LOCAL ELECTRONIC STRUCTURE IN SIMPLE ALCOHOLS STUDIED IN X-RAY EMISSION

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LOCAL ELECTRONIC STRUCTURE IN SIMPLE ALCOHOLS STUDIED IN X-RAY EMISSION

J.-E. Rubensson, N. Wassdahl

To cite this version:

J.-E. Rubensson, N. Wassdahl. LOCAL ELECTRONIC STRUCTURE IN SIMPLE ALCOHOLS STUDIED IN X-RAY EMISSION. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-793-C9-796.

�10.1051/jphyscol:19879140�. �jpa-00227251�

JOURNAL DE PHYSIQUE

Colloque C9, suppl6ment au 11'12, Tome 48, d6cembre 1987

LOCAL ELECTRONIC STRUCTURE IN SIMPLE ALCOHOLS STUDIED IN X-RAY EMISSION

J.-E. RUBENSSON and N. WASSDAHL

Department of Physics, Uppsala University, PO Box 530, 5-751 21 Uppsala, Sweden

Lw opecfiw d ' e m k i o n

C K

dc m b a n e , de &mot, dl&thmot e t de prtopanol, e.t l w hpecfieh dleminnion 0

K

d'eau, de methanol et d'ethanol, gazewr, ~ c h . % . b pan l l h p a c t d d e c t d w 6lec;tnona ~ o n t anal god^ how incidence &anante avec

un

4pecO~o- m&e a

~t&m

want un a q o n de

10

m. I& oont 626 i n t u p x & % avec 18&e de c d w l RHF, l e model dlintenn& de neut cent;lru, et compan&on de photoemkion. Lw

e n u g i w d e s niveaux

K

ont 626 de.tuminw e.t dam ten &coo.& now avona mo&d que t l o a b L W dloxygen a lle.xfi&mite' w t 6ortXemeont mehngh avec l l o a b L W coa~t~pontlant de canbon.

Electrron excL&d cahbon

K

e m k i o n hpectta

06

meAhwie, methanol, ethanol

and

ptopanol, and oxygen

K

e m a i o n 4pec;tna

06

a, methanol and ethanol, i n -the gan- p h e , h v e been xecoxded uoing a

70

m gtrazing Lnchiencc ~p,pectkornefm.

The

npecaa m e intuprteXed w a f i e czid 06 Rw;tnicZed Hm;tnee Fock c & M o n a , .the o n e - c e W intern@ modet, and pho;taeLec;tnon opecaa. Coincididsncw

and

di66uencw in .the 4pec;tna

06

Ahe v m i o w moleculw m e d&c&ul,

14

binding e n u g i w m e de.tmmined

and

companed

;to XPS

data,

and

dart .the &coho.&

i.t &

&own that .the k i g k u t occupid oxygen out-06-plane o~tb.&& ~ b ~ a h a X d @

m i x

w a Ahe cot~twponding cmbon ortbi-

m.

Koster / I / recorded h i g h r e s o l u t i o n oxygen K emission s p e c t r a o f some s o l i d a l c o h o l s i n 1971, u s i n g Bragg d i f f r a c t i o n f o r t h e wavelength d i s p e r s i o n . Recently Yumatov e t a l . /2/ measured t h e K emission s p e c t r a o f methanol. I n t h i s paper we p r e s e n t t h e X-ray emission s p e c t r a from gas-phase methanol, e t h a n o l and propanol.

For comparison we i n c l u d e t h e carbon K emission spectrum o f methane, and the, p r e v i o u s l y p u b l i s h e d / 3 / , oxygen K emission spectrum o f water. The emission was e x c i t e d by a 7 keV e l e c t r o n beam, and recorded on photographic p l a t e s i n a I 0 m g r a z i n g i n c i d e n c e spectrometer /4/. The w e l l e s t a b l i s h e d C and 0 K emission bands o f carbon d i o x i d e were used as c a l i b r a t i o n standards. The carbon s p e c t r a were recorded i n t h e f i r s t o r d e r o f d i f f r a c t i o n w i t h a r e s o l u t i o n o f 0.1 eV, and t h e oxygen i n t h e second w i t h a r e s o l u t i o n o f 0.2 eV.

To f a c i l t a t e t h e i n t e r p r e t a t i o n , ab i n i t i o R e s t r i c t e d H a r t r e e Fock c a l c u l a t i o n s were made u s i n g t h e MOLECULE/ALCHEMY program package / 5 / . According t o t h e one- c e n t e r i n t e n s i t y model t h e t r a n s i t i o n from a c o r e h o l e s t a t e i t o a f i n a l s t a t e j i s

where E i s t h e t r a n s i t i o n energy and ( C J, i s t h e MOLCAO expansion 2p c o e f f i c i e n t f o r o r b i t a l j / 6 / . The p o p u l a t i o n probab$?ity, Pi, and t h e t o t a l Auger decay r a t e of t h e i n i t i a l s t a t e , Wi Au

,

need o n l y t o be considered where s e v e r a l c o r e h o l e s are i n v o l v e d . W i t h t h e ~ x c 2 ~ E i o n o f t h e H20 case were more accurate c a l c u l a t i o n s a r e a v i a l a b l e /3/, ground s t a t e o p t i m i z e d o r b i t a l s a r e used throughout t h e discussion.

T h i s s i m p l e model has been shown t o work w e l l f o r a rumber o f molecules /7/. I n s p e c t r a where i n t e n s i t y from s e v e r a l c o r e h o l e s c o n t r i b u t e i t i s assumed t h a t t h e i r i o n i z a t i o n c r o s s s e c t i o n s and Auger r a t e s a r e i d e n t i c a l .

The ground s t a t e valence e l e c t r o n c o n f i g u r a t i o n s of t h e i n v e s t i g a t e d molecules can be seen i n Table 1, and t h e recorded K emission s p e c t r a a r e shown i n F i g . 1-3.

F o r d e t e r m i n a t i o n o f t h e energy p o s i t i o n s o f t h e i n t e n s i t y b a r s t h e most narrow peak

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879140

JOURNAL DE PHYSIQUE

is in all cases assumed adiabatic, giving the core electron binding energy, which is then combined with UPS data.

To understand the alcohol spectra it is instructive to first consider the spect- rum of the water molecule, since this molecule can be thought of as a hydroxyl group with an attached hydrogen atom, and discuss the changes in the local electronic structure which are invoked by replacing hydrogen atoms with methyl groups.

The previously discussed /3/ K emission spectrum of H20 is shown in Fig. la.

There are three orbitals from which we expect electrons fllllng up the core hole, giving rise to X-ray intensity. The highest occupied molecular orbital (HOMO) is the Ibl orbital, which gives rise to the peak at 526.8 eV, is antisymmetric with respect to reflection in the bonding plane, and is to a large extent of oxygen lone- pair character. The peak at 525.1 eV originates from the symmetrical 3a, orbital, and the 520.4 eV peak from the Ib2 orbital, which has the node in the symmetry plane perpendicular to the bonding plane. The vibrational broadening

and energy shifts reflect the.differences in equilibrium geometry between the core and valence hole states.

In methanol the methyl group is addding two valence orbitals and breaking the Cpv symmetry. The

0

K emission spectrum, shown in Fig Ib, is thus becoming more complicated, however many of the spectral features are preserved. The 527.8 eV peak originates from transitions between the oxygen core hole state and 2a" hole state.

The 2a" orbital is directed perpendicular to the symmetry plane and is mainly of oxygen 2p character. Compared to the corresponding H 2 0 peak vibrational broadening, is more significant, which indicate a bigger equilibrium geometry difference between the core and valence hole states. The methanol lav-2a" peak is shifted 1 eV as compared to the la -1b peak of water, thus implying a differential core to valence ionization pote~lal'shift upon methyl substitution. The core electron binding energy derived from the position of the lar-2a" peak is 538.7 eV.

The peak at 526.2 eV is assigned to- -

the lat-7a' transition. According to the calculation the 7a' orbital is mainly a combination of the carbon and oxygen 2p orbitals in the symmetry plane, with a pi-antibonding and a sigma-bonding cont- ribution. Locally at the oxygen atom site it can be related to the 3al orbi- tal in H20,

The peak at 521.4 eV is attributed

TAle 1. Symnetry

End groud state valence

elect- mnfignaticm of

the investwted

rmlecules.

H2°

c ~ v

M4

Td It

5a721al

12&15albl

^lZ

??"

^cS

7a~21a1 128a129a12h1 1z10a123a1 12

Cw ^Cs

Crp1 c1 (9)-(17)

Table 1: The inner orbitals have been omitted since they are of no importance for the spectra discussed. For propanol the C symmetrical trans form is report-

OXYGEN K EMISSICA SPECTRA

C. ETHANOL : . .. ..,

8. METHANOL

++ ;'.:

: . j .:

: <T

A. WATER

520 525 530 C.VI

ed toSbe less stable by only 0.0002 Har- Fig. 1

:

The oxygen K emission spectra of

trees

/8/.

For simplicity the calcu- water, methanol and ethanol. The bar

lations are performed with Cs symmetry. heights are one-center intensities.

t o t h e l a 1 - 5 a ' t r a n s i t i o n . The Sat o r b i t a l corresponds, a t the oxygen atom s i t e , t o a water I b 2 o r b i t a l mixed t o form a bond w i t h t h e carbon 2p o r b i t a l s .

Substituting another hydrogen atom f o r a methyl group, t o get ethanol, gives seven valence o r b i t a l s from which one can expect X-ray i n t e n s i t y , and accordingly t h e spectrum i s f u r t h e r complicated. Nevertheless i n c e r t a i n respects the spectrum can be compared t o those o f methanol and water.

I n ethanol t h e C symmetry i s preserved i n a staggered geometry. The HOMO, 3aW, and t h e next highes? occupied o r b i t a l , 10ag, l o c a l l y simulate t h e two h i g e s t o r b i - t a l s i n t h e previous molecules, and g i v e r i s e t o comparable s t r u c t u r e i n t h e 0 K emission. The peak a t 528.0 eV i s assigned t o the l g ' - 3 ~ ' ~ t r a n s i t i o n , and t h e 526.4 eV peak corresponds t o t h e l ~ ' - l O f i ~ t r a n s i t i o n , though the l a t t e r assignment i s not unambigously supported by t h e one center i n t e n s i t y model. These peaks are both s h i f t e d 0.2 eV t o higher energies compared t o t h e methanol peaks. The oxygen core e l e c t r o n b i n d i n g energy derived from t h e p o s i t i o n o f t h e la1-3a" i s 538.6 eV.

The carbon K emission wectrum o f , t

methane and methanol are shown i n Fig.

2. S u b s t i t u t i n g a methane hydrogen atom f o r a hydroxyl group breaks the symmetry and adds a t l e a s t three peaks t o the C K emission soectrum. The considerable i n - t e n s i t y i f the l i n e a t 281.2 eV corre- sponding t o t h e 2 a 1 ' o r b i t a l i m p l i e s t h a t t h i s o r b i t a l , though o f t e n r e f e r r e d t o as oxygen lone p a i r o r b i t a l , has appreciable carbon 2p character. The carbon core e l e c t r o n b i n d i n g energy determined from t h i s peak i s 292.1 eV.

The s t r u c t u r e a t 279.4 i s assigned t o t h e 2a'-7a1 t r a n s i t i o n , and the broad band a t 276.9 eV has i t ' s o r i q i n i n a superposition o f t h e 2ag-6a' -and

x-

l a ' t r a n s i t i o n s .

-

CARBON K EMISSiON SPECTRA

B. METHANOL <$$;:

9 . i,

.*'I

*&,.

,..

, ,

"k.

C- .!brirh

' i.

,,?"

^{<,. .&- .,.;*, .,. y,v* : *;;r 5 !}

.,

1

274 276 278 280 282 tmV3

CARBON K EM1 SS ION SPECTRA OF S IWLE ALCOHOLS

i

The carbon K emission spectrum o f ethanol, shown i n Fig. 3b, i s a superposition of i n t e n s i t y from t h e two inequivalent carbon core holes.

Fig. 2: The carbon K emission spectra o f

270 275 280 285(eV)

Fig. 3: The carbon K emission spectra o f methanol, ethanol and propanol. The b a r heights are based on equal Auger r a t e s and population f o r t h e various core holes, and t h e c o n t r i b u t i o n s from t h e i n d i v i d u a l core holes are shown above

methane and methanol. t h e associated spectra.

C9-796 JOURNAL DE PHYSIQUE

The peak a t 281.4 eV i s a t t r i b u t e d t o t h e

&'-zl'

t r a n s i t i o n , where 2a1 mainly i s t h e c e n t r a l carbon ? s o r b i t a l , and 3a" i s t h e HOMO, though q u i t e i n t e n s e i n t h e carbon spectrum mainly o f oxygen 2p character. From t h e p o s i t i o n o f t h i s peak t h e 2a1 b i n d i n g energy i s evaluated t o 292.0 eV.

The C K emission spectrum o f propanol, shown i n F i g . 3c, c o n s i s t s o f overlapping i n t e n s i t y from t h r e e carbon core holes. The complexity i s considerable b u t s t i l l some f e a t u r e s a r e preserved as compared t o t h e e t h a n o l spectrum. The two h i g h energy peaks, o r g i g i n a t i n g from t r a n s i t i o n s were t h e two outermost o r b i t a l s f i l l t h e c o r e h o l e l o c a l i z e d on t h e carbon i n v i c i n i t y o f t h e oxygen atom, a r e d i s t i n c t and s i m i l a r those found i n methanol and ethanol.

I t i s concluded t h a t the l o c a l e l e c t r o n i c s t r u c t u r e around t h e hydroxyl group i n a l c o h o l s g i v e r i s e t o s i m i l a r f e a t u r e s i n t h e oxygen K emission spectra when t h e carbon c h a i n i s prolonged, though t h e i n t e n s i t i e s a r e n o t doninated by t r a n s i t i o n s i n v o l v i n g t h e HOMO. T r a n s i t i o n s t o t h e two outermost o r b i t a l s g i v e i n f a c t an OH group f i n g e r p r i n t i n t h e carbon spectra.

The core e l e c t r o n b i n d i n g enerqies obtained i n USX, compiled i n Table 2, are g e n e r a l l y lower than those neasured i n XPS. The d i f f e r e n c e v a r i e s from 0.2 eV up t o 0.5 eV, and can be explained i n terms o f v i b r a t i o n a l e x c i t a t i o n s .

Table 2.

Core

U & m Bindvlg Enerqies (eV) Table 2: The

*

s u p e r s c r i p t r e f e r s t o t h e core i o n i z a t i o n atom s i t e , and t h e Molecule KEmissim

- -- ~~

l e t t e r s designate t h e references i n t h e

CHP*H 538.7, ~ 3 8 . 4 ~ 539.09, 539.14' f o l l o w i n g way; a: Values d e r i v e d from

M~YO*H 538.6 Ref. 2, b: Ref. 9, c: The values o f Ref.

".81b, 538*Bf 10 combined w i t h t h e values o f Ref. 11,

c * ~

m.1, 292.8

m.d,

m.4Ze d: The CH4 v a l u e o f Ref. 12 combined

CHjZ

H;!W 292.0 2 9 2 Y w i t h t h e chemical s h i f t r e p o r t e d by Ref.

13, e: Ref. 14, and f: Ref. 15.

References:

1. A. S. Koster, Applied Phys. L e t t . ,

2,

170 (1971)

2. V. D. Yumatov. A. V. Okotrub. L. N. Mazalov. G. S. B e l i k o v a and T. M. Okhri- menko, Zhurnal s t r u k t i r n o i Khimi,

6;

59 (1985).

'

3. J-E. Rubensson, L. Pettersson, N. Wassdahl, M. Backstrom, J. Nordgren, 0. M.

Kvalheim and R. Manne, J. Chem. Phys., 82, 4486 (1985).

4. J. Nordgren, H. Agren, L. ~ e t t e r s s o n , L. Selander, S. Griep, C. N o r d l i n g and K. Siegbahn, Phys. S c r i p t a , 20, 623 (1979).

5. J. Almlof, P. S. Bagus, B. L i u , D. MacLean, U. I Wahlgren and M. Yoshimine, IBM San Jose Research Laboratory

6. R. Manne, J. Chem. Phys.,

2,

5733 (1970).

7. H. Agren and J. Nordgren, Theor. Chim. Acta, 58, 111 (1981).

8. K. Kimura, S. Katsumata, Y. Achiba, T. ~amazaki, S. Iwata, Handbook o f He1 Photoelectron Spectra o f Fundamental Organic Molecules (Japan S c i e n t i f i c Soc. Press, 1981 )

9. S. Svensson and D. Nordfors, p r i v a t e communication.

10. N. MArtensson, P-A. Malmqvist, S. Svensson, E. B a s i l i e r , J. J. Pireaux, U.

G e l i u s and K. Siegbahn, Nouv. J. Chim., 1, 191 (1977).

11. 8 . E. M i l l s , R. L. M a r t i n an3 D. A. S h i r l e y , J. Am. Chem. Soc., 98,2380 (1976).

12. L. Asplund, U. Gelius, S.Hedrnan, K. Helenelund, K. Siegbahn and P. E. M.

Siegbahn, J. Phys. B: At. Mol. Phys.,

18,

1569 (1985)

13. D. W. Davis, M. S. Banna and D. A. S h i r l e y , J. Chem. Phys,

0,

237 (1974).

14. J. E. Drake, C. Riddle, H. E. Henderson, B. Clavincevski, Can. J. Chem,

55,

2957 ( 1 977).

15. K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P. F. Heden, K. Hamrin, U.

Gelius, T. Bergmark, L. 0. Werme, R. Manne and Y. Baer, ESCA Applied t o Free Mole- cules, N o r t h Holland P u b l i s h i n g Company (1971 ).

-

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