<title>Dual-pulse LIBS: why are two lasers better than one?</title>

Abstract

Laser-induced breakdown spectroscopy (LIBS) is a very attractive form of elemental analysis which holds much promise as a remote technique. LIBS, however, has problems with sensitivity and reproducibility compared to alternate forms of analysis. One way in which our laboratory is addressing these problems is through the use of dual-laser pulses which have been shown to produce enhanced LIBS signals. Our studies include a systematic approach to the factors affecting the dual-pulse LIBS signals and the mechanisms behind the enhancements observed. For optimizing emission signals, we have found that the laser beam geometry plays a large role. This paper focuses on results solely from a perpendicular beam orientation where a pre-spark is formed above the sample surface by a laser pulse before any material ablation occurs from a second laser pulse. Using this pre-ablation spark, enhancements in copper and lead signal to background ratios are found to be as high as 2 to 6 fold for copper and lead, respectively. The reason for this increase in sensitivity is still unclear but is related to an increase in the extent of sample ablation that occurs. Factors contributing to the LIBS signal are also being investigated by acquiring time-resolved plasma images of the dual-pulsed plasmas. These experiments show changes in the overall size and shape of the plume when using a pre-ablation spark. In addition, the enhancements in lead signal to background ratios appear to be highly region specific.