No abstract

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.

No abstract

A laser pulse, when focused on a metal sample, produces characteristic elastic waveforms, which depend on whether thermoelastic or ablative/evaporative mechanisms dominate the generation process. In the thermoelastic regime, with an unconstrained surface, the predominant axial displacement is opposite to the direction of propagation (negative), but there is typically a small transient positive displacement. This precursor is not predicted by elastic point source models, but is predicted by models including thermal diffusion. A recent formulation of pulsed photoacoustic generation is used to show how the precursor arises from interaction of the thermal and elastic modes at the illuminated surface.

The problem of photoacoustic pulse generation is treated using generalized thermoelastic equations, specifically incorporating the hyperbolic heat conduction equation to avoid an infinite thermal propagation velocity. The assumption of equality of longitudinal and thermal velocities leads to a simplification of the solution in certain limiting cases, enabling insights into the character of the solution, without appreciably affecting the numerical results. The effect(s) of approximations made by previous authors may also be assessed. Numerical Hankel–Laplace transform inversion is shown to be practical for the general case, allowing such calculations to be duplicated by others.