Collision Mechanisms of Particles in the Al–Ti Plasma Plume Induced by Pulsed Laser Ablation

The dynamics of pulsed laser ablation plumes strongly influence thin-film deposition quality; however, pressure-dependent collision accumulation and component-resolved transport in binary metal plumes remain poorly understood. In this study, a kinetic-statistical model was employed to investigate the propagation of an Al0.75Ti0.25 plume in a low-pressure inert Ar background at a laser fluence of 8 J/cm2. The results show that, at t = 0.56 μs, the cumulative number of particles that have experienced at least one collision increases with pressure in the range of 0.001–1 Pa and follows an approximately power-law dependence. Across the entire pressure range and throughout the 0.08–0.56 μs interval, the collision fraction of Ti remains consistently higher than that of Al. Based on a Ti-normalized cumulative collision index, the propagation regime can be classified into a near-free-flight region, a transition region, and a collision-influenced region, with only minor temporal variations in the corresponding boundary pressures. Further analysis of the initial velocity spectrum shows that Ti contributes more strongly to the high-velocity tail, which explains its greater propensity for collision during propagation. These findings provide a quantitative framework for understanding pressure-dependent collision accumulation and species transport in binary metal plumes under inert low-pressure conditions.