Showing papers on "Photoacoustic spectroscopy published in 1974"

Limitation to optoacoustic detection of atmospheric gases by water vapor absorption.

Abstract: Optoacoustic spectroscopy provides a highly sensitive method for the detection of molecules in the ambient air. Kreuzer and Patel have employed this technique to mea­ sure nitric oxide concentrations of 10 parts/billion (ppb). In later work Kreuzer et al. detected 5 ppb of ethylene in air. Recently, Dewey et al. have demonstrated that acoustic gain can increase the detection sensitivity. If the sensitivity improves to the point that 1 ppb or less can be monitored, this method will become a prime candidate for the detection of trace gases of the atmosphere, drugs, and explosives. However, absorption of the laser excita­ tion radiation by other constituents of the ambient air may limit the minimum molecular concentration that can be monitored. Indeed, water vapor has been identified as an interfering species. In this report, the sensitivity for optoacoustic detection of light and heavy gases in the presence of strong and weak water vapor bands has been calculated. The existence of strong molecular absorption lines cou­ pled with the availability of numerous laser sources render the 5-10-μm spectral range particularly suitable for op­ toacoustic detection. In this range water vapor displays more than 4000 absorption lines. The average line width at atmospheric pressure is 3 GHz with little variation from line to line. Very little spectral space is left unoccu­ pied by the pressure-broadened water vapor absorption lines. The strength of these lines extends from 10 – 4 cm – 1 / g / cm 2 to 10 cm – 1 / g / cm 2 . A strong band is char­ acterized by a line strength of 10 cm – 1 /g /cm 2 ; average and weak bands exhibit line strengths of 10–1 c m – 1 / g / cm and 10 – 3 cm – 1 /g /cm 2 , respectively. At less than 15-Torr total pressure the water vapor lines are predomi­ nantly Doppler broadened with full widths at half-maxi­ mum intensity of ~0.15 GHz. Therefore, at low pressure, interference from water vapor is reduced at excitation wavelengths that do not coincide with the centers of water vapor lines.