Characterization of laser-induced plasmas by emission spectroscopy with curve-of-growth measurements. Part II: Effect of the focusing distance and the pulse energy

Laser-induced plasmas generated with different focusing distances and pulse energies have been characterized by a method based in emission spectroscopy that includes the measurement and calculation of curves of growth. An infrared Nd:YAG laser is used to generated the plasmas from Fe–Ni samples placed in air at atmospheric pressure. The characterization method provides a reduced set of plasma parameters (N_e, T, N′l, αA) that describe the line emission in optically thin and optically thick conditions. For a pulse energy of 100 mJ, the plasma parameters for varying focusing distances are obtained. The apparent (population averaged) temperatures for neutral atoms and ions are shown to be different in the plasmas generated with all the focusing distances. For each pulse energy (in the range 20–100 mJ), the plasmas generated with the optimum focusing distance, which corresponds to a constant value of irradiance, have been investigated. In these conditions, simple laws have been obtained for the variation of the plasma parameters with the pulse energy E: the electron density N_e and the apparent temperature T are independent of E while linear relations with E are obtained for the parameters N′l, αA. These simple laws lead to a quadratic dependence on E of the line intensities in the optically thin limit and to a variation of the intersection concentration C_int that characterizes self-absorption as E^− 1.