Abstract

If a spectrophone containing an infrared active gas is placed near a cold surface and the path between the two is interrupted, the periodic flow of radiation from the spectrophone causes a sound wave to be produced. Experiments were done with a 3.4- × 1.7-cm diam acoustic cell that was fitted with thallium bromoiodide windows. The cell (maintained at room temperature) viewed a liquid nitrogen Dewar whose interior was coated with soot from an acetylene flame. At a pressure of 1 atm signals were found (in increasing order) for the following gases: CO, CH₄, SF₆, CO₂, C₃H₆, C₂H₄. The phases of the acoustic signals were π/2 relative to the modulation (but π relative to the normal optoacoustic signal). Further experiments were carried out to determine the magnitude of the acoustic signal as a function of gas concentration. As the concentration of SF₆ was lowered from a mole fraction χ of 1 the signal amplitude increased, peaking at χ ≅ 0.1. The signal was linear with mole fraction for χ < 10^-4. The amplitude response is not simple since, depending on wavelength, the gas is both optically thick and thin at the same time. For the optically thick part of the gas, the acoustic signal is generated in a small region near the window of the spectrophone cell.

© 1985 Optical Society of America