Measuring concentrations with laser-induced thermalization and electrostriction gratings

Laser-induced thermal acoustics is used to measure non-intrusively the concentration of a resonantly excited seed species diluted within a non-resonantly excited species by comparing the contributions from thermalization and electrostriction in the combined signal. The speed of sound is measured simultaneously. It is shown theoretically and experimentally (for iodine vapor in nitrogen) that the ratio of the grating magnitudes of the thermal and of electrostrictive gratings is proportional to the concentration of the resonantly excited species. The proportionality constant only depends on the two species. The uncertainty is 5% when thermal and electrostrictive contributions are comparable in magnitude. The speed of sound is measured simultaneously with an uncertainty of 0.35%.