Quantitative Photothermal Deflection Measurement Of Thermal Diffusivity

Abstract

Recently it was pointed out that photothermal deflection spectroscopy (PDS) is an important tool capable of measuring either very small absorption coefficients or low sample concentrations 1-5. In this method, two laser beams are focused in an interaction region of the sample. One laser, a pump laser which has relatively high power is pulsed and can be tuned to an absorption of the gaseous sample. The advantages of using pulsed laser are that much higher power is available and also measurements related to a transient effect can be made. The other laser, a probe laser, has a lower power, low divergence and high stability. When the pump laser is turned on, the temperature of the gas begins to increase, initially having the same spatial profile as the pump laser, resulting in a temperature gradient. This temperature gradient causes a gradient in the index of refraction due to the density gradient. The index gradient bends the probe laser beam and the deflection is measured by a position sensitive detector. © 1986 SPIE.