HAL Id: jpa-00227321
https://hal.archives-ouvertes.fr/jpa-00227321
Submitted on 1 Jan 1987
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.
Si K-SHELL ABSORPTION SPECTRA OF
CHLOROMETHYLSILANE MOLECULES IN THE GAS PHASE
J. Hormes, R. Chauvistre, U. Kuetgens, U. Fischer, I. Ruppert
To cite this version:
J. Hormes, R. Chauvistre, U. Kuetgens, U. Fischer, I. Ruppert. Si K-SHELL ABSORPTION SPEC-
TRA OF CHLOROMETHYLSILANE MOLECULES IN THE GAS PHASE. Journal de Physique
Colloques, 1987, 48 (C9), pp.C9-1113-C9-1116. �10.1051/jphyscol:19879202�. �jpa-00227321�
JOURNAL DE PHYSIQUE
Colloque C9, supplement au n012, Tome 48, decembre 1987
Si K-SHELL ABSORPTION SPECTRA OF CHLOROMETHYLSILANE MOLECULES IN THE GAS PHASE
J. HORMES, R. CHAWISTRE, U. KUETGENS, U.
FISCHER*and
I.RUPPERT*
Physikalisches Institut der Universittit, Nussallee 12, 0-5300 Bonn 1, F.R.G.
' ~ n s t i t u t fiir Anorganische Chemie der Universittit Bonn, Gerhard-Domagk-Strasse, 0-5300 Bonn 1, F.R.G.
R6smii
Nous avons mesure les spectres de photo absorption des mol6cules chloromethylsi- lane dans la region du seuil Si Is ( entre 1840 et 1900 eV ) avec une trhs haute resolution et une statistique amelioree. Cette etude systematique facilite l'interpretation des resonaces et devrait aider
a
comprendre l'environnement chimique donnant lieue des resonances de forme.Abstract
The photoabsorption spectra of the chloromethylsilane molecules (CH3)nSiC14-n ( n = 0
-
4 ) have been measured near the Si Is-
edge ( between 1840 and 1900 eV) with high resolution and improved statistics. This systematic investigation facili- tates the interpretation of the observed resonances and i s believed to help in the understanding of the chemi cal environment giving rise to shape resonances.1.Intraduction
Though recent theoretical /1,2/ and experimental,/3/ results show promise of XANES being a new powerful structural technique, it should be pointed out that there are still several open questions.
A
final prove is still needed to show wether it is possible really to deduce a complete structure from a XANES spectrum.The best candidates for such an attempt are stable molecular gases for which their coordination geometries are well known from other experiments
.
Here, no perturb- ing effects are expected from non-next neighbour shells as for solids. In this paper we report our new high resolution spectra of theK
- shell photoabsorption of silicon in simple chloromethylsilane molecules.It is the main purpose of this contribution to demonstrate the advantages of systematic measurements for a consistent interpretation of the
K -
shell near edge structure. Thus we confine ourselves to a qualitative interpretation of spectra and observed systematic trends. A more detailed quantitative analysis of the data will be published in a following paper.2. Experimental
Our measurements were carried out at the Bonn 2.5 GeV synchrotron running at 2.0 GeV with an electron current of about 20 mA / 4 / .
A
double crystal vacuum monochromator of the Lemmonier type /5/ was used equipped with InSb (111) crystalsArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879202
C9-1114 JOURNAL
DE
PHYSIQUEg i v i n g an estimated r e s o l u t i o n o f about 0.5 eV i n t h e energy r e g i o n o f i n t e r e s t . For a l l s p e c t r a a s t e p w i d t h o f about 0 . 1 eV was used. Our energy scale was c a l i - b r a t e d a g a i n s t t h e w h i t e l i n e o f SF6 (2485.9 eV) and/or against t h e w e l l known w h i t e l i n e o f Argon a t 3203.54 eV. By using both l i n e s a l s o the accuracy o f t h e c r y s t a l constants used could be checked. The value used f o r our measurements was
2deff=7.48 a
f o r InSb (111). The number o f photons was aboutl o 9
per r e s o l u t i o n i n t e r v a l and second. Spectra were taken w i t h t h e standard X-ray absorption technique. The monochromatized r a d i a t i o n i s monitored by an i o n i s a t i o n chamber b e f o r e passing t h e sample c e l l . A second i o n i s a t i o n chamber i s used as a detec- t o r . The p r e a m p l i f i e d c u r r e n t s are given on VFCs which are read o u t by a VME - bus computer c o n t r o l l i n g a l s o t h e scanning o f t h e monochromator. The sample c e l l separated by kapton windows from t h e i o n i s a t i o n chambers i s about 20 cm long. The pressure i n t h e c e l l i s v a r i e d between some t e n t h s and about 50 T o r r . The p u r i t y o f t h e i n v e s t i g a t e d molecules was i n a l l cases higher than 99 %.3. Results and Discussion
As an example o f our r e s u l t s , XANES spectra are shown i n f i g u r e 1 of a s e r i e s o f molecules s t a r t i n g from SiC14 and going t o Si (CH3 ) 4 (TMS) by successively r e p l a c i n g a c h l o r i n e atom by a methyl group. The spectra a r e a l i g n e d a t t h e S i Is i o n i s a t i o n l i m i t s and thus a r e presented on a common term value scale. As some o f t h e resonances above t h e i o n i s a t i o n l i m i t are r a t h e r weak as compared t o t h e reso- nances below t h e IP, t h e energy r e g i o n above t h e IP i s shown i n f i g u r e 2 on an enlarged scale. Our spectra show c l e a r l y serveral systematic trends:
-
as i s expected, t h e i o n i s a t i o n p o t e n t i a l i s moving continuously down i n energy from TMS t o SiC14 ;-
w i t h t h e i o n i c i t y i n c r e a s i n g from TMS t o SiCll t h e term value o f t h e resonances below t h e I P increases;-
i n a l l spectra a t l e a s t two d i s t i n c t shape resonances a r e observed w i t h i n 15 eV above t h e IP. The stronger one o f these two features ( a t higher energies has a term v a l u e which seems t o be constant w i t h i n our e r r o r margins f o r a l l molecules.The low energy resonance which i s j u s t a shoulder f o r TMS gains i n t e n s i t y when going t o SiC14 ending w i t h about t h e same i n t e n s i t y as t h e h i g h energy resonance.
T h i s shape resonance changes i t s energy p o s i t i o n and t h e corresponding term v a l u e decreases from TMS t o SiC14 where t h e l i n e i s j u s t a t t h e i o n i s a t i o n l i m i t . T h i s o b s e r v a t i o n o f two shape resonances i s believed t o be c h a r a c t e r i s t i c f o r a l l t e t r a h e d r a l molecules as was a1 so demonstrated from mu1 t i p 1 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 GeCll / l b / ;
-
f o r a l l molecules w i t h S i bound t o C as w e l l as t o C1, two seperated 1 in e s a r eobserved i n t h e spectra below t h e I ? . From systemtatic changes i n i n t e n s i t y and energy, these l i n e s can be assigned as t o belong t o a t r a n s i t i o n t o an ( S i - C l ) r x o r b i t a l and t o an ( ~ i - ~ l n * o r b i t a l , r e s p e c t i v e l y , whereby t h e 1 in e w i t h t h e l a r g e r term v a l u e corresponds t o t h e t r a n s i t i o n t o t h e (Si-C1) o r b i t a l ;
-
by a p p l y i n g t h e bond l e n g t h-
energy c o r r e l a t i o nAnu
R*= const. as r e p o r t e d by N a t o l i /9/ we f i n d f a i r agreement w i t h theory when t h e a* resonance w i t h t h e h i g h e r terms value i s c o r r e l a t e d t o t h e $ ( ~ i - ~ l ) t r a n s i t i o n . For t h e low energya
resonance such as c o r r e l a t i o n cound n o t be proved;-
t h e r e a r e f u r t h e r resonances above the I P which are c l e a r l y seen i n t h e SiC14 spectrum i n f i g u r e 2. The strongest o f these s t r u c t u r e s decreases i n i n t e n s i t y from SiC14 t o TMS where i t could n o t be detected a t a l l . Thus, a connection t o t h e C1-
atoms seems obvious b u t a d e f i n i t e i n t e r p r e t a t i o n i s n o t y e t p o s s i b l e ;-
from t h e we1 1 r e s o l v e d 1 ines i n t h e d i s c r e t e p a r t o f t h e spectrum o f (CH3 I3SiC1 i t i s p o s s i b l e t o o b t a i n a r e l i a b l e value f o r a Gaussian h a l f w i d t h . Based on t h i s v a l u e a l l spectra were deconvoluted. It turned o u t t h a t i n a l l cases more l i n e s were r e q u i r e d than expected from t h e symmetries o f t h e various ground s t a t e s . T h i s s t r o n g l y supports an i n t e r p r e t a t i o n o f t h e s i l i c o n h a l i d e spectra on t h e b a s i s o f a Jahn-
T e l l e r E f f e c t as was reported by Bodeur e t a l . /6,7/.The spectra o f SiC14 and Si(CH3I4 were already r e p o r t e d before /6,7/ and our r e s u l t s a r e i n good o v e r a l l agreement w i t h those data. However, s t a t i s t i c s seems t o be b e t t e r i n our data so t h a t some f i n e r d e t a i l s can be observed which were n o t r e p o r t e d i n previous s t u d i e s . The second r a t h e r weak shape resonance, f o r exam- p l e , as observed i n TMS about 4 eV above t h e IP, has n o t been seen b e f o r e . This f i n d i n g o f a second shape rsonance, however, i s c o n s i s t e n t w i t h t h e data o f t h e continuum s t r u c t u r e above t h e Si 2s edge which were r e c e n t l y r e p o r t e d by Bozek e t a1
.
/8/.There i s another discrepancy i n t h e spectrum o f SiC14 as compared t o spectra r e p o r t e d by Bodeur e t a1./6/. We observe a resonance j u s t a t t h e I P which was assigned before as a d i s c r e t e s t a t e w e l l below t h e IP. The p o s i t i o n r e p o r t e d here i s n o t o n l y i n agreement w i t h t h e S i 2s spectrum o f SiC14 as r e p o r t e d by Bozek e t a1./8/ b u t i s a l s o supported by the systematic trends as discussed above. It i s assumed t h a t t h i s discrepancy i s caused by an i n c o r r e c t 2d
-
value used i n t h e p r e v i o u s study and n o t by a wrong value f o r t h e i o n i s a t i o n p o t e n t i a l as was sug- gested by Bozek e t a1./8/.T h i s work has been p a r t l y funded by t h e German Federal M i n i s t e r f o r Research and Technology (BMFT) under c o n t r a c t number 05 333 AX 85.
C9-1116
JOURNAL DE
P H Y S I Q U EL , I
1840 1850 1860 1870 1880
ENERGY t eVI ENERGY CeVI
Figure 1: The Si K-shell XANES of Figure 2: The Si K-shell absorption Chloromethylsilane molecules Spectrum of Chloromethylsilane
molecules above the ionisation potential
References
l e g . : a)P.J.Durham.J.B.Pendry,C.Hodses.Comp.Ph~s.Comm.25 193 (1982); b ) C.R.Natoli,
D.K.Misener,S.Doniach,~.~.~utzler,
~ h y s - ~ e v . ~ , 1104, (1980); c )J. L
.Dehmer, Phys. Rev.Letters 35, 213 (1975)/2/A.Bianconi in: EXAFS for inorganic systems, Daresbury Report, DL/SCI/R 17
( 1981
/3/ F.Sette, J.Stohr.A.P.Hitchcock, Chem. Phys .Lett.
110,
517. (1984) /4/ J.Hormes. Physica Scripta36.
36 (1987)/5/
M.Lemmonier.O.Collet.C.De~autex,J.M.Esteva.D.Raoux
Nucl. Instr. Methods.
-
/7/
S.
~ o d e u rI
. ~ e n n e r ; J. Physique, ~ o l l o q ~ ~ 8 , 47, C8-79 ( 1986 )/8/
