Atomic emission spectroscopy of laser-induced plasmas generated with an annular-shaped laser beam

A new optical configuration for focusing laser radiation in laser ablation has been tested. The system uses a combination of an axicon and two plano-convex lenses placed along the beamline. With this set-up, the second harmonic of a flat top pulsed Nd∶YAG laser beam was used to generate an annular plasma on the sample surface. The morphology of the axicon-generated craters was studied for variable focal positions. The annular plasma images were acquired with an intensified charge-coupled device with the imaging spectrograph in zero order. Time-resolved features of these images were studied as a function of beam focal conditions. A spectroscopy characterization of the annular plasmas in terms of electron temperature and electron density was carried out. These parameters presented similar trends and fell from 11800 K to 8400 K and from 2.1 × 1017 cm3 to 1.3 × 1017 cm3, respectively, within a 100–400 ns delay time interval. The results demonstrate that at early times (200 ns) the electron density inside the plasma can be sufficient for secondary excitation of particulate matter.