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 ∼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 ∼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 P₀ and reservoir temperature T ₀.