Generalized theory of the photoacoustic effect

Abstract

The photoacoustic effect is the production of an acoustic signal when a sample in an enclosed cell is illuminated with chopped light. The absorbed light produces a periodic heat flow from the sample to the surrounding gas and backing material, causing pressure variations in the gas which are detected by a sensitive microphone. A theoretical treatment will be presented which involves simultaneous solution of thermal-diffusion equations for the sample and backing material and fluid-dynamic equations for the gas. The resulting acoustic signal depends on the chopping frequency, the optical absorption coefficient and thermal properties of the sample, and on other material and system parameters. The conditions under which the approximate treatment of Rosencwaig and Gersho [J. Appl. Phys. 47, 64–69 (1976)] is valid will be discussed. Application of the theory to solid and liquid samples will be considered.

Effect of Sample Preparation Route on the Thermal Diffusivity of Nd2O3 - A Laser Induced Photoacoustic Study

“…A material prepared by two different methods shows slightly different surface as well as chemical properties. Kuo et al have shown [10] by mirage effect that the thermal diffusivities of differently prepared specimens of SiC differ. In the present paper the effect of method of preparation on thermal diffusivity of Nd 2 O 3 has been determined by photoacoustic technique.…”

“…The photoacoustic (PA) effect has been revived as a very useful technique for measuring the optical [1][2][3][4][5][6][7] and thermal [8][9][10] properties of materials because of its high sensitivity and non destructive nature. The basic principle of photoacoustic effect is that a sample illuminated by periodically modulated (chopped) light undergoes optical absorption and gets heated by nonradiative transitions.…”

Terahertz Time-Domain Spectroscopy of Metallic Particle Ensembles

“…A disadvantage of this method is that an invasive physical contact is required to accurately measure heat flow through the metallic sample. To overcome this constraint, several groups [9,12,13] have employed the photo-acoustic effect to non-invasively probe metallic phase transformations. In such experiments, phase transition modulates the acoustic signal generated at the surface of a sample when a surrounding ambient gas has been heated by a periodically modulated light beam.…”

Monitoring Power Plant Efficiency Using the Microwave-Excited Thermal-Acoustic Effect to Measure Unburned Carbon

“…The product of µ and β is useful in predicting the relationship between the thermal sampling depth and the optical absorption depth [6,7]. If µβ <<1, absorption is unsaturated, the response of the instrument is linear with concentration of the absorbent.…”

“…Many theoretical models exist for the thermal-acoustic response of solid and condensed samples have developed theories on the subject [2][3][4][5][6]. Following the derivation of [2], the thermal-acoustic signal (PAS), q, is given by: and ρ is the density, C p is the specific heat, k is the thermal conductivity, β is the optical absorption coefficient, µ is the thermal sampling depth, I o is the incident radiation intensity, γ is the ratio of specific heats, P o is the gas pressure, λ is the length of the cell, T o is the gas temperature, and f is the modulation frequency of the radiation.…”