The sensitivity and dynamic range of the photoacoustic effect enables an investigation of the gas phase kinetics and of the adsorption kinetics onto heterogeneous surfaces for the chemical system formed by anhydrous nitric acid vapor and ammonia. By studying the laser absorption spectroscopy of these gases and corresponding photoacoustic responses, the photoacoustic signals can be interpreted in terms of absolute concentrations. By illuminating the cell at different CO laser frequencies that are uniquely absorbed by one component and by slowing the reaction by using low concentrations of the reactants, a gas phase rate of 2X10-18cm3/sec is found. A total pressure of 50 Torr using nitrogen as a buffer gas allowed sufficient signal without excessive pressure broadening of the component spectral lines. Wall effects were included by studying the adsorption of ammonia and nitric acid onto cell surfaces. Present capabilities will allow measurement of bimolecular reaction rates approximately four orders of magnitude greater than the one already determined.
L. A. Farrow,
R. E. Richton,
"Photoacoustic Applications To Chemical Kinetics", Proc. SPIE 0286, Laser Spectroscopy for Sensitive Detection, (8 September 1981); doi: 10.1117/12.965810; https://doi.org/10.1117/12.965810