Acetylene adsorption on silver films: A Raman vibrational study

Journal of Electron Spectroscopy and Rehted Phenomena, 29 (1983) 409-412 Elsevler Scientific Pubhshmg Company, Amsterdam - Printed m The Netherlands

ACETYLENE

I.

ADSORPTION

POCKRAND,

ON SILVER FILMS: A RAMAN VIBRATIONAL

C. PETTENKOFER,

Physikalisches

Institut

D-4000 Dusseldorf

409

STUDY

and A. OTTO

III,

Dusseldorf,

Universitat

1, Fed. Rep. Germany

ABSTRACT 'Coldly' evaporated silver films exposed to acetylene at 120 K exhibit characteristic, enhanced Raman spectra. Peaks are identified by isotope line shifts. The results point to weak interaction of acetylene with silver.

INTRODUCTION So far most vibrational been dedicated (ref. l),

studies

to single crystal

(using EELS) of acetylene

faces of transition

W(110) (ref. 2), and Ni (111) with the metal

strongly

bond order

(to e.g. - 0.25 for C2H2 on W (110)

paper we discuss

leading

to a considerable

(ref.

(to -sp3 for adsorption

results

of a Ram$n vibrational

on silver, where only weak distortion?

adsorption

2))

acetylene

decrease

of the C-C

and, correspondingly,

on W (110)

(ref. 2)). In this

study of acetylene

of the adsorbed

have

like e.g. Pt (131)

(ref. 3). On these surfaces

interacts

to strong rehybndisation

metals

molecules

adsorption

are expected.

EXPERIMENTAL The optical paration

arrangement

Raman scattering prepared

and the UHV-facilities

of the 'coldly' evaporated

(SERS) (ref. 5), have been described

silver surfaces

sure corrected

(ref. 4) as well as the pre-

silver films, which exhibit

have been exposed

for ion gauge

sensitivity).

elsewhere.

to 36 L of acetylene Measurements

surface

enhanced

The freshly at 120 K (expo-

have been performed

with

the sample at 120 K.

RESULTS AND DISCUSSION Fig. 1 displays isotopes mental

conditions

adsorbed

Raman spectra from silver films exposed

as indicated.

molecules

As there IS no multilayer

(ref. 6), the Raman signal and hence enhanced.

to various

condensation

acetylene

under our experi-

is due to at most a monolayer

The SER spectra

exhibit

several

of

remark-

able features: (1) the number of observed molecule

peaks 1s surprisingly

has five fundamentals,

0368-2048/83/0000~000/$03.00

high (the isolated

of which three are Raman active

0 1983 Elsevler Sclentlfic Pubhshmg Company

acetylene (ref. 7)).

410 (ii) the most intense features

are composed

of two peaks (e.g. 1934/1888

cm-',

789/756 cm-l, and 673/635 cm -I for C2H2); (ill) the C-H stretching

vibrations,

which show up strongly

(or IR) spectra

(ref. 7), give rise to only weak features

(e.g. 3245/3317

cm-I for C2H2),

species are stronger (iv) several

(2385/2655

An assignment

adsorption.

of the SER features

cm-'); by a dot in the C2H2 SER spectrum,

of this weakly

energies

isolated acetylene

SER line frequencies

molecules(ref.

ly published

lated at low temperatures normally

frequencies

-all five fundamentals No definite

exposure

llnes,and

of acetylene

on silver films at 11 K

features

for the other SER peaks

C-O stretch of adsorbed

or adsorption

Ag films

13C12CH2

The observation species,

interfaces

(e.g.

observed

sites at the

The peak at - 2110

as 'impurity'

line in SER

(ref. 12), has been attributed IS, however,

questionable

to the

(ref. 14,

in fig. 1) is due to

in the gas. The assignment

of the other

of double peaks points to a second, differently

presumably

on'speclal' sites. Without

one might think of 'active' defect sites, which, tion on 'coldly' evaporated

(fig. l),

for the first time,

on 'special'

filnls might be responsible.

CO (ref. 13), which

-10% impurity of

bonded acetylene

due to C-H stretching

(fig. 1) can be given at

ref. 15). The weak feature at 1906 cm -' (13C2H2 spectrum

lines is unclear.

and isoAll spec-

, chemical reactions of acetylene molecules

1s quite frequently

spectra from 'coldly' evaporated

v2 of a

(ref. 11).

matrix

on Ag.

in the used gases

surface of 'coldly' evaporated

to recent-

formed by gas aggregation

in SERS from solid/gas

(at e.g. hot filaments)

cm-l (fig. 1). which

(ref. 9),

of the molecule.

11) or only weakly pronounced

phenomena

for the

for compa-

on Pt(ll1)

not clear). Our SER spectra display,

explanation

Impurities

layers condensed

silver particles

(ref. 10, ref.

upon

agree with the

species are similar

in a solid adsorbate/argon

which both are quite general ref. 10, interpretation

distortion

of the weakly adsorbed

Raman-forbidden

modes are either absent

present.

isotope shifts derived

8). Corresponding

SER data from acetylene

is data of

of C2D2 and 13C2H2 may be calcu-

leads to considerable

energies

(ref. 10) and from colloidal

tra exhibit

species

of the molecule

data from an EELS study of acetylene

interaction

The vibrational

vanish

of the

to corresponding

values quite well as shown in table 1. Table 1 displays

rison also vibrational where strong

bonded acetylene

are very similar

lated from the C2H2 data by using the well-known

experimental

desorption

(ref. 7).Thls points to little dlstortlon

Therefore,

in SER spectra

for these peaks.

given ln table 1. Vlbratlonal acetylene

Raman

lines of the deuterated

the sample to - 145 K; this indicates

species responsible

gaseous

the corresponding

peaks, which are marked

when annealing

dunng

in ordinary

knowing details,

in the case of oxygen adsorp-

silver films, lead to different

dloxygen

species at

411

Raman shift

km-‘)

Fig. 1. SER spectra from 'coldly' evaporated silver films exposed to 36 L of various acetylene isotopes (ZOO mW of 514.5 nm radiation, 4.5 cm-1 bandpass). Peaks assigned to weakly adsorbed acetylene are marked by dots.

Table 1. Comparison of SER features attributed to weakly adsorbed acetylene with corresponding data from gaseous acetylene and acetylene on Pt (111). SERS

I

514 (531)+I

gasa)

612 623 (651)+I

505 539 1762

3287 3218 2385 (2381)+ (3235)+

2427

3374

2701

2655 (2645)+ (:%)+

I

I

+jsotope shifts calculated after ref. 8 a)after ref. 7 b)after ref. 9

ELS Pt(ll1) b)

Vibrational mode

the surface and hence various characteristicSER peaks (ref. 16, ref. 17). SER acetylene spectra might be interpreted analogously.

SUMMARY ANDCONCLUSION Enhanced Raman signals have been observed from silver films exposed to acetylene (Ts = 120 K). The most prominent features with vibrational energies close to those of gaseous acetylene have been attributed to a weakly bonded, almost undistorted species (Tdes - 145 K). The observation of double peaks point to a second, somewhat stronger bonded species, whose nature is unclear at present. Nevertheless, this investigationreflects the potential of surface enhanced Raman scattering as a surface analytical tool.

REFERENCES H. Ibach, H. Hopster, and B. Sexton, Appl. Phys. 14 (1977) 21 C. Backx and R. F. Willis, Chem. Phys. Letters 53 (1978) 471 J. E. Demuth and H. Ibach, Surf. Science 85 (1979) 365 I. Pockrand and A. Otto, Appl. Surf. Science 6 (1980) 362 I. Pockrand and A. Otto, Sol. State Commun. 35 (1980) 861 Landolt-Bornsteln,II. Band, 2. Teil; Spring= (1960) 8

G. Herzberg , ‘Infrared strand (1945)

and Raman Spectra

of Polyatomic

Molcules’,

van Nor-

W. F. Colby, Phys. Rev. 47 (1935) 388 H. Ibach and S. Lehwald,J. Vat. Sci. Technol. 15 (1978) 407 1; M. Moskovits and 0. P. DiLella in 'Surface Enhanced Raman Scattering', R. K. Chana and T. E. Furtak (eds.). Plenum (19821 11 K. Manzel, W. Schulze, and MI-Moskovits, Chem. Phys. Letters -85 (1982) 183 12 I. Pockrand, to be published 13 0. P. DiLella, A. Gohln, R. H. Lipson, P. McBreen, and M. Moskovits, Chem. Phys. 73 (1980) 4282 14 :: H. Wood, M. V: Klein, and D. A. Zwemer, Surf. Science -107 (1981) 625 15 I. Pockrand and A. Otto, to be published 16 J. Eickmans, A. Goldmann, and A. Otto, to be published 17 c. Pettenkofer, I. Pockrand, and A. Otto, to be published