UC San Diego

Laser-Induced Breakdown Spectroscopy (LIBS) is an analytical tool that can be used in a wide range of applications. By focusing a laser pulse onto a small area, material is ionized and heated to 10,000 to 20,000 K. As the plasma cools, atoms emit light. The light contains atomic information about the sample and is analyzed by a spectrometer. In this work, a fundamental study will examine the relationship between ablation and LIBS enhancement in dual-pulse LIBS. Also, an application of LIBS to identify trace metals in molten salt will be presented. The first experiment will look closely at how spectral enhancement of zinc and copper in brass is influenced by plasma temperature and ablation particles from a dual- pulse laser induced breakdown spectroscopy (DP-LIBS) compared with single-pulse LIBS. The work presented will look at a dual-pulse scheme using two pulsed Nd:YAG laser operating at a fundamental wavelength of 1064 nm. First, a pulse was focused parallel and above the surface forming a pre-ablative plasma in air. A second pulse is then fired to form an ablative (analytical) plasma on a surface while intersecting the volume of the pre-ablative plasma. Two parameters were studied in the DP -LIBS experiments, the inter-pulse delay and the pre- ablative flunce. Both these parameters have an effect on the emission intensity of zinc and copper and the ablation volume. Single-pulse experiments were also conducted by varying the fluence, which also has an effect on the emission intensity of zinc and copper and the ablation volume. In experiments varying the inter-pulse delay, a 90 mJ pre-ablative laser pulse followed by a 30 mJ ablative (analytical) laser pulse were used. Using this scheme with an inter-pulse delay of 20 [mu]s resulted in a 5 fold increase in intensity for Cu at 521 nm and a 7 fold increase in intensity for Zn at 481 nm compared to single- pulse LIBS. A thirty fold increase in ablation was observed in this DP-LIBS scheme compared to single-pulse LIBS at an inter-pulse delay of 20 [mu]s. With a constant inter-pulse delay there is a mild increase in emission intensity for both zinc and copper with increasing pre- ablation fluence, however, a decrease in ablation volume is also observed with increasing fluence. The single-pulse experiments involved the use of only the ablative analytical pulse. Emission intensity increased with increasing fluence for both Cu and Zn; however, there was a sharp decrease in ablation volume with increasing fluence. The electron temperature was calculated for all experiments using the Boltzmann plot. It was found that emission intensities of Cu and Zn correlated well with the electron temperature; however, considering ablation particle volume along with the electron temperature improved this correlation. The results of this experiment suggest that both particle volume and electron temperature play a significant role on the emission intensity. The second project involves the use of LIBS to analytically detect trace elements in a molten salt environment. Here an apparatus was built to simulate an electrorefiner and its enclosure. Electrorefiners are used to reprocess nuclear fuel for recycle through electrolysis in a molten salt bath. This is an important application that demonstrates ability of LIBS to analytically detect elements in hostile environments and on liquid surfaces. Chromium, cobalt, and manganese where measured in a eutectic potassium-lithium-chlorine molten salt mixture. Calibration curves were successfully constructed for cobalt (CoCl₃) and manganese (MnCl₃), while chromium (CrCl₃) was used to demonstrate the resolution of the spectrometer. Theoretical detection limits were determined to be 0.04, 0.5 and 0.3 percent mass for CrCl₃, CoCl₃ and MnCl₃, respectively