Fig. 1. Sketch of the experiment setup.

Fig. 2. Optical emission images in ambient Ar with pressure of 20 Pa. The logarithmic false color intensity scale is shown at the right of the image.

Fig. 3. (a) Spatial distributions of emission intensity at different time delays via vertical cuts along the z axis within −0.05 ≤ r ≤ 0.05 mm through such images as Fig. 2; (b) temporal evolution of the shock front position z derived from the experimental data (black spheres) along with the line of best-fit, assuming z ∝ t^b.

Fig. 4. Emission spectra measured at the pressure of 10⁻⁵ Pa (top row), 2 Pa (middle row), and 20 Pa (bottom row). These images show the evolving spatial distribution at Δt = 40 ns (left column) and 70 ns (right column).

Fig. 5. Emission spectra at Δt = 70 ns obtained by taking horizontal cuts through the 10⁻⁵ Pa image in Fig. 4 at z = 0.8 mm (top left panel) and 3 mm (top right panel) and the 20 Pa image at z = 0.8 mm (bottom left panel) and 3 mm (bottom right panel).

Fig. 6. (a) Spatial distribution of Si II 488.320 nm (red), Si III 455.262 nm (blue), and Si IV 465.432 nm (magenta) emission lines measured at Δt = 40 ns with pressure of 2 Pa; (b) z-dependent intensities of Si IV 465.432 nm (2p⁶6h¹–2p⁶5g¹ transition) at the pressure of 0.5, 10, 20, 50, and 100 Pa when Δt = 40 ns; (c) the evolution of the front position with pressure p at Δt = 40 and 70 ns; (d) the evolution of the average velocity with pressure p at Δt = 40 and 70 ns.

Fig. 7. Representative energy levels of Si and Ar considered in this paper.

Fig. 8. Spatial distribution of N_e and T_e derived from the spectra of plasma produced by laser ablation at a pressure of 20 Pa when Δt = 40 ns (black) and 70 ns (red).