Analysis of solids using laser-induced plasma spectroscopy in double-pulse mode

We investigate plasmas formed from solid aluminium alloys in air using a Nd:YAG laser in double-pulse mode, in view of the possibility of enhancing analyte line emissions. In particular, time-resolved studies of emission characteristics and plasma properties are carried out. It is found that the Al II 281.6 nm line is considerably enhanced when using a double-pulse burst instead of a single pulse of equal energy. However, the electron density is found to be approximately the same in both cases, and the plasma temperature is less than 10% higher with the double-pulse burst. The line enhancement is rather explained by the formation of a larger volume of emitting gas. This, in turn, can be linked to a greater ablated mass, as well as to the presence of a preplasma into which the second laser pulse is absorbed. The influence of the interpulse interval on the peak intensity of the Al II line and of several neutral lines of different elements is also studied. For neutral lines, a maximum enhancement factor of 3-4 is attained with an interval in the range 0.5-1 mu s. Finally, it is found that the relative standard deviation of 20 consecutive intensity measurements is reduced by a factor of 2-3 when going from single- to double-pulse mode. (C) 1998 Elsevier Science B.V.