Spectral properties of pyrazine adsorbed on silver electrodes and cold silver films from surface enhanced Raman scattering (SERS)

Journal of Electron Spectroscopy and Related Phenomena, 30 (1983) 197-202 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

SPECTRAL COLD

PROPERTIES

SILVER

FILMS

OF PYRAZINE FROM

ADSORRED

SURFACE

ON SILVER

ENHANCED

RAMAN

197

ELECTRODES

SCATTERING

AND

(SERS)

R. DORNHALJS I. Physikalisches

Institut

der RWTH

Aachen,

D-5100

Aachen

(FRG)

ABSTRACT Surface enhanced Raman spectra of molecules adsorbed on electrode surfaces, metal-island films, colloidal particles and UHVevaporated low-temperature substrates often show considerable differences in mode strength and pcsitions between each other and and their possible the bulk molecular spectrum. These differences origins are discussed fcr pyrazine adsorbed on silver electrodes and cold silver films.

INTRODUCTION The observation cules

upon

esting

of greatly

enhanced

adsorption

on certain

metal

and intriguing

phenomenon

both

Raman

intensities

surfaces

experimentally

its discovery

in 1974.

Nevertheless

considerable

controversy

concerning

the origin

the enormous

intensity

cally

since

and the majcrity aspects

relatively in surface

led processes (ref.2).

in the electric

rules

advantage

adsorbed

review

a of

see ref.1)

on different

at electrode

layer

or corrosion

fcr spectra

is obtained,

knowledge

to determine

nature

processes

of adsorbed

and Raman

is that molecular,

should

help

to use the new possibi-

double

of infrared

molecules

aimed

for the study

infcrmation

of detaisurfaces or

molecules.

spectroscopy

rather

of vihraticnal

and orientation

than

As

of sur-

elemental,

selection

rules

of adsorbed

mole-

species.

The unambigous Raman

remains

spectroscopy

of catalytic

of selection

an important

cular

there

has concentrated

few investigations vibrational

the study

the study

face

of investigaticns

and theoreti-

and mechanism

(for a recent

mole-

an inter-

of this question.

Only lities

enhancement

frcm

has been

selection

interpretation rules

6) has been hampered, (i) Selection coupling

rules

between

of the observed

free and adsorbed

however,

for several

are expected

mechanisms

betweer

differences molecules

in

(ref. 3-

reasons:

to he different the exciting

for different

electromagnetic

036%2048/83/0000-0000/$03.00 0 1983 Elsevier ScientificPublishing Company

radia-

198 tion and the vibrating

molecule-metalsurface

(ii) Even in the case of a predominantly between

light

gradients terms

and adsorbed

very

close

to become

are described different

important

might

are normally

(ref.3.7).

dipol

the large

surface

which

coupling

electric

field

cause quadrupole neglected.

These

whose

transformation

properties

of the usual

polarizability

tensor,

as hyper-Raman

scattering

by a tensor,

from that

electric

molecule,

to a metal

complex

lead to the same selection

rules

are

and

(ref. 8). (iii) The well

red and specular may be obscured necessary

However, tional

as nearly

perfect

modes

polarization

of different

has been

(ref.12)

shown

apply

Bearing

it seems of mode

for several

pyridine-Ag

with

an oscillating should

mode

positions,

example

will

be considered

on silver.

PYRAZINE

ADSORBED

ON SILVER

Pyrazine

and several

by several

SER-spectra

groups

from

examples

D2h-symmetry

with

vibraconcer-

(ref.ll),

selection larger

bands

than about

corresponding

dipole

it

rule"

moment

to

predominant-

and changes

systems. in many

compa-

in selection

As the standard publications,

in this article

diazines

mentioned

a more detailed

on silver

already,

in some detail:

have

been

investi-

(ref.3-5,13,14).

pyrazine

upon

adsorbed

a lowering adsorption.

all fundamental

to silent s

tra obtained

other

indicating

of the free molecule

features

difficulties

strengths

has been discussed

adsorbed

active

not be conclusive

to attempt

adsorbate-substrate

pyrazice

either

fre-

be suppressed.

the interpretational

system

of Raman

particles

that those

still worthwhile

rison

might

on metal

to the surface in mind

rules

first

metals

in the visible

that the "metal-surface

to adsorption

vibrations

ly parallel

gated

conductors

due to roughness

20 8 in diamet.er. It predicts

another

(ref.q-12)

spect,roscopy by the

the fact that most

properties

symmetry

orientation

above,

Raman

in infra-

region.

although

molecular

and/or

rule"

spectroscopy

enhanced

roughness

ning molecular

should

selection

electron-energy-loss in surface

surface

do not behave quency

"metal-surface

established

Ramanelectrode

on silver-electrodes of the molecular While

vibrational z

fcr a molecule

frequencies

infrared-active substrates

were

symmetry

modes

the D2h

with

correspond both

and low-temperature

SER-specevapors-

199 ted silver assigned This

films

show pronounced

to modes

which

is especially

stretch

modes.

on Ag is given assignments

can unambiguously

C-H-wag.

and the v.,~~ and ~,~b

of the different

1. It should

of fundamental

be noted

vibrational

be

for the free molecule.

for the vjbb out-of-plane

A comparison in Table

which

are Raman-forhidden

apparent

the v,, out-of-plane

peaks

hend,

in-plane

results that

ring

ring

fcr pyrazine

some of the

frequencies

are controver-

sial.

TABLE

Sym.

Raman

1 dilson node lumber

Bulk (IR + Raman)

Raman aqu. solut,

this work t ref.3

ref.1

ref .f ref.1:

6a

596-620

615

635

636

615

1

1015

1017

1018

1020

1015

1241

1242

1237

1233

1225

1597

1578

1592

3066

3055

9a

1230-1239

8a

1578-1593

1594

1590

2

305L-307L

3060

3050

363

362

74.4

16a

(340-363)

silen

17a

(950-960)

Big

l@a

754

(927")

758

6b

641

(699")

(677)

3

1118(1346"

1120

1121

1123:

8b

1523-1529

1529

1520

1519

1522

7b

3040-3060

4

703

5

920

12

1018-1021

1038

18a

1130-1135

(1164?)

1483-1490

1485

AU

*ef.lL

352

(966)

972 743

753"

Raman B2g

Raman B3g Raman Blu

19a

IR B2U

IR B3u

IR

(983%)

13

(3066

16b

1416-418

f(

535,662

$:_

3031 703

700

698

700"

922

916

897

922% 1031

1488

1484

(3012+*)

1-

436

440

417

420

11

797

800

792

810

15

1069

10883

1088

1317

13473

14

1346(1149")

-

1340

19b

1411-1418

-

1420

20b

3066(?013*)

-

3060

* assignment

different

from

that given

1407

1430 (3080:

in ref.5

or ref.4

200 The agreement Ag-electrode

in vibrational

between

mode positions

the different

for pyrazine

investigat,ions

on an

is relatively

good, alt,hough the deviations are in general somewhat larger than e.g. for pyridine reported in the literature. In some details, larger

however,

differences

occur

(e.g. for B3u-type

modes);

the

-mode, very strong in ref.3, obviously has not been observed '19b in mode positions and relative intenat all in ref.4. Differences sities

between

'electrode

bed from

the gas phase

are more

pronounced.

sitive

on the cold silver

Some out-of-plane

) seem to be more

v11 case of pyridine,

spectra.' and those from

enhanced

also

considerably

or 'cold silver

to the bulk

'Electrode potential,

spectra'

Ag-type

modes

tion-reduction

spectral

rapidly

affect

spectra

vc,

to the sen-

they appear

components

sensitive values decay.

all modes

They also appear

Similar

are particularly

compared

first

to the applied

than

-.4V

Intensity

(versus changes

to the same extend.

(v,, vSa' V9a) are relatively

tial variations.

In both

hand,

however,

(v,~~, vlOb,

spectra.

or negative

SCE) pyrazine-SERS-intensities on the other

case.

into different

are particularly

at more positive

do not,

in the latter

surface'

(ref.5).

surface

For the electrode

shift.ed.and split

adsor-

vibrations

the v6a,6b -vibrations

to the environement.

pyrazine

insensitive

after

to poten-

the initial

oxida-

of pyrazine

exhibit

cycle(ORC).

type

changes

of investigations upon laser

SER-spectra

irradiation

on a t,ime scale

of several

hours. in the introduction

As mentioned ties to interpret

the apparent

there

changes

are several

in selection

possibili-

rules

compared

to the free molecule. Assuming exposed terms

an adsorbed

molecule

to st,rong electric

into play

could

field

explain

that is only weakly gradients

bringing

the activation

perturbed quadrupolar

of additional

of Au -type since these are hyper-Raman and Blu' B2Ll' B3u for D2h (ref.15). One might then, tentatively, interpret apparence While

of these during

additional

the initial

the electric

double

layer

only through

their

normally

with

change

a large

lecular more

plane

closely

like

modes phase

near

during

the electrode

in the polarizability v,, vYa, v8a ), these

to the large

electric

field

modes active the

the ORC as fellows:

of the ORC molecules

Raman-active

but

surface

modes

entering exhibit

(especially

derivative molecules gradients

along

SERS

those the mo-

are attached at the surface

201 during

t.he final

activates

potential

increase

only quadrupolar

(hyper-Raman)

"18a' "19a' "19b' It would, on the other new selection sorbed take

rule?

into account

atoms

to which viewed

surface

selection depending

usually

the local

compared

molecule

on a microscopic

on the other hand,

explain

to

If the surface

and hence

molecule.

t,he 'met.al-

group

of the adsorbed

to the free

the

for the adrequire

plane

symmetry

in symmetry

15'

for t.he substrate

is attached.

site

v

like

to interpret

symmetry

symmetry

in turn,

modes

however,

homogeneous

on the orientation

rules

pyrazine

adsite

molecule

lead to an increase

selection

a different

as an unstructured rule'(

allowed

This would,

a particular

the pyrazine

would.be

would,

complex.

This,

also be possible

hand,

by assuming

molecule-metal

of the ORC.

(ref.11))

molecule,

more restrictive

Adsorption

site of suitable

the changes

j.n selection

interpreting

the results

of the

symmetry rules

could,

equally

well. Avouris EELS

et.al.(ref.lh),

spectra

of pyrazine

adsorbed

flat

(ref.15)

in their

orientation

on Ag(lll),

on the Ag(ll1) study

which,

adsorption

'standing-up'

thermal

geometry

energy

similarly

however, at high

Moskovits

assumed

might

is et.al.

a flat

be converted

coverage

for yeorientation

of vibrational

that pyrazine

UHV conditions.

of s-triazine

of s-triazine

sufficient

under

concluded

to a

and providing

at a cold

silver

sur-

facp. Assuming modes

t.hat the observed

are mainly

essence

of the

scattering changes

due to orientational

'metal-surface

intensities

The sole appearance modes

to the metal

surfaces

cate that

the majority

ding-up'

seems

does

explained

that the large

polarizability apply,

our results

in another

way:

of pyra-

approach with

orientation.

chemisorbed

by special (adatoms)

the a 'stan-

By rising

potential

sites

large

that

to indi-

the applied tion

surface,

namely

of the

phase

surface

and assuming

rule',

with

of vibrational

of A g-type

reduction

electrode

effects

are more readily

at the beginning

zine molecules

variations

selection

are correlated

perpendicular

on electrode

intensity

they

are

Ag-adsorp-

and stabi-

Fig.

1. Model

for chemisorbed

pyrazine adatom.

at an Agt

202

lized

by coadsorbed

milar

to the suggested

appearence

the relaxed

also

selection

Raman-forbidden plex

by small

reorientational

allowing

atoms.

effects

symmetry

equivalent

for

of surface

more

irradiation

com-

case

lone-pair

is therefore

or laser

in

of normally

than in the pyridine

The balance changes

(siThe

is responsible

of this type

two essentially

potential

orientation in ref.2).

the appearance

in the pyrazine

contains

complex

(e.g. v,) is favoured

site

The stability

modes.

at its nitrogen

perturbed

vibrations

the local rules

is more delicate

as the fcrmer trons

pyridine-Ag'-Cl-

of out-of-plane

this orientation,

ions in an inclined

chloride

eleceasily causing

to occur.

CONCLUSIONS At the present of the possible tween

time no definite

bulk pyrazine

tention

of this

and SER-spectra

short discussion

of the SER-spect,rum combined

of pyrazine

and more detailed

possibilities face-adsorbate

conclusions

explanat.ions for the pronounced

difference

can be drawn.

of some observed on Ag, however,

investigations

of SERS to obtain

in favour

structural

of one be-

It is the inpeculiarities

to stimulate

to develop information

the full on sur-

complexes.

REFERENCES 1

YO 11 12 13 14

15

R.K.Chang and T.E.Furtak, edts., Surface Enhanced Raman Scattering, Plenum Press, New York/London, 1982. M.Fleischmann and I.R.Hill, in ref. 1, p.275-292. R.Dornhaus, M.R.Long, R.E.Benner and R.K.Chang, Surface Sci. 93(1980) 240-262. rR.Erdheim, R.L.Birke and J.R.Lombardi, Chem.Phys.Lett. 69 (1980) 495-498. KMoskovits and D.P.DiLella, in ref. 1, p.243-274. M.Moskovits and D.P.DiI#ella, J.Chem.Phys. 12. (1980) 6068. +.;t;~~sAppl.Surf.SCi. 6 (1980) 309. H.Neff, M.Moskovits and S.Holloway, J.Phys.Chem. 8; i1981j 621-623. KA.Pesrce and N.Sheppard, Surface Sci. 59 (1976) 205-217. R.M.Hexter and M.G.Albrecht, Spectrochimxcta 15A (1979) 233-251. H.Nichols and R.M.Hexter, J.Chem.Phys. 76 (1981) 3126-3136. R.G.Greenler, D.R.Snider, D.Witt and R.xSorbello, Surface Sci. 118 (1982) 415-428. T-J-Sinclair, Proc. VIIth Int.Conf. on Raman Spectroscopy, Ottawa 1980, W.F.Murphy, ed., North-Holland, Amsterdam, 1980, p. 408-409. Ph.Avouris and J.E.Demuth. J.Chem.Phys. 75 (1981) 4783-4794. S.J.Cyvin, J.E.Rauch and J.C.Decius, J.Chem.Phys. 42 (1965) 4083-4095.