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AUGER ELECTRON SPECTRA FOR THE FLUOROMETHANES
F. Larkins, L. Tulea
To cite this version:
F. Larkins, L. Tulea. AUGER ELECTRON SPECTRA FOR THE FLUOROMETHANES. Journal de
Physique Colloques, 1987, 48 (C9), pp.C9-725-C9-728. �10.1051/jphyscol:19879123�. �jpa-00227232�
JOURNAL DE PHYSIQUE
Colloque C9, supplbment au n012, Tome 48, dbcembre 1987
AUGER ELECTRON SPECTRA FOR THE FLUOROMETHANES
F . P . LARKINS and L.C. TULEA
Department of Chemistry, University of Tasmania. Hobart, 7001 Tasmania, Australia
Hbscract
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'1Ae carbon and fluorine Auger spectra t o r the fluoromethane s e r i e s of molecules, CH4-nFn, r ~ 1 . . . 4 , have been calculated using a s a n i q i r i c a l molecular o r b i t a l model. The model i s satisfactory f o r t h e prediction of the spectral shape profiles, hmever energy s h i f t s t o account for hole-hole electronic relaxation and localization e f f e c t s , especially on the fluorine atan, a r e warranted. 'Ihe previous analysis f o r CF4 by Rye and Houston using only ism values f o r the hole-hole interaction energy term is sham t o be inadequate.Mhile the Auger electron spectra of polyatomic molecules a r e c q l e x ;
tortunately, for i n i t i a l core holes the spectra a r e dominated by intra-atomic terms such that as a f i r s t approximation a one-centre model can be used t o estimate spectral intensities
111.
Ab i n i t i o energy calculations are possible, but because of the many possible f i n a l double hole symmetry s t a t e s . they are r e s t r i c t e d t o smallmolecules. Hence, semignpirical approaches have been developed /2,3/ following e a r l i e r work f o r atoms. Our approach has been discussed elsewhere /3,4/. A l l the semiempirical models ignore the effect o t molecular electronic relaxation on transition r a t e s , an effect sham t o be veryimportant f o r absolute X-ray rates 151, and may over-emphasise delocalization effects. The l a t t e r may be serious, i n energy terms, when more than one l i k e atom i s present i n the same symmetry environment.
The carbon and fluorine Auger spectra associated with the fluoromethane s e r i e s CH4_,E;, have been calculated with our semiempirical approach /3,4/. Molecular geometries /h/ and experimental ionisation potentials /7/ have been used along with IMX) wavefunctions and atomic Auger matrix elements 181. ATI adiabatic relaxation correction n t 2.2. eV has bem applied t o the spectral enemies sham.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879123
JOURNAL DE PHYSIQUE
Fig. 1. Carbon Auger spectra for the fluoromethanes.
Bar spectra, 0.25 eV, resolution, based upon INW wavehctions.
1.0 * ~ , ~ , ~ , , , 1 , , 0.9
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CH3F-F K-W0.8 i
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0.9 CH2F2-F K-W2
0.8I
Fig. 2. Fluorine Auger spectra for the fluoromethanes.
Bar spectra, 0.2 5 eV , resolution, based upon INDO wave functions.
C9-728 JOURNAL DE PHYSIQUE
Experimental r e s u l t s a r e limited t o same poor resolution spectra f o r t h e s e r i e s by Moddeman /7/ and some more detailed work f o r t h e CF4 molecule /Y/.
I I RESULTS AND DISCUSSION
The carbon and fluorine Auger spectra f o r t h e CH4-nFn s e r i e s a r e shown i n t i g . 1 and 2 respectively. The l i n e spectra include t h e sun of a l l t r a n s i t i o n s with an i n t e r v a l of 0.25 eV. Detailed discussion of t h e conplex spectra is presented elsewhere / 4 / . It must s u f f i c e t o address a t w general features here. There a r e broadly three groups of l i n e s corresponding t o t h e involvement of outer-outer, outer-inner and inner-inner valence o r b i t a l s i n t h e double hole t i n a l s t a t e s . The hole-hole interaction energies < w ' > ~ * ~ T span a wide energy r a m e within a p a r t i c u l a r molecule, e.g. CF4 6.4-18.5 eV, and hence a l s o contribute t o t h e spread of t r a n s i t i o n energies. It is not s u f f i c i e n t t o s e l e c t only one o r two values representing delocalization and localization e f f e c t s a s was t h e case i n t h e previous analysis f o r CF4 /g/. T r i p l e t s t a t e s make a negligible contribution t o t h e s p e c t r a l profiles. They a r e mainly delocalized s t a t e s . Calculated
i n t e n s i t i e s f o r processes involving inner valence o r b i t a l s a r e i n general too large. This i s principally due t o the neglect of many electron e f f e c t s which a r e most important f o r these o r b i t a l s and thus cause a sharing of t h e associated
t r a n s i t i o n i n t e n s i t i e s over several s t a t e s .
The carbon spectra (fig. 1) r e f l e c t t h e changing symmetry of t h e molecule with t h e 1~ S F t T CH2F2 molecule being t h e most complex. The nmber o t main C peaks decreases from CH3F t o CF4. This probably accounts f o r t h e progressive sharpening of t h e experimental p r o f i l e s /7/. The experimental E' spectra present t h e opposite trend with an increase i n t h e width of the main peak trom CH3F t o CF4. This observation a l s o correlates well with t h e increase i n t h e number of t r a n s i t i o n s predicted t o be i n t h e region.
The calculated C spectra align with t h e experimental spectra t o within +3 eV.
The agreement i s reasonable and supports t h e conclusion t h a t t h e model adequat- e l y describes t h e hole-hole interaction contributions and t h e localized nature of t h e t r a n s i t i o n processes. Tnis i s because there i s only one carbon atom i n each molecule. The calculated F spectnm f o r CH3F is close, i n energy terms, t o t h e experimental one; hmever, f o r t h e other CHbenFn molecules of t h e s e r i e s with two o r more equivalent fluorine atoms t h e calculated F spectra must be
s h i f t e d t o l m e r k i n e t i c energy by -6 eV t o achieve a l i m e n t . '&is observation supports the conclusion t h a t t h e m.0. model has over-emphasised delocalization e f f e c t s , when two o r more F atoms a r e present, consequently hole-hole inter- a c t i o n energies <w'> f o r the Auger process a r e too l m , compared with t h e t r u e s i t u a t i o n where t h e fluorine Auger process i s predominantly localized on a s i n g l e fluorine atom. This point i s substantiated by recomising t h a t t h e maximum value f o r <wl> i n any of the fluoromethanes i s 22.7 eV, whereas t o r t h e
HF molecules values up t o 28.3 eV a r e calculated and t h e INW atomic F value i s 25.7 eV. It i s desirable t o transform t h e M.0.s t o localized o r b i t a l s before calculating t h e hole hole interaction terms i n such cases.
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