Laser photoacoustic spectroscopy: principles, instrumentation, and characterization

Laser photoacoustic spectroscopy has emerged over the last decade as a very powerful investigation technique, capable of measuring trace gas concentrations at sub-ppbV level. Recent achievements in this field have made it possible to fully characterize the method and improve the design of instrument components in view of the task they are expected to fulfill. The basic principles of laser photoacoustic spectroscopy are reviewed and compared with those of other spectroscopic techniques. Photoacoustic signal generation is discussed, covering different aspects regarding resonant cells, that are resonance frequencies, dissipation processes, quality factor, pressure amplitude, cell constant, optimal PA cell geometry, voltage signal, and saturation effects. Noises and other limiting factors which determine the ultimate detection sensitivity are presented, and various types of noises are measured. The interfering gases play an important role both in limiting the sensitivity of the method and in the multicomponent analysis of the atmosphere. We therefore discuss the experimental measures for reducing the influence of interfering gases in a single component measurement and the methods used in multicomponent analysis. Based on a general schematic, the main components are described in detail. Special emphasis is laid on the home-built, frequency-stabilized, line-tunable CO2-laser source and the resonant photoacoustic cell. All of the parameters that are characteristic of the photoacoustic cell, including the limiting sensitivity of the system, are measured and compared with the best results reported by other authors. Approaches to improving current sensor performance are also discussed. Other aspects of a functional photoacoustic instrument, such as the gas handling system and data acquisition and processing, are outlined. As a typical application, we present a precise measurement of the absorption coefficients of ethylene at CO2 laser wavelengths. While the values obtained for the 10-µm band excellently agree with other measurements reported in the literature, important differences were found for the absorption coefficients in the 9-µm band. Other applications are briefly reviewed in the last section.