The theory of the photoacoustic effect is extended to include the contribution of mechanical vibration of the sample. Coupled equations for thermal and acoustic waves are solved in both sample and gas. It is shown that the pressure signal in the gas may be significantly affected by acoustic coupling in the sample, and experimental confirmation of this extended theory is given. The results of the fully coupled treatment are shown to be accurately reproduced by an extension of the Rosencwaig piston model: the pistonlike motion of the gas boundary layer adjoining the sample is superimposed on the mechanical vibration of the sample surface to give a composite piston displacement which then produces the pressure signal in the gas. The composite-piston model provides relatively simple algebraic results applicable to many cases of physical interest.

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.

A new method for calculating signals in photothermal beam-deflection imaging is reviewed and applied to the case of vertical interfaces (cracks or other thermal barriers) in opaque solids. The generality of the approach and the effect of finite probe-beam size are emphasized.On prtsente une nouvelle methode pour le calcul des signaux en imagerie photo-thermique par dtflection de faisceau, qui est Cgalement appliqute au cas d'interfaces verticales (fissures ou autres barrikres thermiques) dans des solides opaques. Le gtntralite de l'approche et I'effet de la dimension finie du fasiceau-sonde sont particulikrement dtveloppts.

We propose and demonstrate that the photoacoustic effect can be used for absolute determination of the optical absorption coefficient. The photoacoustic signal is measured as a function of chopping frequency and compared to the theory of the photoacoustic effect. The essential agreement of theory and experiment over a restricted frequency makes possible the determination of the optical absorption coefficient (to within 10% in a test case). Observation of a characteristic leveling off of the photoacoustic signal at low frequencies for several materials is also reported.

Capillary oscillations on modulated liquid jets have been investigated using laser illumination and electronic detection of the magnified jet shadow. The amplitudes of several wave harmonics of a growing spatial instability were measured as a function of distance from the orifice for a range of jet velocities and initial-disturbance amplitudes. The experimentally determined growth rates at the fundamental frequency are compared with theories of capillary-wave propagation. An empirically derived explanation of the suppression of satellite formation is given. Experimental evidence for infinite-wavelength capillary oscillations is reported; a description of these oscillations in terms of the Rayleigh theory is presented.