Principles of the Photoacoustic Effect

1. photothermal and photoacoustic

Photothermam science encompasses a wide range of techniques and phenomena based upon the conversion of absorbed optical energy into heat. Optical energy is absorbed and eventually converted into thermal energy by an enormous number of material –solids, liquids, and gases. In fact, the optical energy is absorbed, the excited states in atoms or molecules lose their excition energy by a series of non- radiative transitions that result in a general heating in the material.

The underlying principles of the photoacoustic effect have been studied for more then a century. it was named photocoustic because the photothermal heating effect was detected by an indirect acoustic method in 1880. Alexander Grshsm Bell (cited in Favier J.P.1997) had discovered the early concept of the photoacoustic effect when he tried to explain the operation of his photophone. He had done a lot of experiments on photoacoustic effect with solids, gases and liquids, where modulated light was used to illuminate the sample. through the experiments, Bell discovered that when a periodically interrupted beam of sunlight shines on a solid in an enclosed cell, an audible sound could be heard by means of hearing tube attached to the cell. the photoacoustic effect discovered by Bell was regarded as a part of the family of photothermal phenomena encompassing many effect produced by the heat generated in a sample due to the absorption of electromagnetic energy.

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In 1881,both Tyndah and Withem Roentgen (cited in Favier J.P. 1997) confirmed Bells experiment on gases. they found that an acoustic signal could also be produced when a gas in an enclosed cell is illuminated with modulation light. But due to the limitation of hearing tubes as detectors in the early experiment, progress in the field of photoacoustic died down. therefore, the photoacoustic technique had lay dormant for almost 50 years, until the advent of a microphone 50 years later the photoacoustic effect with gases was reexamined. then it had become a well-established technique for gas analysis. photons in the photoacoustic cell absorbed by the gas was converted into kinetic energy of the gas molecules, thus it gives rise to pressure fluctuations within the cell.

All the discoveries in the photoacoustic effect originate from 1938 was entirely limited to gases only. the phoyoacoustic effect on solid matter did not occur until the early of 1970. it is almost 90 years after Bells discovery, since 1973, photoacoustic effect has strongly reemerged on the solid sample. ti has revived with the development of a very useful technique spectroscopic investigation of solid material.

The photoacoustic effect in condensed matter may be detected by microphone absorption of modulated light by a solid sample produce a modulated heating of the sample surface. This heating cause pressure waves to be created in a gas in contact with the sample, producing an acoustic signal in the gas, which may be detected by a microphone. the sample, coupling gas, and the microphone are enclosed in a gas light photoacoustic cell. the cell acoustically isolates the microphone from external noise and contains a window enabling the modulated light to illuminate the sample (Almond and Patel 1996)

Bells first experiment on a condensed matter sample, revealed the fact that the loudest signal were produced by sample with the darkest colours. a few other experiments were performed in the years following, however condensed matter photoacoustic died out until the 1970 Parker (1073) carried out the experiment about to measure the phootoacoustic effect in solid when he tried to carry out the experiment on the photoacoustic effect in gases. in his experiment, he worked on gas phase photoacoustic, attributed an anomalously large PA signal to absorption of light by the windows of his cell.

Three years later, Rosencwing and Gersho (1976) derived the one-dimensional theory for a photoacoustic effect from asolid material, which has become known as the R-G theory and which has basis for the most other theories on microphone photoacoustic detection from a solid sample. in his hypothesis, the primary source of the photoacoustic signal result from a periodic heat flow from the solid to the surrounding gas. these can be explained when the beam of light falls onto sample, the heat produced by the light absorbed in the sample will diffuse from the sample to the gas through the sample –gas interface. by modulating the light beam, we are actually causes the expansions of the gas layer which near to the sample and this will finally create a sound wave.

The R-G theory has been shown to be agood model by subsequent experiment work. Therefore, it led to direct expansions in photoacoustic research in 1970. The publication of the R-G theory, stimulated work in photoacoutic and number of papers have been written on the subject, describing various light sources, sample, cells, modulation technique and frequencies, detection methods and signal processing apparatus. This theory will be discussed in detail in the next chapter

The photothermal and photoacoustic research was investigated since 1970 due to three major factors.

i) Devlopment of intense light sources, such as laser and high pressure arc lamps,such as xenon arc lamps.

ii) Development of sensitive detection equipment, such as condenser and electret microphones and piezoelectric detectors

iii) Development of more sensitive signal processing equipment such as filters and sensitive lock -in amplifiers.

The improvement in the above three areas enabled the photoacoustic effect to be studied and hence higher sensitivity photoacoustic spectroscopy could be performed.

Schemes of Photoacoustic Detection.

All photothermal system employ a modulated source of electromagnetic radiation usually a light source, to generate modulated heating in a sample medium. the system rely on the absorption by the medium of electromagnetic energy and its subsequent conversion into thermal energy. this heating result in a number of physical changes in and around the sample, figure 1.1 is schematic illustration of the phenomena resulting from the exposure of sample surface to a localized periodically modulated light source.

in addition to a change in the temperature of the sample, it is also produced infrared, acoustic waves, surface expansion, thermoelastic waves, surface reflectivity modulation and refractive index gradient in the medium in contact with the heated surface. all of these effects could be used to probe the photothermal response of an enormous number of materials –solids, liquids, and gases. the thermal wave detection method were classified in to acoustic and thermal detection techniques. acoustic detection technique employ either a gas condenser microphone for the detection of pressure variation in air or a piezoelectric transducer for the detection of thermoelastic waves in solid media. thermal detection method includes the use of thermocoupe or pyroelectric transducers (photopyroelectric detection, PPE)to detect waves directly. (Murphy et al. 1992)

Incident

Refractive index gradient

Infrared emission Surface Reflectivity modulation

Acoustic waves Surface expansion

thermal thermoelastic

waves waves

sample

figure 1.1 photothremal effect caused by illumination of a surface by a modulated beam of light (Almond and Patel 1997)

1.3 Objective of the present study.

CHAPTER 2

THEORY

2.1 Introduction.

In this chapter, will discuss the theory of photoacoustic effect in the condensed matter.the formulation of Rosencwaing -Gersho (R-G)theory from the acoustic detection technique employ a gas condenser microphone for the detection of pressure variation in air are described. According to R-G model,when heat is created by means of non radiative transition with a boundary layer of the gas in the cell. Since the light is chopped, the photoacoustic signal is generated due to an acoustic pressure disturbance at the sample -gas interface transferred from the gas medium to the microphone.

2.2 Rosencwaing -Gersho Theory

The Rosencwaing -Gersho theory, known as R-G theory, is an one -dimensional analysis of the production of a photoacoustic (PA) signal in a simple cylindrical cell resulting from the absorbed light energy. The model is schematically shown in figure 2.1 from the figure, the photoacoustic cell has a diameter D and length L

. It is assumed that the length L is small compare to the wavelength of the acoustic waves and the microphone detecte the average pressure produced in the cell. In the present case, they also assumed that the light is not absorbed by the gas and backing material. The sample has thickness and diameter D. The sample is mounted so that it’s front surface is expose to the gas (air)within the cell and back surface is against a backing material of thickness . the length. . Of the gas volume is given by.

=

When a sinusoidally chopped monochromatic high passing through are window of the cell and incident upon the solid sample, the intensity I at the depth x is given by