Computational study of the angular distribution of species in the expansion of laser-produced metallic plasmas*

The forward peaking of nanosecond laser ablation plasma plumes is a hydrodynamic phenomenon that depends strongly on the photo-physics of the laser-plasma interaction. In pulsed laser deposition, heterogeneities in the angular distribution of plasma species may affect the properties of the resulting films. In this work, we use a laser ablation/fluid plasma expansion model to simulate the angular distribution of species in laser plasmas. We apply the model to plasmas produced by the ablation of metallic targets. The simulation is carried out in an adaptive Cartesian mesh framework over centimeter distances for proper evaluation of the angular distributions. The compressible solvers available in this framework were adapted to include the equations of state of the plasma, which link local Saha equilibrium with augmented ideal gas expressions that include the internal degrees of freedom of the atomic species. For the ablation of an FeSe target with 248-nm, 25-ns pulses over a spot area of 30 mm², the model predicts an angular distribution for Fe⁺ ions with FWHM of ~30° at a fluence of 1.4 J/cm², which decreases to ~12° for 2.6 J/cm², in qualitative agreement with typical angular distributions observed in film growth. We will discuss differences in elemental vs. compound metallic plumes and subtle variations in predicted angular distributions of different chemical species in compound plumes, which are critical for reproducible film synthesis.