Laser ablation plume thermalization dynamics in background gases: Combined imaging, optical absorption and emission spectroscopy, and ion probe measurements

Combined diagnostic measurements are employed to characterize the penetration of energetic ablation plumes through background gases during a key transitional regime in which the ion flux is observed (with fast ion probes) to split into distinct fast and slowed components. This apparently general phenomenon occurs over a limited range of distances at ambient pressures typically used for PLD (as reported for YBCO ablation into O-2) [1-5] and may be important to film growth by PLD because a 'fast' component of ions can arrive at the probe (or substrate) with little or no delay compared to propagation in vacuum (i.e., high 10-100 eV kinetic energies). At longer distances, this 'fast' component is completely attenuated, and only slowed distributions of ions are observed. interestingly, this 'fast' component is easily overlooked in imaging studies because the bright plume luminescence occurs in the slowed distribution. Time- and spatially-resolved optical absorption and emission spectroscopy are applied to experimentally determine the composition of the 'fast' and 'slow' propagating plume components for a single-component target ablation (yttrium) into an inert gas (argon) for correlation with quantitative imaging and ion probe measurements. The yttrium/argon system was chosen because optical absorption spectroscopy of both Y and Y+ was simultaneously possible [9] and the inert nature of argon. Experimental results for several other systems, including Si/Ar, Si/He, YBCO/O-2 are presented to illustrate variations in scattering mechanisms. Species-resolved imaging of YO* and Ba* is presented for the YBCO/O-2 system to illustrate the similarities and differences in the spatial regions of observed luminescence, These measurements confirm that, in addition to the bright significantly-slowed front which has been described by shock or drag propagation models [1], a fast-component of target material is transmitted to extended distances for some ambient pressures with near-initial velocities.