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.