Photoacoustic effect

The photoacoustic effect, which transforms electromagnetic radiation into acoustic waves, is also used as an IR detection technique in both qualitative and quantitative analyses of various materials. Alexander Graham BeU (1847-1922) discovered the effect in 1880. He observed that when a beam of sunlight was interrupted rapidly, sound was produced from the diaphragm he used in his experiment. 

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The photoacoustic effect was first discovered by Alexander Graham Bell in the early 1880s [18], but it was not applied to Fourier transform infrared (FTIR) spectroscopy until a century later [19,20], Significant advantages of FTIR photoacoustic spectroscopy (PAS) include the following (1) Spectra may be... 

The basic theory of the photoacoustic effect was described by Tam and Patel [279,280] and some of its applications were presented in a review by Braslavsky and Heibel [281], The first use of PAC to determine enthalpies of chemical reactions was reported by the groups of Peters and Braslavsky [282,283], The same groups have also played an important role in developing the methodologies to extract those thermodynamic data from the experimentally measured quantities [282-284], In the ensuing discussion, we closely follow a publication where the use of the photoacoustic calorimety technique as a thermochemical tool was examined [285],... 

Figure 13.2 The photon is the maestro of the photoacoustic effect. It produces a local heating and therefore a local thermal expansion. The propagation of this expansion through the medium is a sound wave.

An approach widely used by atmospheric scientists is to infer the imaginary part of the refractive index k from measurements of the absorption coefficient a of particulate samples. Diffuse reflection, the photoacoustic effect, and integrating plates have been used for determining absorption even in the presence of considerable scattering these methods are discussed briefly in the following section. The relation (2.52) between a and k, a - 4nk/, is, of course, strictly valid only for homogeneous media. But under some circum-... 

The Photoacoustic Effect ( 7). The modulated absorption of light by material in a cell leads to the production of a sound wave at the modulation frequency. The sound wave is due to modulated pressure pulses in the cell arising from the liberation, as heat, of a portion of the absorbed light. The sound wave thus produced can be detected with a sensitive microphone and associated electronics, i.e., a spectrophone. 

Photoacoustic effect Generation of heat after absorption of radiation, due to radiationless deactivation or chemical reaction. 

Photoacoustic spectroscopy A spectroscopic technique based on the photoacoustic effect. A photoacoustic spectrum consists of a plot of the intensity of the acoustic signal detected by a microphone or a piezoelectric detector, against the excita-... 

This combination of near-held imaging with the photoacoustic effect is very promising if the time delay between the laser pulse and signal detection is... 

The recent revival of interest in the photoacoustic effect in condensed media (6,7) led us to consider the possibility of detecting natural circular dichroism photoacoustically. 

Certain aspects of the photoacoustic effect suggest that this technique might be generally applicable to all chiral solids regardless of crystal class, size or perfection, or strength of absorption. Although subsequent theoretical developments and experimental results have caused us to limit considerably the predicted scope of this method, nevertheless, it is possible now to say clearly that the experiment does work and offers prospects for unique results. In this paper we review briefly the nature of the theory and practice of condensed phase photoacoustic spectroscopy and its extension to the measurement of natural circular dichroism, and present initial results for single crystals and powders. 

The above statements of the nature of the photoacoustic effect are drawn primarily from the conclusions of the theory developed by Rosencwaig and Gersho (R-G) (9), based on a onedimensional thermal piston model. Since this is the starting point for extending the theory of photoacoustic spectroscopy (PAS) to include photoacoustic circular dichroism (PACD), a brief survey of the salient factors and important parameters of that treatment follows. 

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