A more complete interpretation of optoacoustic data taken with fixed-frequency lasers

The response of the cylindrical optoacoustic cell when filled with an absorbing gas and irradiated by a laser beam passing along its axis has been studied theoretically and experimentally as a function of the gas pressure and composition. The pressure on the cell microphone has been calculated by solving numerically, for the experimental parameters of interest, the general heat-diffusion equation with a forcing function which is shown to depend upon both space and time for the realistic case of 50% modulation of the laser beam. It is further shown that if the experimental optoacoustic signal at a given pressure is divided by the signal calculated for that pressure, the ratio is expected to be a constant. The results of a series of experiments using a CO laser to irradiate mixtures of nitric oxide (the absorbing gas) and nitrogen are presented and are shown to confirm the theoretical predictions.