High-resolution spectroscopic imaging of atoms and nanoparticles in thin film vaporization

Abstract

We introduce a spectroscopic absorption imaging method in the UV regime (225–400 nm) to study tin vapor created by irradiating a thin film with a low intensity 108 W cm−2 nanosecond laser pulse, a case inspired by current developments around “advanced target shaping” in industrial laser-produced plasma sources for extreme ultraviolet light. The 4-ns-time-resolved, 10-μm-spatial-resolution images contain a 10-cm−1-resolution spectrum of the vapor in each pixel 100 ns after the vaporization. The images allow us to reveal a homogeneous temperature profile throughout the vapor of around 3000 K. We obtain a density map of the atoms (with a peak density of 5 × 1018 cm−3) and nanoparticles (1012 cm−3 for the best fitting 20 nm radius case), which both are shown to be present in the vapor. For each free atom, approximately three appear to be clustered in nanoparticles, and this composition is invariant over space and density. The density and temperature maps of the free atoms are combined to estimate the electron density (peaking at 1013 cm−3) in the vapor.