CORE ELECTRON SPECTROSCOPY OF SURFACE LAYERS ON LIQUID SUBSTRATES

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CORE ELECTRON SPECTROSCOPY OF SURFACE LAYERS ON LIQUID SUBSTRATES

S. Holmberg, R. Moberg, O. Bohman, H. Siegbahn

To cite this version:

S. Holmberg, R. Moberg, O. Bohman, H. Siegbahn. CORE ELECTRON SPECTROSCOPY OF SURFACE LAYERS ON LIQUID SUBSTRATES. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-955-C9-959. �10.1051/jphyscol:19879172�. �jpa-00227287�

JOURNAL DE PHYSIQUE

Colloque C9, supplhment au n 0 1 2 , Tome 48, decembre 1987

CORE ELECTRON SPECTROSCOPY OF SURFACE LAYERS ON LIQUID SUBSTRATES

S. HOLMBERG, R. MOBERG, 0. BOHMAN* and H. SIEGBAHN Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden

* ~ e p a r t m e n t of Organic Chemistry, Uppsala University, Box 534, S-751 21 Uppsala, Sweden

Abstract: Adsorbed layers of octanol and bromooctan.ol on top of formamide, are studied using core electron spectroscopy. In particular it is noted that the binding energy shift of the Br 3p line when going from a gaseous state t o an adsorbed state is not the same for Zbromooctanol and 8-bromooctanol. This is concluded to be mainly due to differences in polarisation energy. By applying a simple electrostatic model of the polarisation energy, these findings suggest that the bromooctanol molecules are oriented at an angle smaller than 90° with respect to the surface.

1. Introduction

We have previously reported on applications of electron spectroscopy to liquid samples, e.g./l/for a review. These applications have been concerned with the nature of solvation forces in electrolyte solutions. The results from our exper- iments have thus been interpreted in terms of models based on bulk structure of ionic solutions 2-41. It is not immediately obvious that this mode of inter- pretation is justi

/

ed in all cases because of the small sampling depth in electron spectroscopy (25-30W). In the cases we have studied so far the electrostatic screen- ing length of the ionic field is smaller than the sampling depth thus justifying a bulk interpretation. Comparisoo between experiment and theory also bears this out. However, it is clear that other situations exist where the surface sensitive character of electron spectroscopy will completely decide the appearance of the observed spectra. This is true particulary for samples exhibiting surface activ- ity. In this report we present results from the investigation of such phenomena, obtained with liquid phase electron spectroscopy.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879172

JOURNAL DE PHYSIQUE

2. Experimental

1500

cn I-

z

3 o

u 500

3. Results and discussion

1500

In I-

z 3

u o

500

Fig.:! shows the C l s and Br3p spectral regions of the formamide surface covered by a layer of 2-bromooctanol. The spectrum also includes t h e Kr3p line of the krypton gas calibrant, simultaneously present via a continuous inlet through the needle valve. T h e figure also shows the gaseous

-

adsorbed state binding energy

Cls

-

^HCONH,

SUBSTRATE

.

.

- . .

-

-%.--.-*:

--

^{*.... -...} ...-...

Our technique for obtaining spectra from a liquid sample is based on the creation of a thin liquid film on a stain- less steel substrate. This technique has previously been described in detail 5-

{

71. The experiments described be ow were performed according t o two differ- ent procedures. In the first procedure the sample, formamide, was cooled to a temperature of 277-278 K. After cool- ing, the sample compartment was evac- uated. This resulted in a final pres- sure of 4

.

loF2 mbar in the sample

295 290 sure that only peaks originating from BINDING ENERGY(eV) the liquid appear in the spectra. Thus, the conclusion from Fig.l(bottom) is

Eia. I C l s spectra bejore(top) and ajter(bot- that some octanol molecules stick to the tom) octanol vapour has been adsorbed on the surface and form an overlayer. Several jormamide substrate. Note: because of the hours after the gas was turned off the applied voltage between the liquid substrate peak essentially un-

and the spectrometer entrance slit there are

n o peaks originating from the vapour phase changed. T h e amount of octanol en-

/6/. tering the spectrometer was very small,

less than one percent of the formamide present. In t h e second procedure investigations were made on 8-bromooctanol and 2-bromooctanol adsorbed on the formamide substrate. Due t o the low vapour pressure of these two bromoalcohols, it was not possible t o let them enter the spectrometer in gaseous form. In this case the adsorbed film was introduced onto the substrate via a syringe.

chamber. The spectra then recorded showed only a single C l s peak as can be seen in Fig.l(top). Next, octanol vapour was introduced into the sam- ple chamber via a needle valve. The result is shown in Fig.l(bottom). By applying an accelerating electric field between the liquid and the spectrome- ter entrance slit, any signal originating from a gaseous sample is smeared out into a flat background and in practice eliminated/6/. By doing this we make

H C O N H 2 SUBSTRATE

-

.

.

- .

.

- .

C.. .

-

^{*; .-*-.-.}

.

.

C8H170H

' ADSORBED 3.5 ev,

..

.

.

_{. . .}

. . _.... - . .

I

-1

.

.

. .

.-

shift of 2-bromooctanol. If it can be assumed that the calibrant lines are unaffected by the presence of the li uid sample, the two spectra can be related to each other via the Kr3p lines (F'ig.2Y. This assumption is expected to be valid t o within k0.1 eV, based on our previous studies of the relation between gas phase and liquid phase levels 171.

B I N D I N G ENERGY (eV)

We find t h a t the Br3p line is shifted towards a lower binding energy when 2- bromooctanol goes from the gaseous state t o the adsorbed state (Fig. 2 and 4)- T h e observed shift is 0.86 eV. This value is set by the presence of the surface when the molecules are adsorbed on the formamide substrate. The adsorption is likely t o occur via an interaction between the polar substrate and the hydrophilic (hydroxy) part of the octanol molecules. There are two contributions t o the ob- served shift t o be considered. First, as the molecules bind t o t h e surface a charge redistribution may occur both internally within the molecule as well a s in the lo- cal surface environment of the substrate. This redistribution may also involve the preferential orientation of the polar substrate molecules towards the hydrophilic end of the adsorbing molecules. Since the adsorption interaction is weak, of the order of a tenth of an eV, this redistribution contributes only marginally to the observed shift. To substantiate this we write the shift as

Fiq 2.1s and Br 3p spectral regiotu of jormamide coo- ered by a layer o/

2- bromooctanol.

The spectrum also includes the KrSp line of the the kry- pton gas calibrant simultaneously present.

C ~ S 2-Bromooctanol on

Kr 3~ 312

HCONH, (gas) HCONH2 substrate

substrate

A E b = Eb(ads)

-

Eb(gas)

= [E$,,(ads)

-

Etot ( a d s ) ] - [ ~ $ t ( s a s )

-

Etot(sas)l (1)

A Eb ⁼

[EL,

( a d s )

-

E& ( g a s ) ] - [ G o t ( a d s ) - G o t ( g a s ) ] (2)

A E ~ =

AH^+^$

-

AH^^^

(3)

300 290 v220 21 0 200 190 180

. . .

' 2-Bromooctand:

-

adsorbate

. .

_{. .} ^2. *

. . . .

. .

,. ..

_0-8 ^.<

-

^J;

.

- ^,

^{a-, I}

where Etot ads) is t h e total energy of the system octanol molecule

-

for-

b

mamide su strate when the octanol molecule is adsorbed on t h e surface. Etot(gas) is the total energy for the same system when the octanol molecule is still in the gas phase. T h e

+

superscript designates the corresponding state when the oc- tan01 molecule is ionized by ejection of a Br3p electron (note t h a t this is a vertical

1;

..

B r 3 ~ 3 / 2

(gas)

. .

- _-

^{26.1 eV}

.

I,

'.

_{Br3~1/2 j}

\*

-4*b#.--***4-

-\:

I I I I I

B r 3 ~ ~ , ~

.

;

; a

%*

I

C9-958 JOURNAL DE PHYSIQUE

tion energy in this position is ^{DISTANCE FROM SURFACE} approximately

$

of the bulk

value rather than as ex- ~9 Calculated polarization energy as a junction of cavity petted from simple consid- distance from a liquid surface. The dashed lines represent erations. ~h~ value of the calculations without inclusion of mirror chargeseoo = 2.05).

The estimated positions for the bromine atom w t h respect to

shift (corresponding the surface in a straight up adsorption geometry are s I A and E ~ O 1 ( z = -w)) found for a -10A for bbrommctanol and 8-bromooctanol, respectivel~

large number of liquids av- (assuming the l-carbon atom to lie at z=O).

erage around 1.5 eV 11-4,8j T h e shift value reported

here for t h e adsorption case, 0.86 eV, thus fits well with t h e prediced value from Fig.3. We have attempted t o verify the depth dependence of the ~olarization en- ergy shown in Fig.3 by substituting the Br-atom in the 8-position of the octanol molecule.

process which only leads t o electronic polarization of the substrate medium). AS can be seen, the shift is given exactly as the difference between two adsorption energies, one related to the adsorption of the neutral molecule and the other t o the adsorption of the positive core-hole ion. We calculate that the main part of the shift is due to A H ~ , i.e. the adsorption energy of the core-hole ion. The fact t h a t AH+ is much larger than AHad, may be attributed t o the presence of the positive g i r g e in the adsorbed core-hole molecule. This positive charge polarizes t h e substrate and lowers the total energy in the adsorbed state substantially with respect t o t h e free state of the octanol core-hole molecule. A similar shift is ob- served also between the gaseous state and the bulk liquid state of a pure liquid or mixture of liquids /I-4,8/.

Since, in the present case, the rnoIecuIes are adsorbed on the surface we expect the binding energy shift t o be smaller than the corresponding bulk shifts observed for pure liquids. Qualitatively, the positive core-hole charge in the adsorption case can only polarize approximately half as many molecules as when the positive charge is located well inside the bulk of the substrate. Considering the substrate as a dielectric medium with optical dielectric constant c,, model calculations can be made of t h e polarization energy vs. distance from the surface. The polariza- tion energy and hence AEb vary according t o Fig.3 as a function of the depth coordinate z (z=0 at the surface and negative towards the bulk).

Calculations were performed

for two radii of the ion- ~ ~ q u ~ d ^t surface

ized particle,

la

(typical of _,v a metal cation) and 2.4A

(typical of solvent molecule S 4

such as formamide). In or- (I ,-R=l.OA

---___

der t o assess t h e importance w

of t h e mirror charge cre-

g

^{3 -}

ated upon ionization, calcu-

5

lations were also performed ^N

with (full-drawn) and with-

2 '-

out (dashed) this refinement.

2 .

^{~ . 2} --- 4 8

It is indeed seen that the

,

mirror charge has a non neg- ligible influence on the po-

larization energy near the ^{I I I , I I I . I}

surface. Thus, the polariza- -8 -6 -4 -2 2 4 6 a f t

LIQUI D

-

VACUUM I

Fia. 4 Binding energy s h i b of ^Br the BrSp line in bromooetanol

when the molceules go from gas phase to the adsorbed state on formamide. To the right is 8- bromooetanol, in the middle is 2-brommctanol and to the lefl is the liquid-gas binding energy shift of the C l s electrons in for- mamide.

T h e shift found in this case was indeed smaller, 0.61 eV (Fig.4), but not as small as would b e expected from Fig.3 provided that t h e hydroxy-carbon is located exactly at t h e liquid-vacuum interface and the molecule is standing straight up.

In such a case, our calculations imply that the binding energy shift for Br3p in the 8-position should be very small. Our explanation for this is that the molecules ( a t least in t h e 8-bromooctanol case) are tilted so t h a t t h e t o p end lies closer to the substrate. The measurement of the gaseous-adsorbed states binding energy shifts (and hence substrate polarization energies) thus provides a possible means t o investigate adsorption geometries on liquid substrates. This possibility is being further pursued for other hydrocarbons. For a more extended series of molecules it is possible t h a t a C l s line shape analysis could even allow the resolution of various carbon atom sites and their distances t o t h e substrate surface.

REFERENCES

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431 H. Siegba (1983h n, M. Lundholm, M Arbman and S. Holmberg, Phys. Scr.

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425, 1984)

H. Siegbahn, S. vensson and M. Lundholm, J. Electron Spectrosc. 24,205

(1981)

L

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28,

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22,

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2,

411 (1975)