Spatial population distribution of laser ablation species determined by self-reversed emission line profile

We propose a method for determining the spatial distribution of population densities for the species in laser-produced plasma. Our method relies on the parameter fittings of the experimentally observed self-reversed emission profiles to the model which is based on the calculation of one-dimensional radiative transfer. Employed parameters in the model represent spatial distribution of emitters, absorbers, and plasma free electrons. Since the density of plasma electrons has a spatial dependence, Stark shifts and broadenings are incorporated in a position-sensitive manner. After a general description of the method, we have specifically applied it to the laser-ablated Al plasma, where Al(I) P-2-S-2 emission line is employed for the analysis. In this specific example, we find that the accuracy of the fittings is significantly improved due to the presence of two emission lines originating from the fine structure, i. e., P-2(1/2)-S-2(1/2) and P-2(3/2)-S-2(1/2). In particular, the depth of the self-reversed structure turns out to be very sensitive to the position-dependent upper and lower level populations, which enables us to accurately determine the spatial variation of the laser-ablated species in these states. Furthermore, the calculated profile is almost unchanged with temperatures employed for fittings. This means that the present method gives reliable values of the parameters for the spatial distributions, even if the temperature is not precisely known. (C) 2002 American Institute of Physics.