Effect of coupling gas viscosity on the photoacoustic signal

The phase angle of a gas-coupled photoacoustic (PA) signal has been measured as a function of coupling gas column thickness lg for a]] one-dimensional" PA cell. The PA signal is excited by a modulated continuous laser beam incident on an opaque solid surface of silicon. We show that the observed data can be explained in a PA model including the consideration of a residual volume in the PA cell, together with the consideration of viscous damping of the PA signal at small lg.