Gas velocity measurements using photothermal deflection spectroscopy

Abstract

This paper demonstrates two different ways in which photothermal deflection spectroscopy (PDS) can be used to measure a single spatial component of gas velocity. The first method is a transit-time velocimetry where the time delay between a rapid pump laser pulse and the resulting deflection of the probe laser beam is measured. The velocity is determined directly from the known displacement of the beams and the measured time delay. The second method uses the shape of the PDS signal to obtain velocities: specifically, the shape of the experimental PDS signature is fit to velocity-dependent PDS signatures computed by numerical solution of the energy equation. The fit of the experimental to the computed profiles is good. The above methods employed signal averaging over a number of pump laser pulses in order to improve signal-to-noise ratio. However, velocities can also be measured using a single pump laser pulse in order to obtain 1-msec temporal resolution. One can also obtain simultaneous velocities and relative absorber gas concentrations by using the second method plus measurements of the amplitude of the PDS signal. This is demonstrated using a room temperature laminar flow of nitrogen-diluted ethylene flowing from a jet into room air.