Probing femtosecond dynamics of laser-heated noble-gas cluster explosions with NIR-wavelength-scanning technique*

Clusters, having near solid density but nanometer scale and being in the transition matter state between molecules and solids, are important objects for studies of fundamental phenomena and applications. Despite a few decades of cluster research, many aspects of laser-cluster interactions remain ambiguous, especially at early stages after ionization. In order to shed light on these processes, we experimentally study femtosecond dynamics of expanding nano-plasmas from laser-ionized noble-gas clusters. Our experimental approach is based on detecting ion kinetic energy distribution in a pump-probe setup with a wavelength-scanning technique. When the frequency of collective motion of quasi-free electrons in the cluster coincides with the frequency of the probe pulse, plasma resonantly absorbs radiation, which results in an enhanced yield and energy of detected ions. A series of pump-probe scans captured at a set of probe wavelengths ranging near- and mid-infrared spectral regions (1–2.5 µm) equates to mapping the time resolved cluster density and the corresponding cluster size. The result is important for optimization of laser-matter/plasma interaction models and applications, e.g. optimization of generation of energetic charged particles and x-ray radiation.