Noncontact photothermal probe beam deflection measurement of thermal diffusivity in an unconfined hot gas

A pulsed CO2 laser beam is used to produce a transient thermal refractive index gradient in a nitrogen gas doped with trace amounts of absorbing Freon 12 at temperatures from 25 to 425°C. The diffusion of this gradient is probed by a continuous helium-neon laser beam parallel to but displaced from the pulsed beam. The observed defletion signal agrees well with theory, and thermal diffusivity or gas temperature can be derived from the signal.