A comparison of the electron component within laser-ablated titanium plumes formed by UV and visible lasers

A Langmuir probe was used as a diagnostic of the temporally evolving electron number densities within a low-temperature laser-ablated titanium plasma expanding in vacuum. Measurements were made following ablation by a KrF excimer laser (248 nm, F=30 ns) and a frequency-doubled Nd:YAG laser (532 nm, F=7.5 ns) for laser power densities between 85 MW cm^-2 and 1130 MW cm^-2 on target. Electron number density data were obtained from the saturation electron current region of the probe (I/V) characteristic. Peak electron number densities in the range 1.5᎒¹⁰ cm^-3 to 1.5᎒¹³ cm^-3 were measured, at a distance of 5 cm along the target normal, for the laser power range investigated. Above ablation threshold the temporally integrated electron flux increased linearly with incident power density for both ablation wavelengths. The ablation thresholds, in terms of peak power density within the laser spot on the target, were found to be 85ᆨ MW cm^-2 for KrF ablation and 300ᇆ MW cm^-2for 2P YAG ablation.