Viscous and Thermal Effects on Resonant Photo-Acoustic Spectroscopy

摘要

The Photo-acoustic effect occurs when a chopped beam of light shining on an absorbing gas produces sound waves. From the amplitude of these waves one can infer the absorption spectrum of a known sample, or detect trace gases in an unknown sample. While the signal strength is inversely proportional to the sample chamber volume, in some cases it is advantageous to design a large cell as an acoustic resonance chamber, and modulate the excitation beam at a natural frequency of the chamber. The resonant amplification factor Q(W) depends upon the system losses. The dominating loss mechanisms are the thermal and viscous boundary layer effects. By rescaling the local coordinates, one can include these losses, neglecting terms smaller than the boundary layer thickness, simply by modifying the boundary conditions for the non-dissipative wave equation. From there one can derive an equation for Q(W) applicable at all sample chamber resonances. This equation will be useful in evaluating the relative advantages of resonant and non-resonant photo-acoustic spectroscopy. An experiment conducted with a Krypton/Argon laser shining on a one atmosphere sample of dry air with .5% NO2 enclosed in a cylinder resulted in amplification factors less than those predicted. An analysis of the data, and the results of later experiments at lower pressures indicated that the unpredicted losses were probably due to imperfect cell construction. (Author)