t h i s work--- Henke e t . ol. --- Henke e t . 01.
-
2E
L
b l c ,
- =
" I l .'500 600 700 800 do0 1000 O 500 600 700 800 goo 1000
Photon energy(eV) Photon energy(eV)
Fig. 2 The a b s o r p t i o n s p e c t r a of 0 2 , CO, C02 and OCS.
It is a p p a r e n t t h a t t h e observed a b s o r p t i o n c o e f f i c i e n t i s comparable t o t h e atomic sum a t e n e r g i e s f a r above t h e edge except i n t h e c a s e of OCS where t h e d i s c r e p a n c y may be a t t r i b u t e d t o t h e decomposition of t h e molecule. The most i n t e r e s t i n g f e a t u r e s a r e t h e o s c i l l a t i o n s e x t e n d i n g w e l l beyond t h e n e a r edge r e g i o n . They a r e i d e n t i f i e d a s EXAFS.
The EXAFS a n a l y s i s of
O2i s i l l u s t r a t e d i n Fig. 3. The l o g a r i t h m of t h e a b s o r p t i o n spectrum i s t a k e n f i r s t (Fig. 3A),
where P B and po a r e t h e background and t h e smooth atomic a b s o r p t i o n
r e s p e c t i v e l y , and c
=po/(po + LIB). Hence
The slower variation of the first term in (2) permits a smoother removal of the background. The removal is performed in two steps. First we fit the spectrum, In y(E), above the edge with a Victoreen-type function of the form:
a, + a1 ln(E), and subtract this fit from the data (Fig. 3B). Next we convert the data into k-space, perform a spline fit and subtract the fit from the spectrum to complete the removal (Fig. 3C). The ratio c(E) varies little over the EXAFS region. We take it as a constant in the analysis.
The Fourier transform of ~ ( k ) (Fig. 3D) peaks at R
=0.98 A, a clear indication of the presence of a single 0-0 distance. The derived phase shift however is not in quantitative accord with the atomic phase shift calculated on the basis of a plane wave approximation.7 Similar situations are encountered in other molecules. This observation confirms our earlier findings1 and is
indicative of the inadequacy of the plane-wave approximation.
Photon energy(eV) 1
u
...
- 2
- 3
Fig. 3 EXAFS analysis of the O2 absorption spectrum (see text).
This work is supported in part by the Nationgl Science Foundation under Grant No. 8540801. The experiment was carried out in part at the NSLS, Brookhaven National Laboratory, which is operated under Contract DE-AC02-76CH00016 with the U. S. Department of Energy.
:IA :
-
1 .O
-
0.5 0.0. 2
X 1
-0.5
500 600 700 800 900 1000 -1 .O 0 2 4 6 8 10
Photon energy(eV) k (A")
