Cavity-enhanced resonant photoacoustic spectroscopy with optical feedback cw diode lasers: A novel technique for ultratrace gas analysis and high-resolution spectroscopy J. Chem. Phys. 133, 044308 (2010) Electromechanical probing of ionic currents in energy storage materials Appl. Phys. Lett. 96, 222906 (2010) Broadening effects and ergodicity in deep level photothermal spectroscopy of defect states in semi-insulating GaAs: A combined temperature-, pulse-rate-, and time-domain study of defect state kinetics J. Appl. Phys. 105, 103712 (2009) Measurement of the redistribution of arsenic at nickel silicide/silicon interface by secondary ion mass spectrometry: artifact and optimized analysis conditions J. Appl. Phys. 104, 024313 (2008) Standoff photoacoustic spectroscopy Appl. Phys. Lett. 92, 234102 (2008) Additional information on Rev. Sci. Instrum.The application of different types of acoustic resonators such as pipes, cylinders, and spheres in photoacoustics is considered. This includes a discussion of the fundamental properties of these resonant cavities. Modulated and pulsed laser excitation of acoustic modes is discussed. The theoretical and practical aspects of high-Q and low-Q resonators and their integration into complete photoacoustic detection systems for trace gas monitoring and metrology are covered in detail. The characteristics of the available laser sources and the performance of the photoacoustic resonators, such as signal amplification, are discussed. Setup properties and noise features are considered in detail. This review is intended to give newcomers the information needed to design and construct state-of-the-art photoacoustic detectors for specific purposes such as trace gas analysis, spectroscopy, and metrology.

Sensitive photoacoustic detection of ammonia with near-infrared diode lasers (1.53 microns) and a novel differential acoustic resonator is described; a sensitivity of 0.2 parts per million volume (signal-to-noise ratio = 1) is attained. To eliminate adsorption-desorption processes of the polar NH3 molecules, a relatively high gas flow of 300 SCCM was used for the ammonia-nitrogen mixture. The results are compared with recent ammonia measurements with a NIR diode and absorption spectroscopy used for detection and photoacoustic experiments performed with an infrared quantum-cascade laser. The performance of the much simpler and more compact setup introduced here was comparable with these previous state-of-the-art measurements.

The pulsed excitation of acoustic resonances was studied by means of a high-Q photoacoustic resonator with different types of microphone. The signal strength of the first radial mode was calculated by the basic theory as well as by a modeling program, which takes into account the acoustic impedances of the resonator, the acoustic filter system, and the influence of the microphone coupling on the photoacoustic cavity. When the calculated signal strength is used, the high-Q system can be calibrated for trace-gas analysis without a certified gas mixture. The theoretical results were compared with measurements and show good agreement for different microphone configurations. From the measured pressure signal (in pascals per joule), the absorption coefficient of ethylene was calculated; it agreed within 10% with literature values. In addition, a Helmholtz configuration with a highly sensitive 1-in. (2.54-cm) microphone was realized. Although the Q factor was reduced, the sensitivity could be increased by the Helmholtz resonator in the case of pulsed experiments. A maximum sensitivity of the coupled system of 341 mV/Pa was achieved.

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An inexpensive resonant optoacoustic monitoring system using near-infrared laser diodes was developed. It was demonstrated that wavelength modulation at the resonance frequency of the cell provides a superior signal-to-noise ratio compared with amplitude modulation and eliminates background drifts and fluctuations. The system was tested out on ammonia. Its sensitivity is 8 parts in 10(9) (S/N = 1) at atmospheric pressure, which corresponds to a minimum detectable absorption coefficient of approximately 3.5 × 10(-11) cm(-1) W(-1). The pressure dependence of the optoacoustic resonance was also investigated. The monitor can be used as a continuous flow-through system up to a flow rate of approximately 3.5 L/min.

A distributed-feedback (DFB) diode laser radiating at 1.53 mum was used for photoacoustic detection of ammonia molecules in the gas phase under flow conditions. The influence of the adsorption-desorption processes that occur at the cell and tube walls on the measured gas concentration was studied. Dramatic differences in the adsorption behavior of a metal and a polypropylene cell are demonstrated. Simulations of the gas flow and adsorption-desorption processes yield the conditions that must be fulfilled for accurate concentration measurements in trace-gas analysis of polar molecules.

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