Photothermal and photoacoustic spectroscopy

Journal of Molecular Structure, 173 (1988) 129-140 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

PHOTOTHERMAL

H. COUFALl

AND PHOTOACOUSTIC

129

SPECTROSCOPY

and J. F. McCLELLAND2

1IBM Almaden Research Center, K33/802, San Jose, California 95120-6099 (USA) Mmes

Laboratory-U.S.D.O.E.,

Iowa State University, Ames, Iowa 50011-3020 (USA)

SUMMARY Photothetmal and photoacoustic spectroscopy rely on the detection of thermal or acoustic waves generated by the absorption of optical radiation and subsequent radiationless deexcitation. The principles of these spectroscopic techniques as well as most recent experimental developments are discussed. The potential and the limitations of these techniques are highlighted with typical applications.

INTRODUCTION In photothermal a modulated

(PT) or photoacoustic

(PA) spectroscopy

or pulsed light source. Subsequent

rise to thermal wave phenomena.

the sample under study is excited with

radiationless

A suitable temperature

radiation induced heating of the sample (Photothermal

decay causes local heating, giving

sensor can be utilized to detect this

effect).

Due to thermal expansion sound

waves are generated at the same time. They can be detected with a microphone detectors. effect.

This detection

scheme is frequently referred to as the photoacoustic

Both, PT and PA spectroscopies

and techniques

complement

conventional

such as specular, diffuse or internal reflection.

field is documented

by a number of rather comprehensive

articles (refs. 5-8). Up to date information

transmission

of international

conferences

on photoacoustics

body of literature and to allow a prospective

assessment

of the potential

compared in detail. applications.

of photothermal

Recent developments

spectroscopy, sample.

spectroscopy

the physical

Various detection schemes are are highlighted with typical

and trends in this field are presented.

optical spectroscopy

to determine the optical properties

this

user of this technique a critical

and photoacoustic

process are described.

(refs. 9-10) and the

(refs. 11,12). To complement

Potential as well as problems of these techniques

With conventional

of this

books (refs. l-4) and excellent review

on the current status of the lield can be found in

substantial

principles of the signal generation

spectroscopy

The growing importance

recent special issues of journals that were dedicated to photoacoustics proceedings

or other sound or optoacoustic

the transmitted

of the sample.

or scattered radiation is analyzed

In photothetmal

and photoacoustic

however, the absorbed energy is detected as radiation induced heating of the

Weakly absorbing

this type of spectroscopy

samples, such as surfaces or thin films, are particularly

difference between incident and transmitted

light intensity.

Highly absorbing samples are another

class of materials, that can be addressed by these spectroscopies, 0022-2860/88/$03.50

suitable for

because the absorbed energy is determined directly and not as the small

0 1988 Elsevier Science Publishers B.V.

due to the fact that no

transmitted

or reflected light has to be analyzed.

considerable

For optical transmission

spectroscopy

effort has to be spent to prepare samples such that the transmitted

analyzed and the optical properties or no sample preparation spectroscopy

at all is required.

light can be

For PT measurements

of the sample derived.

This makes photothermal

the method of choice for biological, pharmaceutical

and photoacoustic

or technical samples.

The fact that besides optical also thermal and in the case of photoacoustics properties

of the sample are involved in the signal generation

these detection schemes. spectroscopic

techniques

these techniques

highlighting

In a prototypal

experiment (Fig. 1) a homogeneous

or pulsed light source.

by a wavelength

dependent

of

in the sample can be

length A(A)

coefficient of the sample.

processes part of the absorbed energy is released as heat.

Due to radiationless

With the incident energy being either

or pulsed, the heat generation will show a corresponding

diffusion then a temperature

sample with thickness T

The light absorption

optical absorption

with /?(A) the commonly used optical absorption

modulation

The physical principles of these the power and the limitations

PROCESS

photothermal

is excited with a modulated

modulated

problems to be adressed,

are discussed in the following paragraphs.

SIGNAL GENERATION

characterized

of spectra considerably.

as well as applications

even acoustic

process is the main drawback of

These effects allow many nonspectroscopic

but can complicate the interpretation

only minimal

profile develops in the sample.

time dependence.

Via heat

For a heat source with the

frequency f the heat diffusion can be described by the thermal diffusion length D D(f)=

k denoting the thermal conductivity,

J

k npcf ’

p the density and c the specific heat of the sample.

amplitude of a thermal wave is attenuated

The

by a factor of e-2’r, i.e.. 2 x 10m3, when diffusing one

diffusion length (ref. 13). Therefore, only heat generated within one thermal diffusion length of a sample surface will be able to reach this surface with an appreciable magnitude. is in contact with another medium thermal waves are reflected and transmitted

If the sample

at the interface.

Heat diffusion extends into the second medium according to Eq. (2) with the material parameters of that medium instead of the sample properties. The transient temperature medium, is accompanied

profde in the sample, and where applicable also in the adjacent

by a stress profile due to thermal expansion.

Sound waves are hence

generated in the sample, at the surface or interface and in the adjacent material. waves, being essentially unattenuated over long distances.

in the frequency range considered here, can then propagate

If the heat generation

addition buckle slightly.

These sound

is sufficiently localized the surfaces or interfaces in

131

Samole Thickness

T

---

’ f

/ f

t

Diffusion

Samole

Absorbed Light

Length

Layer within diffusion Length of Surface

D

Layer that Causes Signal

Fig. 1. Schematic of the photothermal signal generation process. absorption and diffusion length please see the text.

For deftitions

All detection schemes that directly or indirectly detect the temperature surface will have to deal with the peculiar character of thermal waves.

of the

at the sample

Due to the fact that

thermal waves are critically damped .

the detector has to probe the temperature

within one thermal diffusion length of the

excited area and .

only energy deposited within this distance is effective in signal generation.

In this regard thermal wave detection is completely different from conventional

optical detection

and leads to the unique features of PT spectroscopy. In an optically opaque sample with thickness T>A(I),

such as shown in Fig. 1, the

radiation induced heating can always be observed at the illuminated side of the sample. back side a temperature

On the

increase is, however, only noticeable when light is absorbed within one

thermal diffusion length of the back side, i.e. the condition T-A(A)
of the sample such as p(A), k, p and c but also an experimental

frequency f. Acoustic detection schemes seem to overcome that

problem, since sound waves are essentially unattenuated

in the frequency range discussed here.

An acoustic detector can therefore be far away from the illuminated area of the sample. however only true for those detection techniques Sample thickness is then of no concern.

It is, however, important

wave serves only as a carrier of the thermal information. signal will depend on the temperature

distribution

This is,

that probe sound generated in a bulk sample. to remember that the sound

The amplitude of the observed acoustic

in the heated sample volume and the adjacent

132 medium, both determined

again by the thermal diffusion length D(l) and the optical absorption

length A(1). For spectroscopy

therefore the relation between these two parameters

has to be

analyzed. Whenever the thermal diffusion length D(I) is larger than the optical absorption A(A), D(f)>A(A), and the sample thickness T is larger than the absorption incident energy is absorbed and contributes absorption

length

length, T>A(J) alI the

towards the observed signal. A small variation of the

coeflicient does in this case not at all change the observed signal! The signal is

saturated and the observed signal is proportional reflectivity of the sample. the modulation

to the incident energy and (l-R), R being the

This condition can be avoided by using a thin sample or by increasing

frequency until D(f)
A sample with an optical absorption

length smaller than the sample thickness A(A)
absorbs all the incident light, hence no light is transmitted. accessible to conventional

transmission

spectroscopy.

Such an opaque sample is not

If the thermal diiusion

length D(f) is,

however, smaller than the optical absorption

length A(1), D(f)
this thermal diffusion length is proportional

to the absorption

coefficient p of the sample.

to the fact that the thermal diffusion length decreases with increasing modulation

according to Eq. (2) this condition can, at least in principle, be fulfiied by modulating enough a frequency.

Photothennal

spectroscopy

Due

frequency at high

thus allows recording of spectra of opaque

samples.

At very high modulation

frequencies

the signal. When using a lower modulation layer generate the signal. By comparing surface absorption

only light absorbed in the surface layer contributes

spectra at various modulation

can be readily distinguished

from bulk absorptions.

profile of a layered structure can thus be obtained in a non destructive

In conventional

optical transmission

spectroscopy

occurs has to be known to allow a determination be cumbersome

to

frequency the same surface layer and the adjacent

In principle the depth manner.

the path length along which absorption

of the absorption

for rough samples, fibers or powders.

frequencies therefore

coefficient.

In PT spectroscopy,

A task that can

however, it is

sufficient to assure that the thermal diffusion length is smaller than the relevant sample dimension such as grain size or fiber diameter, to obtain a qualitative absorption practical applications required.

a qualitative analysis is sufficient.

For quantitative

procedures

spectrum.

For many

In that case no sample preparation is

analysis, however, also in PT spectroscopy

extensive calibration

are required.

In spectroscopy of particular interest.

samples with a wave length variable absorption

coefficient are of course

Different parts of the spectrum might then have an absorption

length

A(1), possibly varying from larger than the selected thermal diffusion length to much shorter (Fig. 2). In this case parts of the spectrum where D(f)>A(I), will be saturated. modulation consideration

i.e. with large absorption

frequency and the thermal properties

of the sample.

spectra of the same sample will be dependent

3). Furthermore be quite different.

coefficient /I,

On the other hand the thermal diffusion length D(f) is a function of the According to the above

on the modulation

frequency (Fig.

spectra of samples with identical optical but different thermal properties might The same holds true for samples with identical light absorption

but diITerent

133

Spectroscopy of Thermally Thick Sample D < T Condition

Optically

Signal

.(1-R) A
Opaque None

HI-R) ._

D
Opaque None

P( 1-R) T-D
Opaque

.-

$1-R)

P(l-RI T
L

-.-.P.-.-

v

Transparent P(l-RI

; H T

Fig. 2. Schematic of the photothermal signal generation process in a thermally thick sample as a function of absorption length A. Shown is the relationship between thermal diffusion length D and sample thickness T for an increasing A (I-IV). For these cases, characterized by the relationship between the parameters, the variation of the photothermal signal with optical reflection coefficient R and absorption coefficient /I of the sample is given for detection on front and back side, respectively. See Fig. 1 for the definition of the symbols and the text for more details.

134

Spectroscopy of Optically Opaque Sample A < T Condition

Thermallv

Signal P(l-RI

Opaque None

(1-R) A<

D
Opaque None

(1-R) T-A<

D< T

Opaque + (I-R)

(1-R) T
IV

Transparent (1-R)

L_________

f

Fig. 3. Schematic of the photothermal signal generation process in an optically opaque sample as a function of thermal diffusion length D. Shown is the relationship between optical absorption length A and sample thickness T for an increasing D (I-IV). For these cases? characterized by the relationship between the parameters, the variation of the photothermal srgnal with optical reflection coeIIicient R and absorption coefficient /I of the sample is given for detection on front and back side, respectively. See Fig. 1 for the definition of the symbols and the text for more details.

fluorescence yields and therefore different heat generation.

Photothermal

and photoacoustic

spectroscopy

thus requires particular care in the selection of the modulation

consideration

of thermal sample parameters

generation

process.

frequency; careful

and thickness; and an understanding

of the signal

135 DETECTION

METHODS

Determining of thermometry

the temperature

and calorimetry.

or the energy of a sample has traditionally

been the domain

In the last years interest in radiation induced thermal and

acoustic processes increased substantially

due scientific and industrial applications

of lasers. A

number of classical detection schemes were adapted for this particular type of application

and

new detection methods were developed.

Detectors tie into various stages of the signal generation

chain described above.

detection schemes the radiation induced temperature

In photothermal

increase in the sample or at the sample surface is monitored by measuring either the temperature directly or a temperature and optoacoustic

dependent

property of the sample or of the adjacent medium.

Photo-

methods detect the acoustic waves caused by the radiation induced heating of

the sample itself or a gas or a liquid that is in thermal and/or acoustical contact with the sample. For experiments thermocouples

with the emphasis on surface temperature

classical temperature

sensors such as

are clearly the method of choice because they are easy to calibrate and convenient

to use. For spectroscopic

measurements,

however sensitivity and convenient

coupling of the

detector to the sample under study are the main concern. The transient heating of a pyroelectric material can for example be detected via the induced electrical charge or current.

As a matter of fact most commercially

available laser power meters

function on this basis. A variation of this technique uses a pyroelectric calorimeter as substrate for the sample of interest. sensitivity of nanojoules

Utilizing a pyroelecttic

thin film calorimeters

combined with a time resolution

(ref. 14) recently a

of nanoseconds

was achieved.

This

technique is, however, restricted to pyroelectxic samples or thin films that can be directly deposited onto a pyroelectric

substrate.

In contrast photothermal radiometry can be utilized with bulk samples and is in addition a noncontact

technique.

Blackbody radiation from the sample is imaged onto a suitable infrared

detector (ref. 15). A change in surface temperature

then effects a change of the observed signal.

These detectors require cryogenic cooling to reach acceptable sensitivities. detector is typically accompanied A number of techniques light source.

High sensitivity of the

by low time resolution.

utilize a probe laser to detect thermal effects caused by another

The power of the probe laser is typically orders of magnitude smaller than that of

the excitation source and different wavelengths

are commonly used. A change in temperature

in

the sample or the adjacent medium is associated with a change in the refractive index of that material.

This change in refractive index causes a change in the reflection or transmission

of the

probe laser (ref. 16). Recently variations of this technique (refs. 17,18) achieved a time resolution of the order of 10 picoseconds!

The refractive index gradient caused by the temperature

gradient

in the sampie cr the adjacent medium forms a transient thermal lens capable of deflecting a probe laser (ref. 19). The same is true for the surface buckling due to localized heating (ref. 20). In a similar fashion acoustic wave fronts give rise to refractive index gradients. surface displacement

(ref. 21). All probe laser techniques are therefore noncontact optically flat samples.

These, as well as

due to acoustic waves can be probed by lasers or other optical techniques techniques.

have the advantage

of optical excitation and probing and

They require, however, careful alignment of two lasers, and

136 Piezoelectric transducers (ref. 22), attached

to the sample under study convert the sound

waves that are generated in the sample into an easily recorded electrical signal. Their main drawback is the requirement in acoustic detection, time resolution.

such as sensitivity to acoustic noise and trade-offs between sensitivity and

Their main area of application

use in spectroscopy transparent

for good mechanical contact with the sample and problems inherent is, therefore, in ultrasonic material testing; their

is limited mainly to weakly absorbing

materials.

samples such as adsorbates

The same holds true for special transducers

or

employed for the detection

of surface acoustic waves (ref. 23). If the sample can be in contact with a gas atmosphere microphones

are frequently used for detection (see, for example, ref. 2). Beside requiring a gas

filled cell to contain the sample the frequency range of microphones suppression

of acoustic noise poses restraints

is rather limited and

on such systems.

An evaluation of the pros and cons of the major photothermal

and photoacoustic

detection

schemes shows that each of the detectors has its merits, making it the prime choice for certain applications.

As should be evident from the above discussion, the signal generation and detection

process can be rather complicated

and may involve a large number of individual processes.

substantial loss in sensitivity and time resolution is associated. process.

These losses become particularly

signal generation chain.

Microphone

with each diffusion or conversion

significant when detection occurs at the end of the

detection in particular has low signal generation

and time resolution and requires the most elaborate theoretical interpretation

of the observed signals.

A

efftciency

models for the quantitative

However, even with these limitations high signal-to-noise

spectra can be obtained using a microphone.

The spectra can be readily interpreted

due to the

long history and large body of literature associated with this approach.

SPECTROMETER. A typical photothermal

spectrometer

consists of a suitable light source, the detector and

signal recovery electronics (Fig. 4). To eliminate the wave length dependence thermal parameters normally employed.

of the sample and the characteristics

of the source, the

of the detector a reference sample is

Reference data are obtained in a single beam spectrometer

the sample spectrum or in a dual beam arrangement Due to the strong influence of the modulation at the same modulation

simultaneously

before or after

with the sample spectrum.

frequency it is imperative to record both data sets

frequency with samples of identical or well known thermal

characteristics. Normally the light source will be intensity modulated by suitable means, such as current modulation advantageous interest.

of a lamp or a mechanical chopper. when small absorptions

Where applicable a modulation

modulation) characteristics

or other physical parameters can be used.

Wavelength

superimposed

or polarization

on a large background

of the absorption (Temperature)

properties

modulation absorption

are are of

of the sample (Stark

that allow a modulation

of the sample

137

Slgnal

Fig. 4. Schematic of a photothermal

spectrometer.

Mostly sinusoidal or square wave modulation

of the incident light intensity is employed

(refs. l-4). In this case a lock-in amplifier is the adequate tool for signal recovery. emphasized

that if a source of constant intensity is modulated in this way the energy per

excitation cycle decreases with increasing modulation frequencies,

It should be

frequency.

The signal at high modulation

desirable, for example, because of the above considerations

on saturation,

will then

be extremely weak.

Excitation with a short light pulse has the advantage of generating a higher

surface temperature

than with the same energy in a longer pulse or a periodic excitation,

the fact that heat loss during the short pulse can be neglected.

signal/noise ratio but might damage the sample irreversibly or affect temperature properties.

With pulsed excitation box-car integrators

another modulation

data by cross correlation advantage

Tie

of the complete time

into the frequency domain (ref. 25). Recently

scheme is emerging using noise modulation

transient digitizers for data recording.

sensitive sample

are frequently used to monitor the signal

in a small time window (ref. 24). Transient digitizers allow acquisition domain signal and if desired transformation

due to

This results in a superior

(ref. 26) of the light source and

domain type results can then be obtained from the

(ref. 27), frequency domain data by Fourier analysis (ref, 25). The

of this new technique is that many modulation

frequencies are probed simultaneously

and therefore depth profnes can be obtained in a small fraction of the time required for recording spectra subsequently

at several modulation

intensity that is modulated

frequencies.

by an external modulator,

of the light intensity than any other modulation interference

noise modulation

scheme.

makes more efficient use

The intensity modulation

due to the

fringes can be utilized when a commercial rapid scanning FT-IR spectrometer

as the light source. modulation

Assuming a light source of constant

is used

One should, however, be aware of the fact that this results in a much lower

frequency for the long wavelength

wavelength part of the same spectrum. wave numbers due to saturation!

side of the spectra as compared to the short

Therefore a spectrum might be partially distorted for high

138 With conventional obtained.

Nevertheless,

light sources only a limited spectral, time and depth resolution high sensitivity, instrumental

preparation

make photothermal

detection an interesting

techniques,

such as diffuse or internal reflectance spectroscopy

however, offers a unique combination

of advantages

alternative

examples will be presented underlining

to more established

(ref. 28). Photothermal

detection,

that is not available with other techniques

when combined with the high spectral and time resolution paragraph

can be

simplicity and a minimum of sample

of laser excitation.

In the following

some of the above features.

APPLICATIONS Among the numerous applications noteworthy:

The state of microphone

of PT spectroscopy

two might be particularly

detection and applications

insurface science.

Largely due to historic reasons and ease of implementation well developed.

detection with microphones

is

Here the sample is either a gas or in thermal contact with a gas. When the

sample, which is normally enclosed in a cell, is excited with light transient heating of the sample occurs which gives rise to pressure fluctuations

that are detected with a conventional

microphone.

With gaseous samples and a suitable laser for excitation trace analysis can be performed. sensitivity of 0.2 ppb of SO2 in air has been achieved (ref. 29) demonstrating

technique for the detection of pollutants. The coupling gas allows a microphone a bulk sample regardless of sample topology. preparation

of experiments

In analytical chemistry

samples ranging from intact Aspirin pills, over thin layer chromatograms demonstrate

detection.

Very few attempts,

however, were successful to date in

Spectroscopy

photothermal

an attractive alternative to diffuse reflectance

ofsurfaces

techniques

and adsorbates is an area of application

(ref 3 1). For adsorbates

perfect mirror or a completely transparent

optical processes and even more so nonideal substrates, real time compensation compensation

techniques,

spectroscopy

(ref. 30).

well suited for

on an ideal, nonabsorbmg

substrate, be it a

the incident light intensity has to be

made large enough to generate a detectable increase in temperature in a typical experiment.

cell

FT-IR spectrometers

substrate, the amount of adsorbate that is detectable

is in principle only limited by the available light intensity:

obtainable

Photoacoustic

are now available from several sources for most commercial

and make PT spectroscopy

These applications

Qualitative analysis is, therefore,

quantifying the results due to the complex signal generation process. attachments

to gel

tissues, and minerals have been successfully examined.

how easy it is to obtain a spectrum of a sample.

facilitated by photothermal

The photoacoustic

in medicine, biology and chemistry.

Spectra of blood, skin, leaves, all of them in viva. have been reported. electropherograms,

to be coupled to

The fact of that therefore virtually no sample

is required has been exploited in a large number of applications.

literature (refs.2,3) shows an abundance

A

the potential of this

in the sample.

Nonlinear

reduce, however, the sensitivities

The signal from a nonideal substrate can be suppressed by such as for example polarization

modulation

schemes make PT surface analysis applicable to many combinations

and substrates in various environments.

With piezo- or pyroelectric

33). or electronic (ref. 34) spectra at coverages of few thousandths

(ref. 32). These of adsorbates

detection vibrational of a monolayer

(ref.

of adsorbate

139

were recorded under well-controlled Germanium

UHV conditions.

Surface states of freshly cleaved Silicon and

single crystals were studied with a laser beam deflection method by monitoring

radiation induced buckling of the crystal (ref. 35.36). Experiments conditions

by studies in situ in an electrolyte, at atmospheric

are complemented

under high pressure (ref. 37). The characterization spectroscopy

(ref. 38) taking advantage

of the depth proffig

Spectra of layered samples demonstrate,

conditions

or

capability of this technique.

however, one of the problems of PT spectroscopy. frequency of the excitation source and

a inversion of spectral lines can be observed (ref. 39-41). The fact that

only light that has been absorbed and converted into heat by radiationless key difference to conventional excitation affect PT spectra. different fluorescence

the

under UHV

of coatings is a natural extension of PT

Observed spectral features depend on the modulation under certain conditions

on adsorbates

optical spectroscopy.

Energy transfer

deexcitation

is another

and quenching of electronic

Spectra of two samples with identical absorption

spectra but

spectra will have completely different PT spectra (ref. 42).

CONCLUSION The principles of PT spectroscopy reviewed.

and several typical detection schemes have been

Their common feature is that only energy absorbed and converted into heat within the

thermal diffusion length of the sample surface contributes feature results in unique capabilities of photothermal preparation,

the potential of depth proftig

towards a photothermal

spectroscopy,

signal. This

such as a minimum of sample

and the utility for analysis of surfaces and thin ftis.

At the same time, however, spectra of samples with identical overall optical properties, different depth profnes or different thermal properties Due to the fact that only nonradiative

can result in completely different spectra.

processes can cause a photothennal

excited electronic states and energy transfer affect PT spectra considerably. make PT spectroscopy quantitative

a very powerful tool for special applications.

interpretation

and detection mechanisms

but

signal, quenching

of

All these features

Qualitative and especially

of spectra requires, however, substantial insight into signal generation and sample properties

and geometry.

REFERENCES

: 3 4 5

6 7 8 9 10 11

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