Laser ablated Ti velocity distribution dynamics

Abstract

Emissive plumes resulting from pulsed ablation of titanium (Ti) targets have been observed using a gated intensified charged coupled device (ICCD) camera to characterize the evolution of velocity distributions as the plume expands into vacuum, Ar, and He backgrounds. The gated imagery allows for the collection of time-of-flight spectra with highly sampled spatial resolution and the characterization of the velocity changing collisional dynamics. Multimodal, shifted Maxwell–Boltzmann distributions with flow speeds of u>0.1 cm/μs are adequate only for expansion into vacuum. Neutral Ti time-of-flight data clearly indicate three distinct distributions, with the fastest component consistent with the ionized Ti velocity distribution and maximum kinetic energies exceeding 20 eV. Expansion into He and Ar are clearly non-Maxwellian, with the highest velocity groups suffering collisions in the shock front. Leading-edge velocities of ∼1.6 cm/μs decrease more rapidly for Ar at rates of ∼30% per cm, consistent with momentum conservation. Expansion into He maintains the appearance of the vacuum distribution at low velocities but shows a decrease in the leading-edge velocity and an enhancement of the intensity of the highest-velocity groups at farther target distances. Determination of velocity distributions from time-of-flight data is complicated by translation-to-electronic excitation rates, intraplume collisional dynamics, and nonhydrodynamic conditions.