MICROCLUSTER IOPTICAL ABSORPTION SPECTRA OF AG-MICROCLUSTERS FORMED BY MATRIX TECHNIQUES

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MICROCLUSTER IOPTICAL ABSORPTION

SPECTRA OF AG-MICROCLUSTERS FORMED BY

MATRIX TECHNIQUES

W. Schulze, Hanna Becker, D. Leutloff

To cite this version:

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OPTICAL ABSORPTION SPECTRA OF AG-MICROCLUSTERS FORMED

BY MATRIX TECHNIQUES

W . S C H U L Z E , H . U . B E C K E R a n d D . L E U T L O F F F r i t z - H a b e r - I n s t i t u t d e r Max-Planck-Gesellschaft,

1 Berlin 3 3 , F a r a d a y w e g 4-6, W e s t G e r m a n y

Résumé. — Des microagrégats d'argent ont été préparés par condensation simultanée d'un faisceau d'argent monoatomique et d'un gaz noble en excès sur une cible de saphir refroidie et analysés par spectroscopie optique à transmission dans le domaine UV-VISIBLE.

En fonction des paramètres de condensation (i.e. température rapport gaz/métal, taux d'admission du gaz et gaz lui-même) les atomes du métal qui se condensent peuvent soit être isolés, soit former des mélanges d'agrégats à la surface de la matrice en train de croître. Les spectres d'absorption optique de tels mélanges présentent plusieurs bandes d'absorption distinctes dans le domaine 2 000-5 000 Â. Les bandes correspondant à l'argent monoatomique et diatomique peuvent être clairement identifiées, ce qui permet d'attribuer les autres bandes à des particules contenant plus de deux atomes d'argent.

Abstract. — Silver microclusters have been prepared by co-condensation of a monoatomic silver beam with an excess of noble gas on a cooled sapphire target and investigated by optical transmission spectroscopy in the U.V.-VIS. range. Depending on the condensation parameters (i.e. temperature, gas/metal ratio, the gas admission rate and the gas itself) the condensing metal atoms can either be isolated as single atoms or as a mixture of aggregates at the surface of the growing matrix layer.

The optical absorption spectra of such mixtures show several distinct absorption bands within the range 2 000-5 000 A. The spectral features of monoatomic and diatomic silver can be clearly identified, allowing the remaining bands to be assigned to multimeric silver particles.

1. Introduction. — T h e m a t r i x isolation of a t o m i c s p e c i e s w h i c h normally exist a s large aggregates is a m e t h o d widely u s e d t o d a y [ 1 , 2 ] . T h e t e c h n i q u e involves t h e c o - c o n d e n s a t i o n of t h e s p e c i e s with an e x c e s s of gas o n t o a c o o l e d s u b s t r a t e . T h e principal f e a t u r e s of t h e m e t h o d a r e t h a t s p e c i e s u n s t a b l e u n d e r - n o r m a l conditions c a n b e stabilised in m a t r i c e s a t l o w t e m p e r a t u r e s , a n d t h a t t h e for^na-t i o n of m a for^na-t r i c e s is a c o n for^na-t i n u o u s p r o c e s s , for^na-t h u s a l l o w i n g an a c c u m u l a t i o n of s p e c i e s t h r o u g h i n c r e a s i n g layer t h i c k n e s s . A n o t h e r a d v a n t a g e of this m e t h o d h a s r e c e n t l y b e e n e m p h a s i z e d in literature [3-6]. U n d e r c e r t a i n e x p e r i m e n t a l c o n d i t i o n s a n aggregation of a t o m i c o r m o l e c u l a r f r a g m e n t s c a n b e a c h i e v e d . T h e s e aggre-g a t e s a r e of aggre-g r e a t r e l e v a n c e t o m a n y p r o b l e m s in c h e m i s t r y a n d p h y s i c s , e.g. in s u c h a r e a s a s h e t e r o g e n e o u s c a t a l y s i s , c h e m i c a l s y n t h e s i s , n u c l e a t i o n p h e n o m e n a a n d solid s t a t e p h y s i c s . A l t h o u g h m u c h w o r k h a s a l r e a d y b e e n c a r r i e d o u t in t h e field of m a t r i x isolation t h e m a i n p r o b l e m h a s not y e t b e e n solved satisfactorily : aggregation o c c u r s easily, b u t d o u b t exists a s t o t h e c o m p o s i -t i o n a n d clus-ter size of -t h e resul-ting aggrega-te m i x t u r e s . T h e s e t w o f a c t o r s d e p e n d o n t h e v a r i o u s c o n d e n s a t i o n p a r a m e t e r s , e . g . t e m p e r a t u r e , g a s / m e t a l r a t i o , gas a d m i s s i o n r a t e a n d gas s o r t . It

is t h e r e f o r e difficult t o assign clearly t h e n u m e r o u s spectral f e a t u r e s t h a t a r e usually p r e s e n t in m a t r i x a b s o r p t i o n s p e c t r a t o specific c l u s t e r s . T h i s is particularly t r u e if m o l e c u l a r aggregates containing m o r e t h a n t w o a t o m s a r e p r e s e n t , for w h i c h t h e c o r r e s p o n d i n g g a s p h a s e d a t a a r e g e n e r a l l y u n k n o w n . T o get a b e t t e r u n d e r s t a n d i n g of t h e aggre-g a t i o n / i s o l a t i o n p r o c e s s [3-9] w e h a v e b e aggre-g u n a s y s t e m a t i c investigation of m a t r i x s p e c t r a in t h e U . V . - V I S . region a s a function of t h e v a r i o u s c o n d e n s a t i o n p a r a m e t e r s u s i n g X e , K r a n d A a s m a t r i x g a s e s a n d silver a s m e t a l [12]. I n this article w e d e m o n s t r a t e t h e effect of v a r y i n g t h e c o n d e n s a -tion p a r a m e t e r s o n t h e spectral f e a t u r e s a n d s h o w h o w s u c h e x p e r i m e n t s c a n greatly aid in t h e a s s i g n m e n t of b a n d s t o specific c l u s t e r s . 2. Experimental. — T h e e x p e r i m e n t a l a p p a r a t u s u s e d in t h e s e e x p e r i m e n t s is s h o w n schematically in figure 1 [10, 11]. A s s u b s t r a t e a s a p p h i r e t a r g e t (area = 4 cm2) w a s u s e d , w h i c h w a s in g o o d t h e r m a l c o n t a c t t o t h e c r y o s t a t . T h e c r y o s t a t a c o m b i n e d b a t h a n d c o n t i n u o u s flow d e v i c e p e r m i t t e d t h e a d j u s t m e n t a n d regulation of t e m p e r a t u r e c o n t i -n u o u s l y withi-n t h e r a -n g e 2-300 K . T e m p e r a t u r e s w e r e m e a s u r e d with a v a p o u r p r e s s u r e t h e r m o m e t e r

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C2-8 W. SCHULZE, H. U. BECKER AND D. LEUTLOFF

Knudsencell ~ u a r t ; - ~ s c ~ l l a t o r

\

Vacuum-Chamber

FIG. 1. - Schematic view of the experimental apparatus.

and carbon resistors. The resistor which was attached to the sapphire target directly had been calibrated sep,arately. Noble gases of high purity were introduced into the vacuum chamber and condensed quantitatively on the sapphire target.

Silver atoms were evaporated from an electrically heated tantalum Knudsen cell. The silver evaporation rate as well as the total quantity condensed was monitored by a quartz oscillator which was located near the sapphire target. The resolution of the quartz balance was approximately 10 ng. During the formation of the matrix layer the silver atoms could condense on only half of the sapphire substrate. This was achieved by a shutter mounted on a translational-motion-feedthrough. The gas atoms, however, could condense homogeneously on the whole substrate. Light of incident intensity 1 was passed through both halves and the transmitted intensities measured separately by two photomulti- pliers. I R ~ ~ is the transmitted intensity of the reference beam which passed the matrix containing no silver, whereas ISIGN is the remaining intensity of the beam which passed the matrix containing silver. Both these signals were fed into a logarithmic d i v i d e r , yielding directly t h e a b s o r b a n c e

A = log

IREFIISIGN.

3. Results and discussion.

-

The yield of isolated atoms strongly depends on the condensation parameters, namely on the condensation temperature

TK, the gas/metal ratio R, the gas condensition rate mGAs and the gas itself [9, 121. Qualitatively it is known that within the series Xe-A the yield of isolated atoms decreases with increasing TK or decreasing R and &GAS. Hence aggregation occurs on the surface of the growing matrix layer. This effect is currently being investigated in our laboratory 1121. The nature of the gas appears to exert practically no influence on condensation at corresponding temperatures, but the gas admission rate, a parameter to which little attention has yet been 'baid, strongly influences the yield of isolated atoms.

Metal atoms can be isolated quantitatively under the following conditions : T ~ 5 0 . 1 TM (TM : melting

point temperature of the respective matrix gas), a gas/metal ration R > 1 000 and a gas condensation rate mGAs

>

1016 ~ m - ~ s-' [12]. Spectra of isolated ~ i l v e r atoms exhibit well known absorption bands corresponding to the P c S resonance transition of silver atoms in the gas phase (see Fig. 2) [lo-11, 13-14].

3600 3300 3200 3100 3000 2 9 0 0

h la

FIG. 2. - Absorption spectra of silver atoms in noble gas matrices below 10 K (solid lines) and in the gas phase (dashed

lines).

The absorption bands in the matrix spectra in the region 3 000-3 400

A

are shifted to higher energies compared to the gas phase lines of Ag and this shift increases from Xe to A but decreases again for Ne. Furthermore the matrix spectra show a substantial broadening of the absorption bands and additional level splitting. These effects which are due to the influence of the surrounding matrix atoms can generally be expected also for small aggregates. We discuss them in detail elsewhere [lo, 111.

Changing one of the above parameters in the reverse direction increases the chance of metal cluster formation. Figure 3 shows a spectrum after the onset of metal clustering as the gas/metal ratio reaches values below 1000. The change in R is

_-

achieved by increasing the silver rate at constant +temperature and fixed gas condensation rate. New absorption bands appear symmetrical to the charac- terized absorption lines of isolated silver atoms, namely a single band at lower energies and an only partially resolved doublet on the high energy side. The matrix layer had been formed at about 7 K in 137 s with a gas/metal ration of 670 : 1 and a gas condensation rate of about 1016 s-I. The total amount of condensed silver is 1.5 x 1016 ~ m - ~ ; i.e. roughly only one monolayer.

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OPTICAL ABSORPTION SPECTRA OF AG-MICROCLUSTERS C2-9

FIG. 3.

-

Absorption spectrum of monomeric and dimeric silver

particles in a Xenon matrix formed at about 7 K .

m x , = 7 . 2 x l o t 6 c m - * s - I , m A , = 1 . 1 x 1 014 ~ m S - I , - ~ m ~ , = 1.5 x l o t 6

conditions. It can therefore be concluded that these bands correspond to the same species. Under the given experimental conditions a statistical formation of molecular aggregates can be assumed. This results from a comparison between experimental data on the yield of isolated atoms [I23 and theoretical predictions of Behringer [7], which show a relative good correspondence. Using these arguments we have attributed the additional absorption bands in figure 3 to dimeric silver. Further- more a comparison with gas phase data for Ag2 1151 shows a good agreement between the matrix bands and the A+-X, B+X transitions of gaseous Ag2.

A further decrease of the gas/metal ratio to a value R = 257 (cf. Fig. 4) by increasing the silver

FIG. 4. - The influence of the temperature of formation

on the xenon matrix spectra. mX, = 7.2 X 1016 cm-* s-',

mAg = 2.8 x lOI4 cm-'s-' ; m,, = 5.9 x 10'6cm-2.

deposition rate results in an increase in the intensitv

trical to the dimer bands together with a broad back-ground absorption in the whole region 2 000- 5 000

A.

By statistical arguments, i.e. a decrease of R favours the formation of larger particles, and by comparison with gas phase data

-

where a progression of bands at about 5 000 due to a polyatomic silver molecule has bedn reported [IS]

-

we conclude that both these bands belong to trimeric silver. The development of the broad back-ground absorption indicates that larger particles are formed.

The clustering discussed so far was achieved only by changing the silver evaporation rate with other parameters fixed. The temperature and the evaporation rate were chosen such that the growth of particles was as closely related as possible to a purely statistical growth mechanism thus assisting in the assignment of the spectral bands. If the temperature and the gas admission rate are changed, further additional bands appear, the assignment of which to specific clusters is not yet possible. Figure 4 shows the influence of the temperature on the matrix spectra. Altering the substrate temperature in the range 6.7-30.3 K results in the growth of a band near the doublet of dimeric silver. Furthermore it can be seen that the quantity of molecular aggregates Agl-Ag-, decreases and hence larger aggregates of unknown size must be formed, the mixture of which shows a broad spectrum. For even higher temperatures the bands due to the small molecular aggregates disappear totally.

Changing the gas/metal ratio by decreasing the gas condensation rate by about one order of magnitude results in the appearance of several new bands and also in the growth of a broad nearly featureless band. This band shows a broad maxi- mum at about 3 500 and the onset of another band below 3 000

A

(cf. Fig. 5). '

2 5 K

I , I

5 0 0 0 LOO0 3000 2 0 0 0

F I ~ , 5 _Ilie influence of the gas/metal ratio R, which is

changed by decreasing the gas rate, at 25 K on the xenon matrix

of bands due to dimeric silver. In addition two new

",P,":2;:8x

_{cm-2 s-l,}

m,g = 5 , 9 x , 0 1 ~ m - 2 ,

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C2-10 W. SCHULZE, H. U. BECKER AND D. LEUTLOFF Finally we summarize the results obtained for the

identified aggregates and show the influence of the matrix gas on these species. Figure 6 shows the

FIG. 6.

-

The peak maxima of the absorption bands of Agl-Ag3

in the gas phase and in Xe, Kr, A and Ne matrices. The results

for Ag3 are to be taken as preliminary.

peak maxima of Agl, Agz and Ag3 in the gas phase and in different matrices. The Ne data are to be taken as preliminary. As already mentioned the progression of bands at 2.5 eV (5 000

A)

in the gas phase formerly attributed [IS] to a polyatomic silver molecule can be assigned to Ag3. Furthermore one of the three transitions attributed to dimeric silver at about 5 eV (2 600-2 400

A)

[15] also possibly belongs to Ag3. All these bands shift to higher energies in the sequence Xe to A, but move back again for Ne. Furthermore we note that with respect to the gas phase transitions of Ag2 there exist shifts to lower as well as to higher eliergies. Another interesting fact is that the energy diffe- rence between the Ag2-transitions is smallest for Xe and increases continuously from Xe to Ne, at which point it nearly equals the gas phase value. Figure 7 shows the spectra of dimeric silver in different matrices. The relative oscillator strength of these bands is approximately 1.5.

4. Conclusion.

-

Mixtures of molecular metal aggregates can easily be formed by co-condensation of the metal component and the matrix gas under

E (eV)

FIG. 7. -The absorption spectra of dimeric silver in Xe, Kr and

A matrices.

suitable experimental conditions. It is difficult to assign the numerous features usually present in such matrix spectra clearly to specific clusters. However a systematic study of the influence of the various condensation parameters and a comparison with gas phase data allowed the assignment of the

' absorption bands to clusters containing up to three

atoms. Further work is necessary to identify the ;emaining absorption bands which are due to multirneric silver particles of more than three atoms.

Acknowledgments.

-

The financial support by the European Recovery Program (ERP) is gratefully acknowledged.

References

[I] MEYER, B., LOW Temperature Spectroscopy (Elsevier, New

York) 1971.

[2] HALLAM, H. E., Vibrational Spectroscopy of Trapped Species (John Wiley & Sons, London) 1973. [3] ANDREWS, L. and PIMENTEL, G. C., J. Chem. Phys. 47

( 1 x 7 ) 2905.

141 BREWER, L. and CHANG, C., J. Chem. Phys. 56 (1972) 1728. [5] FRANCIS, J. E. Jr., and WEBER, S. E., J. Chem. Phys. 56

(1972) 5879.

[6] KUNDIG, E. P., MOSKOVITS, M. and OZIN, G. A., Angew. Chem. 14 (1975) 292.

[7] BEHRINGER, R. E., J. Chem. Phys. 29 (1958) 537.

[8] HULSE, J. E. and MOSKOVITS, M., Can. J. Chem. (in press). [9] PIMENTEL, G. C., Angew. Chem. 87 (1975) 220.

[lo] SCHULZE, W., KOLB, D. M. and GERISCHER, H., J. Chem.

Soc. Faraday Trans. 71 (1975) 1763.

[ l l ] FORSTMANN, F., KOLB, D. M. and SCHULZE, W., J. Chem.

Phys. 64 (1976) 2552.

[12] SCHULZE, et al. (to be published).

[13] BREWER, L., KING, B. A., WENG, J. L., MEYER, B. and

MOORE, G. F., J. Chem. Phys. 49 (1968) 5209.

[14] BREWER, L. and KING, B., J. Chem. Phys. 53 (1970) 3981.

[15] SHIN-PIAW, C., LOONG-SENG, W., YOKE-SENG, L., Nature