Characterization of cluster/monomer ratio in pulsed supersonic gas jets

We determine cluster mass fraction f _c(r,t) at position r within, and time t after firing, a pulsed supersonic gas jet by measuring femtosecond evolution of the jet's refractive index by single-shot frequency domain holography. A fs pump pulse singly ionizes monomers, while quasi-statically ionizing and heating clusters to a level at which recombination remains negligible as clusters expand. Under these conditions, index evolves in two simple steps corresponding to monomer and cluster contributions, allowing recovery of f _c without detailed cluster dynamic modeling. Variations of f _c with t are measured.