Spatio-Temporal Profiling Of Cluster Mass Fraction In A Pulsed Supersonic Gas Jet By Frequency-Domain Holography

We present an in-depth study of a rapid, noninvasive, single-shot optical method of determining cluster mass fraction f(c)(r, t) at specified positions r within, and at time t after opening the valve of, a pulsed high-pressure pulsed supersonic gas jet. A similar to 2 mJ, 40 fs pump pulse ionizes the monomers, causing an immediate drop in the jet's refractive index n(jet) proportional to monomer density, while simultaneously initiating hydrodynamic expansion of the clusters. The latter leads to a second drop in n(jet) that is proportional to cluster density and is delayed by similar to 1 ps. A temporally stretched probe pulse measures the 2-step index evolution in a single shot by frequency-domain holography, enabling recovery of f(c). We present a model for recovering f(c) from fs-time-resolved phase shifts. We also present extensive measurements of spatio-temporal profiles f(c)(r, t) of cluster mass fraction in a high-pressure supersonic argon jet for various values of backing pressure P-0 and reservoir temperature T-0. (C) 2013 AIP Publishing LLC.