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 electr on number densities within a low-temperature laser-ablated titanium plasma expanding in vacuum. Measurements were made following ablation by a KrF exc imer laser (248 nm, tau = 30 ns) and a frequency-doubled Nd:YAG laser (532 nm, tau = 7.5 ns) Tor laser power densities between 85 MWcm(-2) and 1130MWc m(-2) on target. Electron number density data were obtained from the satura tion electron current region of the probe (I/V) characteristic. Peak electr on number densities in the range 1.5 x 10(10) cm(-3) to 1.5 x 10(13) cm-3 w ere measured, at a distance of 5 cm along the target normal, for the laser power range investigated. Above ablation threshold the temporally integrate d electron flux increased linearly with incident power density for both abl ation wavelengths. The ablation thresholds, in terms of peak power density within the laser spot on the target, were found to be 85 +/- 20 MW cm(-2) f or KrF ablation and 300 +/- 50 MW cm(-2) for 2 omega YAG ablation.