High harmonics generation and characterization of clusters from noble gas jets

Noble gas clusters from pulsed gas jets might offer efficient sources of high-harmonic generation, although the ionization might limit their application. Also the reproducibility of cluster experiments is often poor. Clusters are generated by the adiabatic cooling of the generally supersonically streaming gas from the gas jets. Most works estimate the sizes of clusters with simple scaling laws. In order to control the experiments we carried out characterization using Rayleigh-scattering, thus determining the sizes of clusters for different nozzles in different gases as a function of backing pressure. The spatial distribution measurements show that supersonic de Laval nozzles can provide flat-top density profiles close to the exit of the nozzles. The concluded cluster sizes show significant deviations as compared with the Hagena or other scaling laws, especially for Xe. Properties of high harmonics from clusters were in agreement with previous results as they show strong intensity dependence and then saturation due to propagation in the already ionized gas. Propagation in the partially ionized gas results in a blue shift of the harmonics. We could however find an intensity range when the presence of free electrons is still not significant, but inside the clusters nanoplasmas are formed. Here spectral shift of opposite direction, namely a red shift was observed. Measuring the red shift proved to be an additional tool to determine the cluster sizes and densities in agreement with the Rayleigh scattering results