Dependence on the Lens-to-Target Distance and with the Laser Energy at Constant Irradiance of the Laser-Induced...

The emission signal-to-noise ratio (S/N) of a laser-produced plasma on an aluminum target at different focusing distances and at fixed irradiances was investigated. The plasma was produced by a 1064 nm nanosecond-pulsed laser and the energy and irradiances were varied in the 6-110 mJ and 0.4-700 GW cm^-2 ranges, respectively. Regardless of the applied laser energy, adjusting the lens-to-target distance, best emission values were obtained for an irradiance of nearly 8 GW cm^-2. At lower irradiances, the signal decreases due to less matter removal, while at higher values, the plasma shielding effect prevents the laser from reaching the sample. This mechanism is surpassed when the lens-to-sample distance is close to the nominal focusing value at about 100 GW cm^-2. The enhancement of the signal with the focusing distance is due to a combination of an increment of the plasma temperature, electron density, and atomized mass. When the irradiance is kept fixed changing simultaneously the laser energy and the ablated area, an increment of the emission was observed. This is basically due to an increment of the ablated mass while both electron density and temperature do not show significant changes, even though the laser energy increased by more than one order of magnitude.