Reflection absorption infrared spectroscopy of adsorbates on metal single crystal surfaces

Journal of Molecular Structure, 173 (1988) 405-418 Else&r Science Publishers B.V., Amsterdam - Printed

REFLECTION CRYSTAL

ABSORPTION

INFRARED

405 in The Netherlands

SPFCTROSCOPY

OF ADSORRATES

ON METAL SINGLF

SURFACES

M.A. CHESTERS School

of Chemical

Sciences,

University

of East

Anglia,

Norwich,

NR4 7TJ, U.K.

SUMMARY The practical considerations for reflection-absorption infrared spectroscopy (RAIRS) are discussed and an apparatus using an FTIR spectrometer is described. The RAIR spectra of ethyne, and of the substituted alkynes, propyne and but-2-yne adsorbed on the CuIlll) surface are described. Analysis of these spectra provides information on the nature of the adsorption site and the orientation of the adsorbate. The spectrum of hydrogen atoms adsorbed on the Cu(ll1) surface is described and related to the nature of the adsorption site.

INTRODUCTION The RAIRS technioue single crystal important

contributions

concentrated limited

surfaces

to surface

on strong

infrared

sensitivity.

extended

has been used for the study of adsorbed since the early 1970's

adsorbates.

Highly

sensitivity

spectrometers transform (ref. 5). transform

scanning

notably

carbon monoxide,

because

in the detection

limit have

of the technique

has been achieved

covering

spectral

to weak infrared

and discuss

absorbers,

of

such as

either by the use of dispersive range

(refs. 2,3,4)

a large proportion

In this paper I shall describe spectrometer

layers on metal it has made

during this time it has largely

absorbers,

a limited

spectrometers

While

In recent years improvements

the applicability

hydrocarbon

science

(ref. 1).

an experimental

the criteria

or with Fourier

of the mid-infrared

which

range

system using a Fourier

govern

the design of the

apparatus. The spectra

of adsorbed

ethyne

and the substituted

but-2-yne

will then be used to illustrate

selection

rule and analysed

rehybridization adsorbed

alkynes,

propyne

of the metal

in terms of the nature of the adsorption

of the adsorbate

and the orientation

of suhseouent

and

surface site, the

physically

layers.

Finally highest

the operation

the spectrum

sensitivity

002%2860/88/$03.50

of hydrogen

achieved

atoms on Cu(lll) will

with the current

0 1988 Elsevier

Science

experimental

Publishers

B.V.

illustrate system.

the

406 EXPERIMENTAL The criteria Greenler mirror

for the RAIRS experiment

(ref. 6) who calculated

resulting

showed a maximum of incidence

from one reflection absorption

(87-88O)

This behaviour conduction

electrons

to the metal surface results

Electric

is similar

atgrazing surface

vector directions

for p polarized

incidence

the electric

angle is - 900).

surface

oscillating

selection

angle

of incidence

electric

reflected

on infrared

to the metal

ray remains

and a node always

- 18001, Fig.

l(b).

light reflection

at near normal

parallel

from a metal

incidence

is nearly perpendicular

The

but

to the metal

Fig. l(a) (the phase change on reflection This phenomenon

perpendicular

for reflection

to the metal

surface will be excited. at a grazing

(typically

1 cm2 in

system in addition

molecules

the

modes producing

radiation

surface

on the optical < lOI

near

leads to what is now called

of p polarized

from a small single crystal

small sample size (typically

at

that the metal

fields parallel

on reflection

vector

angle

absorption

radiation.

radiation

area) is a quite severe limitation rather

and negligible

rule viz. that only vibrational

dipole moments

The requirement

The results

of the angle of incidence

(phase change

and can be supported,

the optimum metal

irrespective

by

AR, at a grazing

when one remembers

vector of an s polarised

at the surface

Fig. 1. surface. result

will neutralise

The electric

surface.

for s polarized

can be understood

radiation.

in reflectivity, radiation

determined

by a thin film on a metal

of infrared

or change

for p polarized

all angles of incidence

were originally

the absorption

to the

in a monolayer).

If we

were to restrict ourselves to a narrow angle of incidence range near the optimum,

say 87 k lo, this would

reflection-absorption

result

in an optical

cell of - 5 x 10e5 cm*.sr which

throughput

of the

is about three orders

of

407 magnitude

smaller

spectrometer. incidence

than the typical

In our experiments

range and we use 84 + 6O which

angle of incidence

AR Arbitrary

(

optical

curve

units

throughput

of a mid-infrared

we have compromised

FTIR

on the angle of

is illustrated

on a typical

AI? vs.

in Fig. 2.

1

Angie

of incidence

(degrees)

Fig. 2. Typical curve of G? vs e based on Greenler's reproduced with permission from ref. 5.

calculations,

300mm MATTSON CYGNUS 25

Fig. 3 Reflection-absorption system. A, detector; B, KBr lens; C& KRr F, Michelson interferometer; , E, sample; window; D, uhv chamber; P, gold grid polarizer, reproduced with watercooled globar source; permission from ref. 5.

408 The resulting

optical

10-s cm'.sr which

throughput

of the reflection-absorption

is sufficient

to provide

(ref. 7) when using a narrow band mercury A complete

reflection-absorption

The FTIR spectrometer,

Mattson

to 32 cm-' and a mirror

interferogram

one thousand

scan accumulation

resolution

limit.

electron

spectroscopy

spectrometer

beam splitter,

with

can be acquired 20 seconds.

cell windows

Spectra

to 400 cm-l but the

diffraction

for surface

The spectrometer

analysis

BEAM

t /?

RATIO

+ 100% AR R

11%

I

3000

I

I

2500

2000

Wavenumbers Fig. 4

Single

beam and ratio spectra.

of 1 x lO-lo

and with a mass

and detector

I(v)

0

the low

(LEED) and Auger

air. SINGLE

I

reported

scans at 4 or 8 cm-'

at a base pressure

electron

(AES) facilities

3500

in 0.2s and a

and lenses are KRr which

range of the spectra

low energy

dioxide-free

in Fig. 3.

range of 0.5 cm-l

cuts out at 800 cm-' so this defines

for gas analysis.

with dry, carbon

is illustrated

~0.01%.

The LIHV system operates

mbar and is equipped

range limit

(MCT) detector.

This means that a

takes 9 minutes

the low wavenumber

narrow band MCT detector wavenumber

telluride

25, has a resolution

at 4 cm-' resolution

with a noise level

restrict

apparatus

Cygnus

cell is N 3 x

at the dynamic

be the result of one or two thousand

The spectrometer would

cadmium

speed of 1.266 cm s-l.

double-sided

here will typically

spectra

1

I

1500

1000

box are flushed

409 The spectra

are reported

reflection-absorption surface water

is shown in Fig. 4.

vapour absorption

monoxide.

This

the clean metal adsorbate

surface

of carbon monoxide

is ratioed

a (relatively)

are still visible

necessary.

There

beam

on a Cu(lll)

It shows a number of sharp bands due to residual

against

and the result

absorptions

condensing

of a monolayer

and the one band (arrowed)

spectrum

against

as ratios of single beam spectra. The single

spectrum

due to the adsorbed

a similar

single beam spectrum

shows the absorption

flat baseline.

but can be reduced

is also a broad absorption

carbon

Residual

of

band of the

water

vapour

to a negligible

level if

band above 3000 cm-' due to ice

on the MCT detector.

EXAMPLES Ethyne adsorbed

on Cu(llll

The vibrational

spectrum

reported

using electron

spectrum

is compared

of ethyne

energy

adsorbed

loss spectroscopy

with the EEL spectrum

on Cu(ll1)

has previously

(EELS) (ref. 81.

in Fig. 5 with excellent

agreement,

2q20 I, t

I 400

I 1 1200

I

I Energy

6CH

Fig. 5.

EELS and RAIRS

spectra

I 2000

LEELS I

1

2800

Loss(cm-i)

"cc

of ethyne

I

"CH chemisorbed

on Cu(ll1).

been

The RAIRS

I-

410 though

now the inherent

higher

resolution

at 420 cm"

(or the vibration

of the absorption

dipole-excited and was assigned

of the molecule

for the metal-adsorbate being allowed

complex,

by the metal

results

(Table I), including

ethyne

smlecule

is strongly

bond order of between ligand

in cluster

compounds

(ref. 81 suggests

to either 3 or 4 metal

local symmetry

leads to the conclusion

possible

point group

deuterated

atoms.

data

molecule,

for the bands observed. stretch

on chemisorption

Comparison

be coordinated

TABLE

assignments

rehybridized

selection

rule and the tabulated

of the carbon-carbon

one and two.

surface

lead to just four fundamentals

for the adsorbed

in reasonable

The rather low frequency

the highest

selection

band wavenumbers

shows that this results

from the metal

must be in a high symmetry

C2,,, would

surface

stretch

the metal surface).

In fact

surface.

bands above

to the sytmnetric metal-carbon

apainst

rule and the fact that the ethyne molecule on the metal

by the

loss feature was found in the EEL

The rather simple RAIR spectrum

environment

bands are revealed

In addition to the three absorption

technique.

800 cm-l, one further spectrum

widths

indicates resulting

with spectra

that the in a CC

of the ethyne

that the adsorbed

ethyne must

This,

to the Cpv

in addition

that the ethyne

is adsorbed

at the

1

Assignment

of the Ethyne/Cu(lIll

Totally symmetric modes for C2,, point group vCH (sym) vcc 6CH (syml vMC (syml

site illustrated

chemisorbed

and but-2-yne

It is instructive substituted

c2n2

2914 1294 920

2189 1276

spectrum

state of ethyne

above 330 K the ethyne

Propyne

C2H2

in Fig. 5 where

So the vibrational

alkynes

RAIR spectrum.

it is coordinated has provided

on this surface.

desorbs without

c,~,

(EELS) 2920 1307 920 420

to four metal

us with a detailed On warming

atoms. model

the adsorbed

of the layer

decomposition.

on Cu(lll)

to consider as this will

the RAIR spectra

illustrate

of the adsorbed

the ability

of the infrared

411 technique

to resolve

layers

closely

on top of the chemisorbed

orientation

of molecules

The RAIR spectra Considering similar

coordination

Fig. 6.

RAIRS

asymmetric

spectra

I

I

2600

the fingerprint

modes

in the CH stretching to the expected

as for ethyne,

and deformation

if we assume

the methyl

I

I

1200

I

1000

(C) ethyne.

modes and a methyl

surface

selection

rule.

In

we see only one band and so clearly

produce

signals

a

too weak to observe.

region we see four bands,

fundamentals

about the

1400

(h) propyne;

by the metal

region of the spectrum

number of these "allowed"

assign

stretching

adsorbed

are shown in Fig. 6.

Fig. 6(b),

Z/cm-l

of (a) but-2-yne;

all be allowed

alkynes

propyne,

surface

2800

and symmetric

rocking mode would

substituted

to the EELS

state.

of the three chemisorbed

I

in contrast

form physically

and we learn something

layer,

to the metal

bands,

adsorbates

in the physisorbed

first the singly

3000

However

spaced absorption

We also find that the heavier

technique.

three of which we

and one to an overtone

of a methyl

deformation. The detailed by the spectrum deuterium

assignment

of the chemisorbed

of the species

(Figure 7).

propyne

with the acetylenic

This unambiguously

spectrum

hydrogen

identifies

is assisted

substituted

the CH stretching

by band

412 due to the acetylenic frequency

hydrogen

at 2855 cm-', perhaps

than the chemisorbed

significant

ethyne

that the wavenumbers

symmetric

surprisingly

CH stretch.

at a lower

It is

of the other bands in the CH stretching

region are unaffected

in the spectrum

indicating

negligible

coupling

acetylenic

hydrogen.

The fact that the other band in the spectrum

between

of the partially the methyl

shift from 1361 cm-' to 1353 cm-' in the spectrum molecule

suggests

its assignment

to the CC stretch

deformation

whose wavenumber

would

be expected

assignment

of the chemisorbed

propyne

of the Chemisorbed

Propyne/Cu(lll)

deuterated

is seen to

of the partially

deuterated

rather than to a methyl

to be unchanged.

spectrum

molecule

group modes and the

The full

is given in Table 2.

TABLE 2 Assignment

spectrum.

CH,CCH 2865

~1, VCH

v3, vcc *probably

coupled

Now turning di-substituted

TABLE

CH,CCD

with overtone

2923

2922

2883

2883

2828

2827

1361

1353

of methyl

to the assignment

alkyne but-2-yne,

2157, 2117*

rocking mode.

of the spectrum

of the chemisorbed

Figure 6(a), we find correlation

with the

3

Assignment

of Chemisorbed

but-2-ynelCu(ll1)

Spectra.

CH,CCCH, v9* VCH3(asy") VCH3( sym)

2930 2880

cn,cccn, very weak bands in 2000 - 2200vl, cm-' region.

2828 vp,

vcc

~11. PCH,

1392 1018

1356

413

(a)

CH,CCH

I I I I

2855 2883 I

/

1

3000

I

I I

2800

I

2600

/

I

I

2200

2000

-i&k-

'U/cm-'

Fig. 7

above

RAIRS

spectra

spectra

virtually

of (al Chemisorbed

simplifies

the task.

the same positions

carbon-carbon wavenumber.

stretching

The methyl

as in the propyne

band is ohserved,

in the propyne

modes are again too weak to be seen. In summary,

the infrared

group related spectrum

shifted

CHaCCD.

bands fall in

and the similar

further

to higher

of carbon-carbon

substitution

(1294, 1361 and 1392 cm-l) with

molecule

the desorption

stretching

increasing

substitution.

is reflected

temperatures

indicates

alkynes

surface

propyne

are fully

as for ethyne.

with increasing

a lesser degree of

This reduced perturbation

in a lower heat of absorption

for ethyne,

deformation

is given in Table 3.

to the Cu(lll) frequencies

to a methyl

The methyl

of the two substituted

with a similar mode of bonding

rehybridization

spectrum.

The assignment

spectra

The sequence

chemisorbed

(b) Chemisorbed

We also observe a band at 1018 cm-' which we assign

rock which was not detected

consistent

CHaCCH;

and but-Z-yne

of the

as evidenced

by

of 330, 270 and

240 K respectively. As mentioned but-2-yne chemisorbed

above continued

led to formation monolayer.

is shown in Fig. 8.

exposure

of a physically

The spectrum

of the Cu(ll1) adsorbed

of the monolayer

The bands are readily

assigned,

surface

to propyne

or

layer on top of the and multilayer

of propyne

Table 4, by reference

to

414

(b)

phase

I

3500

GO00

3000 3000

2500

2000 2000

1500

1000

g/ cm-'

Fig. 8 RAIRS spectra of (a) Chemisorbed physically adsorbed propyne. the spectrum carbon

of the gas (ref. 91.

skeleton

linear,

of the molecule

the molecular the acetylenic apparent

physically

CH stretch

to the molecular

adsorbed

The spectrum

molecules

of physically

than expected

to the molecular

Fig. 9.

state.

randomly

adsorbed

The spectrum

the solid (ref. 101.

parallel

is assigned

deformation

species.

rule this indicates

to

which

in mind

that the

with the carbon axis parallel

is, by contrast,

infrared

to the molecular

surface

somewhat

active modes parallel axis (e').

is a strong indication

to the metal

(e)) are

Bearing

oriented.

but-2-yne

axis (a,") and perpendicular

axis remains

methyl

adsorbed

Again we expect

fact only the e' modes are observed molecular

into two

axis (al) and perpendicular

are not oriented

they are probably

that the

state will be

active modes divide

(al) and the asymmetric

selection

adsorbed

plus

that strong bands of both type (e.g.

of the physically

of the surface

to the surface;

The infrared

It is notable

in the spectrum

the operation

simpler

parallel

axis (e).

(b) Chemisorbed

Here we must remind ourselves

in the physically

as for the free molecule.

sets with dipoles

propyne;

In

that the

in the physisorbed

in Table 5 by reference

to the spectrum

of

415 TABLE 4 Assignment

of the Physically

Propyne/CuIlll)

Physically* Adsorbed

Symmetry (C,,) point group)

Mode VI,

Adsorbed

vu

'a* VCHa(asymJ

Spectrum.

Gas

(ref. 9)

a1

3271

3305

e

29S7

2?71

v2*

‘CH3(sym)

al

2933

2941

2v7

2 x KH3(,sym)

a 1e

2870

2867

"3, vcc

ai

2118

2141

'73 'CH3(asym)

e

1440

1448

“49

dCH3(Sym)

a1

1370

1382

"8~

pCH3

e

1039

1035

* the bands due to the chemisorbed listed here.

species

remain in the spectrum

but are not

1%

g/+rl-1 Fig. 9 RAIRS spectra of (a) Chemisorbed physically adsorbed layer.

but-2-yne;

(b)-(d) Growth

of

416 We have observed physically attempt

adsorbed

difference

layers of propyne

to explain.

lack of permanent

an interesting

in the structure

and but-2-yne

It is likely to be related

dipole moment

but one could easily

which we shall not

to the higher

of the but-2-yne

use these properties

of the

molecule

to rationalize

symetry

compared

and

to propyne

the reverse

observation. At this point it is worth vibrational appear

spectra

to be a routine exercise

For the surface

scientist

such vibrational infrared science

TABLE

‘7, ‘113

to spectroscopists

working

it has only very recently

is rapidly

becoming

would

with bulk phases.

become

possible

resolution.

as important

of the

it provides

to record

Conseouently

a tool in surface

5 of the physically

adsorbed

but-2-yne/Cu(llll

VCH3(asym) “”

spectrum.

Physically* Adsorbed

Symmetry (DsV1 point group)

Solid

2963

e-

3 ( syml

(ref. 101 2963

a 2"

2922

e'

1443

1442

+ ‘16

e' + e"

1416

1414

GCH3(sym)

aq"

'10 GCHa(asym) vll

and the information

spectra with a few wavenumbers

spectroscopy

Mode

v63

alkynes

as it has been in the study of bulk phases.

Assignment

‘99

noting that the above analysis

of adsorbed

1363 1041

e'

PCH3(sym)

* the bands due to the chemisorbed listed here.

Hydrogen

atoms adsorbed

As a final example

species

remain in the spectrum

but are not

on Cu(ll1) to illustrate

the highest

achieved

by our apparatus

hydrogen

atoms at 91 K is shown in Fig. 10.

atoms since adsorption

1040

the spectrum

of molecular

too slow at temperatures

of a Cu(ll1)

hydrogen

sufficiently

sensitivity surface

It is necessary on copper

low to maintain

currently

saturated

with

to use hydrogen

is activated a monolayer

and is

in ultra

high vacuum. Firstly on the Cuflll)

it should be noted that for an adatom surface,

atoms or bridging infrared

such as on-top of one metal atom, bridging

three metal atoms,

active according

sytnnetric metal-hydrogen

in any high-symmetry

only one vibrational

to the metal stretch

surface

or vibration

selection

two metal

mode will be rule, i.e. the

of the adatom against

site

the

417

2xM

Y\

iHs M

M

M

f

I

1200

900

U/cd Fig. 10

RAIRS

surface.

spectra

of hydrogen

We see two bands in a region characteristic

two metal atoms

(- 900 - 1100 cm-')

result obtained

with

(ref.

12)

infrared

where Chabal stretch

metal-hydrogen

deformation.

of the overtone stretch.

always allowed

by the metal

selection

of coupling rule.

on Cuflll).

of hydrogen

This is analogous

for hydrogen

the lower frequency

and the higher

metal-hydrogen

capability

(ref. 11).

spectroscopy

assigned

metal-hydrogen

appearance

atoms chemisorbed

frequency

chemisorbed

are totally selection

with fundamentals

on W(100)

band to an overtone

as the result of Fermi

surface

to the

band to the symmetric

We adopt the same assignment

Overtones

bridging

resonance

symmetric

of the

and we explain

the

with the

and as such are

rule and will always

which are allowed

have the

by the surface

418 The strongest was achieved

that the hydrogen the stretching adsorbed

band in this spectrum

by accumulating is bridging

frequency

two metal

is unexpected

has recently

been a theoretical

since other examples

analysis

frequencies

which concludes

frequencies

obtained

from cluster

chemisorbed

hydrogen

(ref.

of adsorbed

that the characteristic

12).

compounds Based

(ref.

layer,

based on

of hydrogen

hydrogen

in a three-fold

hydrogen hydride

site.

There

vibrational

stretching

11) are not transferable

in this theoretical

of 1045 cm-l for the metal-hydrogen

chemisorbed elsewhere

The conclusion

atoms in the adsorhed

(111) surfaces place hydrogen in three-fold bridging sites.

on

frequency

is < 0.05% and the low noise level

4DilO scans at A cm-l resolution.

analysis

stretch would place This question

to

the

the

is discussed

further

(ref. 14).

CONCLUSXDN The above examples as a major further

analytical

improvements

will be extended

show that infrared

tool in surface in sensitivity

spectroscopy

science.

may now take its place

It is to be expected

will follow and that the spectral

down to about 200 cm-l by use of more specialised

that range

photon

detectors. Another reactions

important

at surfaces

area for the future will be the use of RAIRS to monitor and to detect

this kind of study are discussed

short-lived

elsewhere

intermediates.

Prospects

for

(ref. 151.

REFERENCES

1 2 3

6" 7 8 9

:': 12 13 14 15.

M.A. Chesters, J. Pritchard and M.L. Sims, J. Chem. Sot., Chem. Con. (1970) 1454. K. Horn and J. Pritchard, Surface Sci. 52 (1975) 437. B.E. Hayden, K. Prince, D.B. Woodruff and A.M. Bradshaw, Surface Sci., 133 (1983) 589.R. Ryberg, Surface Sci., 114 (1982) 627. M.A. Chesters, J. Electron Spectrosc. & Related Phenom. 38 (19861 123. R.G. Greenler, J. Chem. Phys. 44 (19661 310 and J. Vat. Sci. Technol. 12 (19751, 1410. M.D. Baker and M.A. Chesters in: R. Caudano, J.-M. Gilles and A.A. Lucas (Eds.), Vibrations at Surfaces, Plenum, New York 1982. B.J. Bandy, M.A. Chesters, M.E. Pemble, G.S. McDougall and N. Sheppard, Surface Sci., 139 (19841, 87. 6. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules, Von Nostrand, New York. 1945. M.A. Chesters and E.M. McCash, in preparation. U.A. Javasooriva. M.A. Chesters. M-W. Howard. S.F.A. Kettle. D.R. Powell and N. Sheppard,d,Surface Sci. 93 (19801, 526: Y. J. Chabal, Phys. Rev. Lett., 55 (19851, 845. Peter J. Feibelman and D.R. Hamann, J. Vat. Sci. Technol. A5 (19871, 474. M.A. Chesters, E.M. McCash, S.F. Parker and J. Pritchard, in preparation. D.H. Chenery, M.A. Chesters and E.M. McCash, submitted to Surface Science.