Dynamics of laser ablation plume penetration through low pressure background gases

The dynamics of laser-ablated yttrium plume propagation through background argon have been investigated with fast time- and spatially-resolved plasma diagnostics in order to characterize a general phenomenon believed to be important to film growth by pulsed laser deposition (PLD). During expansion into low-pressure background gases, the ion flux in the laser ablation plasma plume is observed to split into fast and slow components over a limited range of distances including those typically utilized for PLD. Optical absorption and emission spectroscopy are employed to simultaneously identify populations of both excited and ground states of Y and Y⁺. These are correlated with intensified-CCD (ICCD) photographs of visible plume luminescence and ion fluxes recorded with fast ion probes. These measurements indicate that plume-splitting in background gases is consistent with scattering of target constituents by ambient gas atoms. The momentum transfer from these collisions produces a transition from the initial, "vacuum" velocity distribution into a velocity distribution which is significantly slowed in accordance with shock or drag propagation models.