This paper reviews the theory and applications of photoacoustic (also called optoacoustic) methods belonging to the more general area of photothermal measurement techniques. The theory covers excitation of gaseous or condensed samples with modulated continuous light beams or pulsed light beams. The applications of photoacoustic methods include spectroscopy, monitoring deexcitation processes, probing physical properties of materials, and generating mechanical motions. Several other related photothermal methods, as well as particle-acoustics and wave-acoustics methods are also described. This review complements an earlier and narrower review [Rev. Mod. Phys. 53, 517 (1981)] that is mainly concerned with sensitive detection by pulsed photoacoustic spectroscopy in condensed matter.

Applications of photoacoustic sensing techniques

Photoacoustic and related photothermal techniques

1 – photo acoustics: spectroscopy and other applications

A new photoacoustic cell for use with Fourier transform infrared spectrometers is detailed. Photoacoustic infrared spectra showing high sensitivity on a variety of chemical systems are presented. Comparison of the photoacoustic infrared spectra with transmission and attenuated total internal reflectance spectra show that saturation effects can play a prominent role in the photoacoustic spectra. Spectral subtractions and study of the situ chemical reactions are possible using the photoacoustic technique. The photoacoustic and diffuse reflectance techniques yield very similar spectra.

Some Applications of Fourier Transform Infrared Photoacoustic Spectroscopy

We show that the photoacoustic signals generated at the high reflection mirror of a pulsed laser depend linearly on the output energy. This allows us to measure acoustically the output energy, the tuning range, and the optimum temperature and concentration of a dye laser, with the advantage that there is no interference of the output beam, as is the case with optical measurements.

Photoacoustic measurement of intracavity energy in pulsed lasers

The construction and operation of a Michelson interferometer that permits Fourier transform photoacoustic spectroscopy of opaque and partially transparent samples at visible wavelengths is described. Multiplexing and throughput advantages are considered. A visible spectrum of Nd(III) doped laser glass is reproduced and potential kinetic applications are described.

Fourier Transform Photoacoustic Spectroscopy

The macrocycles 18-crown-6 and 15-crown-5 in aqueous solution undergo at least one rapid conformational change detectable by ultrasound. A correlation between ring size and rigidity of such macrocycles and the rapidity of the conformational change is suggested. A linear inverse correlation exists between the rate constant for formation of crown ether-alkali and alkaline earth metal cation complexes and the charge density of the cation. 2, 6-dimethylylbenzoic acid-18-crown-5 does not significantly accelerate cleavage of glycine p-nitrophenyl ester in aqueous solution whereas crown ethers with functionalized side arms do catalyze this type of reaction. Ultrasonic absorption kinetic data are reported for aqueous amino acid-crown ether complexation reactions that indicate the complexation step is unlikely to be rate limiting in any such crown ether catalyzed process in waterlike solvents.

Crown Ether Reaction Kinetics

A fully automated laser Debye-Sears ultrasonic absorption apparatus is briefly described and its relationship to Brillouin scattering is mentioned. The utilization of mini-computerized optical techniques at lower ultrasonic frequencies than the present lower limit of ~15 MHz is also considered.

Edward M. Eyring

Papers

Fourier Transform Photoacoustic Spectroscopy

Crown Ether Reaction Kinetics

Optical Ultrasonic Techniques