Photoacoustic Spectroscopy in a Supersonic Flow

Photoacoustic spectroscopy (PAS) is a sensitive broadband absorption technique relying on simple and inexpensive components. Despite this, PAS has never been applied in a cold supersonic environment. The reason for this is clear: how can one detect an acoustic signal under supersonic conditions? The flow provided by a Laval nozzle expansion offers a solution: such flows are thermal collisional environments propagating much faster than the local speed of sound, but inside which one may produce a photoacoustic signal by suitably chopped laser excitation of molecules in the flow. Downstream, a microphone can respond to the pressure oscillations induced by the laser excitation as the flow sweeps by after the absorbed radiation has been converted to translational energy of the molecules. Such a strategy offers a near-universal approach to spectroscopy and low-temperature kinetics in uniform flows. Here we show preliminary results demonstrating the feasibility of this technique and discuss future prospects.