SURFACE AND THIN-FILM STUDIES BY PHOTOACOUSTIC SPECTROSCOPY APPLIED TO INORGANIC AND BIOLOGICAL MATERIALS

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SURFACE AND THIN-FILM STUDIES BY

PHOTOACOUSTIC SPECTROSCOPY APPLIED TO INORGANIC AND BIOLOGICAL MATERIALS

G. Kirkbright, R. Miller, D. Spillane, I. Vickery

To cite this version:

G. Kirkbright, R. Miller, D. Spillane, I. Vickery. SURFACE AND THIN-FILM STUDIES BY PHO- TOACOUSTIC SPECTROSCOPY APPLIED TO INORGANIC AND BIOLOGICAL MATERIALS.

Journal de Physique Colloques, 1983, 44 (C6), pp.C6-243-C6-247. �10.1051/jphyscol:1983638�. �jpa-

00223197�

J O U R N A L DE PHYSIQUE

Colloque C6, suppl6ment au nOIO, T o m e 44, octobre 1983 _{page C6-} 243

SURFACE AND THIN-FILM STUDIES BY PHOTOACOUSTIC SPECTROSCOPY APPLIED TO INORGANIC AND BIOLOGICAL MATERIALS

G.F. Kirkbright, R.M. Miller, D.E.M. Spillane and I.P. Vickery

Department of Instrumentation and Anazytical Science, UMIST, P.O. Box 8 8 , Manchester M60 I Q D , U . K .

Rgsumg - Ce papier pr6sente des r6sultats pour l'analyse en profondeur d'gchantillons en utilisant deux techniques PA diffgrentes : modulation sgquentielle diff6rentes frgquences et modulation pseudo algatoire (mgthode des corr6lations).

Abstract - The techniques of simultaneous multi-frequency and sequential single frequency PAS are used to obtain depth-related spectral information for a number of samples.

INTRODUCTION

There are a large number of techniques employed for the examination of surfaces.

Many of these e.g. Secondary Ion Mass Spectrometry (SIEIS), Auger Electron Spectroscopy (US) will allow analysis only of the top few layer of atoms. Those which will allow examination at greater depth into the sample e.g. diffuse reflectance spectroscopy, will present only an integrated picture of the layer examined. PAS is unique in that it has the potentiality to allow non-destructive spectroscopic study of the distribution of material below a sample surface.

Access to this type of information is of great use in the study of protective coatings, where information on the diffusion of contaminants through the coating and the processes occurring at the coating/substrate boundary is very difficult to obtain. It is also of interest for the examination of multi-layer systems (such as food wrappings) in terms of assessing the composition and thickness of the individual layers.

This paper presents preliminary results for the depth profiled analysis of samples using two different photoacoustic techniques.

1. Sequential Single Frequency

This is conventional PAS where a sinusoidally modulated optical beam is employed. The modulation frequency is then altered sequentially to prove spectra relating to layers of decreasing thickness (1).

2. Simultaneous Multifrequency (Correlation)

This employs a pseudo-randomly modulated light source and cross- correlation techniques to produce an impulse response of the system at a single wavelength ( 2 , 3 , 4 ) . The spectrum is then built up by obtaining the impulse response at a number of different wavelengths.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983638

JOURNAL DE PHYSIQUE

INSTRUMENTATION

The PA spectrometer used for modulation frequency studies has been described elsewhere (5). This system was modified to produce the correlation PA

spectrometer shown in Figure 1. The conventional rotating sector was replaced by an assymetric device which produced an optical PRBS signal having a 127 bit word length. This signal was detected by a photodiode based device housed in

the monochromator. The signal from the PA cell was taken via a low-noise preamplifier (Model 4505, E.G.

G.Brookdea1 Ltd, U.K.) to the signal analyser

(Solartron 1200, Solartron Instruments Ltd, U.K). The signals detected optically and by photoacoustics were then cross-correlated by the analyser and the data collected and stored in a computer (MINC 11/23, DEC Ltd, U.K.). Both the wavelength drive and the signal analyser were under the control of the computer which was then able to synchronise the spectral scan and data collection. Hard copy output was obtained via an X-Y recorder (Model 5221C, Hewlett-Packard Ltd).

Figure 1. Schematic of a Correlation PA Spectrometer

Plotter

I ^x-Y I

PRBS Chopper

-.--- u Optical Path

- Electrical Connections

PA Cell

FIG=I.=fl-G-.

RESULTS AND DISCUSSION

M o n o c h r o m t o - . - -

Figure 2 illustrates the conventional depth-profiling approach employing

sequential single frequency PAS. The sample consisted of a film of poly(urethane acrylate) which had been homogeneously doped during casting with Bromothymol blue.

The surface was then coated with a thin layer of blue ink applied by marker pen.

The sample thus consisted of a thermally transparent surface layer on a thermally thick substrate. The peak observed at z. ³⁹⁴ nm arises from the Bromothymol blue and that at z. 625 nm from the blue pigment. The results show that, as the modulation frequency was increased, the signal arising from the surface absorption became relatively more intense than that arising from the bulk; this result would be expected from earlier work (1). The results are, however,

\ I /

r

, ,

Controlling computer

I

. ^Signal _Amplifier

Analyser

d i f f i c u l t t o i n t e r p r e t and t h e measurement t i m e i s l o n g (E. 3 0 m i n u t e s t o a c q u i r e t h e f o u r s p e c t r a - t h i s m i g h t be reduced t o E. 5 m i n u t e s i f measurements were t a k e n o n l y a t 3 9 4 nm and 626 n m ) .

F i g u r e 3 shows t h e i m p u l s e r e s p o n s e s a t 3 9 4 nm and 626 nm f o r t h e same sample.

I t i s i m m e d i a t e l y c l e a r from t h e s h a p e s of t h e c u r v e s t h a t t h e a b s o r p t i o n a t 3 9 4 nm a r i s e s from a b u l k a b s o r p t i o n and t h a t a t 626 nm from a t h i n s u r f a c e l a y e r . Each impulse r e s p o n s e r e q u i r e d 25 s e c o n d s t o a c q u i r e and t h e t o t a l measurement t i m e was c a . - 2 m i n u t e s .

F l o u r e 2 . PA S v c c l r u o r o Ycl!o\< ~ o l y n l e i f l l n ~ Cootcd W l l h o 111111 I c y e r of

b l u e 11l0rr.sn:.

I t i s c l e a r from t h e r e s u l t s shown i n F i g u r e s 2 and 3 t h a t t h e m u l t i - f r e q u e n c y t e c h n i q u e p e r m i t s more r a p i d d e p t h - p r o f i l i n g a n a l y s i s and a l s o p r o v i d e s more e a s i l y i n t e r p r e t a b l e r e s u l t s .

A s a f u r t h e r i l l u s t r a t i o n o f t h e d e p t h p r o f i l i n g c a p a b i l i t i e s o f c o r r e l a t i o n

t e c h n i q u e s , F i g u r e 4 shows t h e s p e c t r u m o f t h e a d a x i a l s u r f a c e o f a laburnum

l e a f . The UV-absorption o f t h e c u t i c u l a r l a y e r c a n b e s e e n w e l l r e s o l v e d from

t h e s u b s u r f a c e a b s o r p t i o n a r i s i n g i n t h e p h o t o s y n t h e t i c a l l y a c t i v e c h l o r o p l a s t

l a y e r .

C6-246 JOURNAL DE PHYSIQUE

Figare 3 . Cross-Correlotlon Fu;:ctiorls f o r o

cooted n i th o t h i n layer of blbe D i q m e c t at 395 and 626 na.

i -

REFERENCES

1. A.Rosencwaig, "Photoacoustics and Photoacoustic Spectroscopy", John Wiley

Sons, (1980).

2. Y.Sugitani, A-Uejima and K.Kato, J.Photoacoustics (1982), 1, 217.

3. - G.F.Kirkbright and R.M.Miller, Anal .Chem. (1983), 2, ^502.

4. G.F.Kirkbright and R.M.Miller, European J.Phys., In Press (1983).

5. C.M.Ashworth, S.L.Castleden, G.F.Kirkbright and D.E.M.Spillanc,

J .Photoacoustics (1982), - 1, 151.