Density mapping of species in low temperature laser-produced plasmas

Spectroscopy and interferometry have been used to quantitatively map the number densities in low temperature laser-produced plasmas. Optical absorption and laser induced fluorescence spectroscopy were used to determine neutral and ion ground state number densities following the 248 nm irradiation of a magnesium target in vacuum. The Doppler broadening of the absorption lineshapes allowed the three dimensional mapping of the plume and the nature of the expansion to be investigated. Single wavelength optical interferometry was used to quantify spatially and temporally resolved electron number densities in the early stages of expansion, i.e. < 100 ns, for laser-produced magnesium, copper and titanium plasma plumes. The electron densities were measured as a function of fluence, distance from target and time delay. The three-dimensional mapping of the plume yielded densities ranging from 0.5 x 1012 cm-3 to greater than 8 x 1012 cm-3 for an incident fluence of 2 Jcm-2 and at a time delay of 1 μs after the ablating laser pulse. Higher number densities could not be probed due to resulting optical densities greater than the detection limit of the system. Two distinct components within the plume were detected, i.e. a fast component of ions and neutrals travelling in a highly forward directed plume with velocities of ∼ 106 cms-1 and a high density region, ∼ 1015 cm-3, composed of slow neutral material peaked at the target surface. Electron densities in the range ∼ 0.5 x 1017 cm-3 to 1 x 1019 cm-3, were measured at distances < 700 μm from the magnesium target surface and for delays < 100 ns after the onset of the ablating pulse using laser influences in the range 3 - 9 Jcm-2. Copper and titanium plasma plumes were also investigated with copper showing lower electron densities than magnesium but higher values than titanium for all fluences. (author)