Influence of gas viscosity on the photoacoustic signal in gas-microphone cells

We report the experimentally observed variation of the phase angle cursive-phi sign of the photoacoustic (PA) signal as a function of coupling "gas column thickness" for a one-dimensional PA cell. The PA signal is excited by a modulated continuous laser beam incident on an opaque solid surface, and the gas column thickness is the distance between the solid surface and the window for the incident laser beam. We show that the observed data can be explained in a PA model (called the extended Rosencwaig-Gersho model) which includes the consideration of a residual volume in the PA cell, together with the consideration of viscous damping of the PA signal at small gas column thickness. The effect of viscosity is visible already for gas column thickness of the order of magnitude of one thermal diffusion length.