Thermal wave decay coefficients

The infrared absorption coefficient and thermal wave decay coefficients, a(v) and flj, respectively, determine the magnitude of the photoacoustic signal. The term ot( exp —[a( +fls]x in the expression for temperature oscillation leads to a linear PA signal dependence on infrared absorption when a( thermal wave decay length, L, although it is sometimes referred to as the sampling depth, penetration depth, or thermal diflusion depth. The sample layer extending a distance L beneath the surface contributes... 

The quantities a, c, f, F, r, and p are the thermal diffusivity, sound speed, heat capacity ratio, bulk viscosity coefficient, shear viscosity coefficient, and density of the sample, respectively and Eo, a, P and Cp are the energy fluence of the laser beam, the optical absorption coefficient, the volume expansion coefficient, and the isobaric heat capacity, respectively, of the fluid. Tlie first and second terms in Eq. 2 describe the time dependences of the thermal and acoustic modes of wave motion, respectively. Since the decays of the acoustic and thermal mode densities back to their ambient values take place on such different time scales (microsecond time scale for acoustic mode and millisecond time scale for thermal mode), they were recorded on the oscilloscope using different time bases. 

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