Simultaneous measurement of absorption coefficient, thermal diffusivity, and flow velocity in a gas jet with pulsed photothermal deflection spectroscopy

Simultaneous measurement of the absolute optical absorption coefficient or species concentration, thermal diffusivity, and flow velocity in a gaseous medium has been demonstrated by using pulsed photothermal deflection spectr oscopy (PTDS) and a multiparameter fitting procedure. The spatial distribut ion of the three parameters in a laminar flow has been simultaneously deter mined and the influence of velocity gradient on the measurement has been di scussed. Saturation behavior of the PTDS signals was observed and found to influence significantly the fitted results. The saturation influences most significantly the absorption coefficient determination, somewhat less signi ficantly the thermal diffusivity determination, and only negligibly the flo w velocity determination. For an absolute and accurate measurement of the a bsorption coefficient or species concentration, the saturation-induced erro r has to be corrected by using a model taking into account the effects of t he saturation. The influence of saturation on the thermal diffusivity deter mination can be significantly accounted for by using a simple model in whic h the pump beam radius is set as a free parameter in the multiparameter fit ting procedure. Both multipulse and single-pulse measurements are performed . For multipulse measurement, the estimated measurement uncertainties are /-2% for the flow velocity, and +/-5% for the thermal diffusivity and absor ption coefficient, while for single-pulse measurement, the estimated uncert ainties are +/-2% and +/- 10%, respectively. (C) 2001 American Institute of Physics.