Photoacoustic Effect In Solids Research Articles

Theoretical Analysis of Pulsed Photoacoustic Effect in Solids

This article presents a one-dimensional theory of a photoacoustic cell, working in the pulse regime. A four-layer system with elements of finite thickness has been assumed to represent consecutive parts of the photoacoustic cell. A parabolic heat equation with an instantaneous, bulk heat source has been solved using the Fourier transform of spatial coordinates. The theory allows one to assume that a heat source is existing in every part of the system and that an arbitrary time profile of the initial pulse is applied. Consequently, the system can be treated as an arbitrary photothermic or photoacoustic one-dimensional system. As a result, one obtains temperature profiles in the entire system at any time instant after its excitation with a light pulse. The gas-pressure evolution is dependent on the thermal and optical properties of the sample, the cell geometry, and duration and shape of the initial pulse.

Theoretical and experimental investigation of the photoacoustic effect in solids with residual stresses

Abstract Modern experiment and theory in the field of residual stress detection by the photoacoustic method are summarized and analyzed. A multimode approach based on the simultaneous application of several photothermal and photoacoustic methods is proposed for the study of thermal and thermoelastic effects in solids with residual stress. Some experimental results obtained within the framework of this approach for Vickers indentation zones in ceramics are presented. The effect of annealing on the photoacoustic, piezoelectric signal for ceramics and the influence of the given external loading on the behavior of the photoacoustic signal near the radial crack tips is investigated. It is experimentally shown that both compressive and shear stresses contribute to the photoacoustic signal near the radial crack tips. The model of the photoacoustic, thermoelastic effect in solids with residual stress is proposed. It is based on the modified Murnaghan model of non-linear elastic bodies, which takes into account a possible dependence of the thermoelastic constant on stress. This model is further developed to explain the photoacoustic signal behavior near the radial crack tips. It is demonstrated that this model of the photoacoustic effect agrees qualitatively with the available experimental data.

Theoretical analysis of pulse photoacoustic effect in gas filled cell

Pulse photoacoustic effect in solids is a foundation of nondestructive data acquisition method describing thermal properties of solid samples. Pulse research may be carried out using facilities developed for photoacoustic, continuous wave investigation, but proper interpretation of results obtained in a particular experimental setup requires that a theoretical model be developed. In this paper a one-dimensional model of four-segment photoacoustic cell consisting of backing, sample, gas, and window is described. Laser pulse passes through non-absorbing window and gas, and is absorbed in the sample that gives up the heat to the surrounding: backing, gas, and window. Due to thermodynamic process the gas changes its pressure what may be recorded with a microphone. The model is based on analytical general solutions of parabolic heat diffusion equations for each area, with boundary conditions of the 4th type at material interfaces.

Study of Photoacoustic Effect in Solids and Liquids

Study of Photoacoustic Effect in Solids and Liquids

Dependence of the x-ray photoacoustic signal intensity of copper on sample thickness and modulation frequency

The hard x-ray photoacoustic signal intensities of copper have been measured at an x-ray absorption near-edge structure (XANES) region as functions of sample thickness (from 5 to 300 μm) and modulation frequency (from 5 to 50 Hz). It is shown that the photoacoustic signal intensities show maxima at the sample thickness close to 10 μm for each modulation frequency. The x-ray photoacoustic signal varies from f−1.0 ( f denotes modulation frequency) up to 20 μm sample thickness, whereas it varies from f−0.5 beyond a 20 μm sample thickness, which is not consistent with the Rosencwaig–Gersho theory for the photoacoustic effect in solids.

Photoacoustic effect in solids: influence of heat loss

The effects of taking into account the heat loss on the production of the photoacoustic signal are presented. A comparison with the previous results is also made.

On the adequacy of one-dimensional treatments of the photoacoustic effect

It is demonstrated both theoretically and experimentally that the one-dimensional theories of the photoacoustic effect in solids are still applicable when the heat flow is three dimensional, provided that the thermal diffusion length in the gas is much less than the radius of the sample chamber.

Circular differential photoacoustic spectroscopy

A theoretical analysis of left/right circular-polarization modulated photoacoustic spectra is presented. The amplitude of the photoacoustic (PA) signal generated by exciting an absorbing sample alternately with left- and right-circularly polarized light is shown to be proportional to sample circular dichroism, Δα=αL−αR (where αL and αR are sample absorptivities for left and right circularly polarized light, respectively). Expressions appropriate for analyzing photoacoustically detected circular dichroism (referred to as PACD) are developed within the general framework of the Rosencwaig-Gersho model [J. Appl. Phys. 47, 64 (1976)] for the photoacoustic effect in solids and liquids. Extensive calculations based on these expressions are carried out for a number of parameter sets reflecting sample properties, sample cell/detector characteristics, and excitation properties. The results of these calculations indicate that PACD signal strengths for a wide variety of sample types should exceed detectability thresholds, and that PACD should be readily observable for many sample types whose circular dichroism cannot be measured using the conventional transmission techniques. Among this latter class of samples are optically opaque solids and highly scattering suspensions. The expressions for PACD signal strengths are shown to take on particularly simple forms for certain sets of sample properties and excitation modulation frequencies.

Photoacoustic detection of circular dichroism

An analysis is presented for an experimental technique involving the measurement of circular dichroism using polarization-modulated photoacoustic spectroscopy. The technique is referred to as photoacoustic circular dichroism (PACD). In the PACD experiment, a photoacoustic signal is induced by using polarization-modulated excitation light which is alternately left-and right-circularly polarized. Expression appropriate for analyzing the PACD experimental observables (signal strength and phase) in terms of sample circular dichroism are developed within the general framework of the Rosencwaig and Gersho model for the photoacoustic effect in solids and liquids. Calculations based on these expressions are reported and the applicability of PACD for measuring the circular dichronic properties of optically opaque samples is discussed.

Three-dimensional heat-flow effects in photoacoustic spectroscopy of solids

All theories of the photoacoustic effect in solids to date have been one dimensional, ignoring lateral heat transport to the cell walls. By studying the dependence of the photoacoustic signal on the position of focused light on the sample’s surface, we have demonstrated experimentally that a three-dimensional heat-flow analysis is required in some cases, especially at low chopping frequencies. These results have implications for techniques where one measures the photoacoustic signal as a function of chopping frequency, such as the depth profiling of layered samples.

Theoretical aspects of photoacoustic spectroscopy

The exact form of the Rosencwaig-Gersho theory for the photoacoustic effect in solids is used to depict how the magnitude and phase of the photoacoustic signal vary with chopping frequency and normalized length. Applications of the theory to studies of film-substrate systems are discussed.

Photoacoustic spectroscopy of solids

There are a great many substances, both organic and inorganic, that, because of their physical state, cannot be readily studied by conventional absorption or reflection techniques. In photoacoustic spectroscopy, light absorbed by the sample is converted into a measurable acoustic signal, and spectra closely corresponding to optical absorption spectra, can be obtained on any type of solid or semisolid material, whether it be crystalline, powder, smear, gel, etc. The physical processes underlying the photoacoustic effect in solids, and a mathematical analysis of the production of the photoacoustic signal will be presented. The methodology of photoacoustic spectroscopy will also be described. Various applications of this technique in the fields of physics, chemistry, biology, and medicine will be discussed. These applications will be treated under three main headings: bulk studies, surface studies, and de-excitation studies.