Surface raman ellipsometry

Journul of Electron Spectroscopy and Related Phenomena, 30 (1983) 43-50 Elsevier Scientific

Publishing Company,

FOURIER TRANSFORM SURFACE SPECIES

INFRARED

Amsterdam

43

- Printed in The Netherlands

REFLECTION-ABSORPTION

SPECTROSCOPY

OF

William G. Golden and David 0. Saperstein IBM INSTRUMENTS, INC., San Jose, CA

ABSTRACT We have been able to demonstrate progress in Fourier Transform Infrared Reflection-Absorption Spectroscopy (FT-IRRAS) by incorporating a switching circuit into the data acquisition electronics of an air-equilibrated IBW IR/98 FTIR Spectrometer such that it is possible to obtain simultaneously both reference and polarization modulated spectrum. These spectra ratio to produce good discrimination against gas-phase species. The operating principles of this approach and sample FT-IRRAS spectra of monolayer assemblies of cadmium arachidate adsorbed on silver will be discussed. INTRODUCTION The advantages conventional infrared further

of Fourier Transform

dispersive

techniques

spectroscopic the potential

Several

take advantage

described

by Greenler

Absorption

Spectroscopy

to achieve

improved

in ultrahigh

of the so-called

successful

for dispersion

modulation

grazing

monolayers

(7-14).

These

in dispersion

infrared,

Infrared

infrared range

technique

in dynamic

incidence

Infrared

photoelastic

(10).

circular

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

(PEM)

have

dichroism

(16,17) ratio

FTIR. our present efforts

monolayers

in the area of

Spectroscopy

of cadmium

arachidate

The spectra were recorded

modulator

modulators workers

range and signal-to-noise

open to the air, i.e. with atmospheric

using a ZnSe photoelastic

Several

Reflection-

to FTIR using the techniques

Reflection-Absorption

on silver are shown.

spectrometer

systems

rules first

IRRAS and vibrational

improvements

of Langmuir-Blodgett

assemblies

vacuum

incidence

selection

dynamic

In this paper we will describe

Spectra

i.r. at grazing

surface

(IRRAS),utilizes

instrumental

the photoelastic

Fourier Transform

to investigate

science applications.

(15).

One of the methods

over conventional

in a wide area of

spectra of thin films (l), Langmuir-Blodget established

efforts,

developed

(FTIR) over

the need exists

have used FTIR and dispersive

(2-6) and monolayers

and have demonstrated

However,

for FTIR in surface

workers

Spectroscopy

have long been utilized

applications.

to obtain vibrational

adapted

Infrared

(CdAA)

with the

CO2 and H20 present,

in the IBM IR/98.

0 1983 Elsevier Scientific

(FT-IRRAS).

New signal handling

Publishing Company

44 techniques acquired

are described

wherein

the sample and reference

spectra are

simultaneously.

EXPERIMENTAL The experimental modulation

technique

of the intensities of the reflected ratioed

approach

for dispersive

in which,

of the parallel

randomly

oriented oriented

modulation

dipole absorbances

involves

tional rotating of modulation

blade chopper

signals of interest. In FTIR,single

Michelson

interferometer

the Genzel design. mathematically

possible

separate

less than 10 KHz, it is quite possible dispersive

double modulation

Figure la depicts technique.

Other configurations

ventional

grazing

described

here (2-14).

were readily

incidence

available

easily within polarizer

experiment

the optical

in the usual fashion

to emulate,

utilizing

to produce

(19).

polarization

of operation

is then focused at grazing monolayer

of these devices

assembly

The signal from the detector Each channel

passed through

the conventional

preamplifier

is demodulated

monolayerc

could be performer' gold wire grid

modulator

were combined

infrared radiation.

(80°+

by using a standard

FTIR ac amplifier

(20).

5') onto the variable

signal processing is physically

separately.

con-

to the methods

can be found elsewhere

incidence

chopper.

for obtaining

A Perkin-Elmer

retro mirror assembly (Harrick Scientific). Figure lb shows a schematic of the FT-IRRAS

two channels.

a separate

applicable

modulated

can be

the

to test this FT-IRRAS

Langmuir-Blodgett

ZnSe photoelastic

The principles

ment.

Since PEM's operate for the mid-IR

(18) and so the entire experiment

International

it is

in many respects,

adopted

IRRAS and are equally

In this case, however,

This radiation

Langmuir-Blodgett

(16).

have been d~onstrated

the FTIR sample chamber

and Hinds

frequency,

modulation

without

e.g. is

into the FTIR system, at a

experiment

arrangement

which

to produce the infrared

modulation

at ca. 50 to 100 KHz while the interferometer

produced

variations,

an interferogram

modulator

larger than the interferometer

(10).

in either a conventional

interferometer

produces

by the Fourier Transform

to produce a double modulation

of the

in analog mode

from the interferogram

or in one of several

a photoelastic

frequencies

demodulation

to a fixed mirror

of

(PEM) and a conven-

such that their respective

This mirror motion

By introducing

spectrum.

which

IRRAS this double

is produced and recorded

relative

demodulated

frequency much

chosen

modulator

beam spectra are obtained

of a mirror

(Ip-Is/Io+IS)in

In dispersive

enough to allow

The ratio

(I,) components

These signals are then

divide out leaving only the spectrum normal.

the use of a photoelastic

are different

by the motion

are collected. beam spectrum

along the surface

a double

both the sum and difference

(Ip) and perpendicular

infrared radiation

to produce a psuedo-double

dipoles

IRRAS incorporates

in analog mode,

angle arrange-

split into

The signal which

and filter circuits,

is

Ipf Is,

45 PFM

_...

switch to circuit zmple

-

and hold and AID

FTIR amplifiers

(b)

Fig.1. (a) Optical configuration of the PEM, polarizer, retro mirror attachment, and KBr plate in the IR/98 sample chamber; (b) schematic of the signal-handling electronics for simultaneous digitization of numerator and denominator signals. hereafter

called

contains

the denominator,

is first demodulated

numerator,

to produce an interferogram

the usual single

The other channel,

of the sample plus spectrometer.

with a phase sensitive

whose amplitude

beam interferogram

hereafter

called

amplifier

is proportional

the

(lock-in)

to Ip- Is.

A

high-pass filter is also employed to remove the large dc offset associated with this signal. is necessary

of the lock-in

amplifier,

time to respond by choosing

the lowest

sufficient

changes

feasible

numerator

interferogram,

dc output, must

lock-in

amplifier

position

cycles must

output

have sufficient This is accomplished

time Constant.

of the interferometer,

be accumulated

2)

a

to produce an accurate

of the value of (Ip-Is).

fashion

utilized

computer

an analog

is changed

and denominator

it is then necessary

spectrometer's

memory.

switch,

at each digital

laser interferometer)

channels

have been produced

to input both signals

The approach

viz. Fig. lb.

here is to

The state of the analog

data point (derived

such that numerator

produces,

in one mirror

data file which contains the digital ferograms

alternately

is this switching both sample

(numerator)

final sequence

and denominator

in either

signal amplitudes

software

This

an interferometer

information,

and denominator

i.e.

inter-

even or odd data addresses.

which allows

and reference

file to produce

and denominatol

has both numerator

of the data handling

the use of conventional the original

stored

technique

switch

from the FTIR's reference

scan of the interferometer,

both numerator

data file produced

in the

into the FTIR

we have adopted

are sent to the FTIR input (sample and hold and A/D) alternately. ideally

it

1) the output

because:

slowly,

of the interferogram.

step in the mirror

number of modulator

Once the numerator above

mirror

being a filtered

to amplitude

For each digitized

average

In order to produce a reliable

to scan the interferometer

for simultaneous

(denominator)

of this dual

collection

interferogram.

interferogram

It of The

involves

to split out the even and odd data from

two new files, each containing

one half of

46 the total number of data points recorded. interferograms

These numerator

are then Fast Fourier Transformed

and denominator

and ratioed

in the usual

fashion. The CdAA monolayer

assemblies

of their ease of fabrication are well characterized prepared

(no vacuum

systems

(3,4).

by the Langmuir-Blodgett

monolayer

were chosen

always established

system

to test the technique is required)

and because they

All of the CdAA assemblies

dipping

technique

with the carboxyiate

because

were

(21,22) with the first

group closest

to the Ag

substrate. The interferograms spectrometer 100 mirror against

at 4 cm

were recorded

-1

resolution

scans were co-added

the FT-IRRAS

spectrum

time was approximately

at an optical

all spectra

are obtained conditions.

Inc.

IR/98

of 0.235 cm/set.

spectrum

and then ratioed

Data acquisition

of a clean Ag substrate.

technique

atmospheric

velocity

for each FT-IRRAS

Since one of the goals of this work is

4 minutes.

to devise an FT-IRRAS

with an IBM Instruments,

which

is insensitive

with the spectrometer

to gas phase absorptions,

completely

open to ambient

RESULTS Figure 2 shows the polarization six monolayer

noted.

CdAA sample before ratioing

The presence

(Ip+Is).

modulation

against

spectrum

are removed

The resultantspectrumfor

by ratioing

six monolayers

the denominator

along with the spectrometer's numerator to denominator

carboxylate

stretch,

-1 , the asymmetric carboxylate the weaker band, at 1577 cm consistent

with the surface

is oriented

selection

rules,

spikes,

in Figure 3a,

spectra.

and

stretch,

i.e. the carboxylate

with its local C2 axis along the surface

sharp downward

spectrum should be

of CdAA is shown in Figure 3a.

band at 1432 cm -l, the symmetric

The strong

(Ip-Is) of the

of strong water vapor and CO2 absorbances

These unwanted gas phase absorptions

background

spectrum

due to water

normal

are group

(23).

The

vapor, arise from the

fact that when the original

interferogram

file is split into numerator

denominator

the amplitude

of the two are not equal.

amplitude

interferograms,

difference

When ratioed

is carried

these amplitude

to water vapor absorption For the purposes technique,resolution the spectral resolution

through

differences

spectra.

small errors corresponding

was decreased

the present capability in order to provide

errors due to gas-phase

and ratioing

produce

maxima.

of evaluating

that when the resolution

the FFT into the calculated

and

This

error

absorption.

is illustrated

is decreased

of the FT-IRRAS

better ratioing

This trade-off

of

between

in Figures 3b and 3c. Note -1 a substantial

(in the FFT) to 8 cm

41

Fig.2. Single beam polarization modulated spectrum of six monolayers of cadmium arachidate on silver. The large absorbances are due to carbon dioxide and water vapor present in the unevacuated spectrometer. improvement

in spectral

information

(e.g. the CH2 wag and twist progression

error

are not as well resolved). completely

is obtained,

-1 bands near 1321 cm -1 the data digitally to ca. 10 cm

Smoothing

blends the progression

but at a loss in spectral

bands but produces

a spectrum

with usable

signal-to-noise. Vonolayer

sensitivity

taking spectra resolution. symmetric

(1439 cm-') and asymmetric

was investigated

spectrum.

by -1

As in Figure 3, the

(1552 cm-') carboxylate

stretching

bands

seen.

In Figure 5 clearly

the FT-IRRAS

contamination

spectrum

of one monolayer

the presence of the symmetric

However,

stretches.

the broad downward

of the reference

Ag substrate

of CdAA on Ag, also

and asymmetric

features

it is clear that signal-to-noise

Otherwise, monolayer

technique

samples of CdAA on Ag at 10 cm

Figure 4 shows the two monolayer

are easily

exhibits

of the FT-IRRAS

of two and one monolayer

carboxylate

in this spectrum

by ambient ratio

are due to

hydrocarbons.

is sufficient

for sub-

spectroscopy.

One other

experimental

to FTIR instrumentation. calculated, is dependent

the apparent

result Because

is discussed

here as it is

peculiar

of the way in which the spectrum

sign of absorption

on the sign of the quality

bands due to surface

(Ip-Is).

is

species

This is because,

in the

48 (b)

fal 2

1432

I $3-4 Ip+l,

5%

1321

1577

I ' Jcx,

'a21

L

m

1

I

12DO 61 D 30 1600 UAVCNull8Lns en-1

2

21

Fig.3. FT-IRRAS spectrum of six monolayers of cadmium arachidate on Ag; calculated at 4cm-1 resolution a {b) calculated at 8 cm-1 resolution C calculated at 8 cm-l resolutio; and smoothed to ca. 10-l resolution: I{ computation computed

algorithm,

spectrum)

represented

the spectrum.

value of the interferogram

is taken so that all single beam spectra

as positive

which affect

component

the absolute

energy spectra.

Ip and Is equally

absorbed,

always

show positive

in the case where

However,

Consequently,

are stored and

all absorptions

normal absorption

in

Ip is the only polarization

the sign of the apparent

absorption

When Ip
on the sign of (Ip-Is).

(or the

1439

-

peak will depend will yield an apparent

1439

I

-1 Ip-‘S 'P + ‘s

1%

1552

I

IP -

1s

'P + 4

1320

Fig.4. FT-IkRAS spectrum of two Fig.5. FT-IRRAS spectrum of one _1 monolayers of CdAA on Ag calculated monolayer of CdAA on Ag at ca. 10 . at 8 cm-l and smoothed to ca. 10 cm-l.

49 "negative

pretation

For the case when Ip>>Is,

absorption."

will occur.

When

Ip>Is

is difficult.

considerations. illustrates

a discontinuity This effect

Table 1 summarizes

these rules

is due entirely these sign rules

By utilizing

(viz. Figure l.), the s ign of (Ip-Is)

normal

positive

will occur and spectral

absorption inter-

to instrumental for FT-IRRAS.

a KBr plate in the optical can be changed

(lo).

Figure

5

beam

Figure 6a shows

Fig.6. FT-IRRAS spectra of six monolayers of cadmium arachidate on Ag showing the dependence of the sutface species absorption on the sign od lip-Is); (a) Ip>Is, (b) Ip
of six monolayers

of CdAA when Ip>I s; Figure 6b is a spectrum

of the same sample but for f
1

FT-IRRAS

Sign of (IpcIsf:

RULES FOR THE SIGN OF ABSORPTION Apparent

BANDS

Sign of the Absorption

Band for:

Gas Phase Species

Surface

Positive

Negative-Linear with absorbance

+

Positive

Positive-Linear with absorbance

0

Positive

Not observed

Species

50 CONCLUSION We have demonstrated spectra of monolayer and sensitivity an understanding with doing

here the feasibility

of the special

surface absorptions;

the utilization

of the interferograms

of mirror

considerations

relaxes

surface

In addition,

has led to a useful rule for

from non-surface

of the switch circuit the constraints

since signal averaging

resolution

associated

rules for the sign of a surface absorption

an easy test for distinguishing

stability,

instrumental

FT-IRRAS

with reasonable

in a short period of time (ca. 4 minutes).

IRRAS with FTIR spectrometers

Finally,

of obtaining

amounts of surface species

bands.

in the data acquisition

associated

can be performed

provide

absorption

with experimental

to any desired

number

scans.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

P. A. Chollet, Thin Solid Films 52, 343 (1978), (b) R. G. Greenler, J. Chem. Phys. 50, 1963 (1968). 0. L. Allara and J. D. Swalen, J. Phys. Chem. 3, 2700 (1982). F. A. Burns, N. E. Schlotter, J. F. Rabolt and J. D. Swalen, IBM Instruments, Inc., Application Note No. 1 (1981). T. Ohnishi, A. Ishitani, H. Ischida, N. Yamamoto and H. Tsubomura, J. Phys. Chem. 82, 1989 (1978). W. Knoll, M. R. Philpott and W. G. Golden, J. Chem. Phys. 77_, 219 (1982). W. G. Golden, C. Snyder and B. Smith, J. Phys. Chem., to be published. A. Crossley and D. A. King, Surface Sci. 68, 528 (1977). at Surfaces, Ed. R. Caudano, M. D. Baker and M. A. Chesters, in: Vibr%ions et al., (Plenum Press, New York, 1982). H. PfniIr, D. Menzel, F. M. Hoffman, A. Ortega, and A. M. Bradshaw, Surface Sci. 93_, 431 (1980); A. Ortega, F. M. Hoffman and A. M. Bradshaw, Surface Sci. 119, 73 (1982).

(a)

W. G. Golden, D. S. Dunn and John Overend, J. Catal. 11, 395 (1981). R. Ryberg, in: Vibrations at Surfaces, Ed. R. Caudano, et al., (Plenum Press, New York, 19E). P. Hollins and J. Pritchard, in: Vibrational Spectroscopy of Adsorbates, 12. Ed. R. F. Willis, (Springer, New York, 1981). J. C. Campuzano and R. G. Greenler, Surface Sci. 83, 301 (1979). '1:. M. Kawai, T. Onishi and K. Tamaru, Appl. Surface xi. 8, 361 (1981). 15: (a.) R. G. Greenler, J. Chem. Phys. 2, 310 (1966). Tb.) S. A. Francis and A. H. Ellison, J. Opt. Sot. Amer. 49, 131 (1959). 16. L. A. Nafie and M. Diem, Appl. Spec'crosc. 33_, 130 (1979); L. A. Nafie, M. Diem and D. W. Vidrine, J. Am. Chem. Sot. 101, 496 (1979). 17. A. E. Dowry and C. Marcott, Appl. Spectrosc., to be published. 18. Samples supplied by M. Jurich and J. D. Swalen. 19. The potential for obtaining FT-IRRAS spectra of optical components is possible - care should be taken to isolate the spectrometer from polarization modulated light. 20. I. Chabay, Ph.D. thesis, University of Chicago, 1972; I. Chabay and G. Holzwarth, Appl. Opt. 14, 454 (1975). (a) G. Gaines, Insoluble Monolayers at Liquid Gas Interfaces, (Wiley, 21. New York, 1966);. Kay and P.T. Bagus,Ed., Topics in Surface 1::

22. 23.

~~n~~~fSumPrP~~ss~o~wS~~rk~e~g7~)~170 15 (1939), (b)K. B. Blodgett, ;hys. Rev. &, 391 (1939j.-' J. F. Rabolt, F. C. Burns, N. E. Schlotter and J. 0. Swalen, J. Chem. Phys., to be published.