Dual-pulse laser induced breakdown spectroscopy in the vacuum ultraviolet with ambient gas: spectroscopic analysis and optimization of limit of detection of carbon and sulfur in steel

Abstract

Laser Induced Breakdown Spectroscopy (LIBS) in the vacuum ultraviolet (VUV) spectral region was applied to standard steel samples in dual-pulse excitation mode with an ambient gas. Two lasers were employed in collinear geometry, one as an ablation laser (Spectron: 200mJ/15ns) and the other a reheating laser (Surelite: 665mJ/6ns). A dual-pulse scheme was applied to the traditional single-pulse LIBS and led to a significant enhancement, increasing both the signal to background ratio (SBR) and concomitantly the limit of detection (LOD). Three types of gases, nitrogen, argon and helium were investigated individually as ambient environment. The variation in signal gain with ambient gas pressure was measured. The feasibility study of dual-pulse (DP) LIBS) in the deep VUV spectral region was carried out by optimizing a number of parameters for limit-of-detection (LOD) calibration. The pulse energy choices of laser beam, the signal recording position for the space-resolved detection system, the CCD exposure time and the number of laser shots accumulated for a spectrum were among the first few parameters optimized for the current LIBS system. The dependence of emission intensity on focusing conditions was investigated for both single-pulse (SP) and dual-pulse (DP) mode. In DP mode, the lens-to- target distance of the reheating pulse was varied while the ablation pulse was held at a fixed focusing condition. The optimal focusing for signal enhancement was found to be about 10 mm under the sample surface for single-pulse laser beam and 5 - 10 mm for the reheating pulse in dual-pulse mode. The effect of inter-pulse delay on the emission intensity were studied in vacuum and in ambient gas background in the dual-pulse configuration. An intensity peak at about 100 ns and an intensity revival in the μs range were observed on the intensity vs. inter-pulse delay curves. The optimal inter-pulse delay was considered to be 100 ns. The μs range intensity plateau led to a discussion on the plasma expansion dynamics in dual-pulse mode. Finally the limit of detection (LOD) of C and S in steel was improved by a factor of 2 or more over single pulse LIBS with the combination of optimized experimental parameters. The initial measurements and results suggest that DP-LIBS in an ambient gas environment for VUV spectroscopy is practicable and brings with it even further improvement of the LIBS performance in the VUV regime