Laser-induced breakdown spectroscopy with improved spectral resolutions through the generation of high-temperature and low-density plasmas

Abstract

ABSTRACT Improved spectral resolutions were achieved in laser-induced breakdown spectroscopy (LIBS) through generation of high-temperature and low-density plasmas. A first pulse from a KrF excimer laser was used to produce particles by perpendicularly irradiating targets in air. A second pulse from a 532 nm Nd:YAG laser was introduced parallel to the sample surface to reablate the particles. Optical scattering from the first-pulse plasmas was imaged to elucidate particle formation in the plasmas. Narrower line widths (full width at half maximums: FWHMs) and weaker self-absorption were observed from time-integrated LIBS spectra. Estimation of plasma temperatures and densities indicates that high temperature and low density can be achieved simultaneously in plasmas to improve LIBS resolutions. Keywords: Laser-induced breakdown spectroscopy (LIBS),high-temper ature and low-density plasma, improved spectral resolution, line width, reheating 1. INTRODUCTION Laser-induced breakdown spectroscopy (LIBS) has been developed into a very popular and useful elemental analysis technique in recent years. When powerful la ser pulses are focused on solid, liquid, or gas targets, luminous hot sparks (or laser-induced plasmas) are generated. By spectrally analyzing the line emissions from the luminous plasmas, the elemental compositions can be deduced. LIBS has been applied in a wide range of applications, such as aerosol detection [1], artwork diagnostics [2], and remote el emental analysis [3]. LIBS is also a potential tool for real-time monitoring of radioactive materials [4]. A number of techniques, such as introduction of purge gas[5] and dual-pulse excitation [6-18], have been used to improve the sensitivity of LIBS. A higher temperature of plasmas is beneficial to the sensitivity of LIBS. However, plasmas in typical LIBS also have high densities correlated to their high temperatures. High plasma density gives rise to the widened line widths and increased self-absorption of atomic lines, therefore, results in lower spectral resolutions. Spectral resolution of LIBS is very important for element analysis. Higher spectral resolution will improve the accuracy of element determination. To improve the LIBS resolution, low density and high temperature in plasmas need to be achieved simultaneously. The profile of a line is the result of many effects, but under typical LIBS conditions the main contribution to the line width comes from the Stark effect (see Gornoshkin et al. [19], for a discussion of the different broadening effects influencing the spectral line shape in LIBS). In fact, the electric field gene rated by electrons in plasma perturbs the energy levels of ions, thereby broadening the emission lines from these upper (or excited) levels. Thus the Stark broadening has a well-established relation with plasma de nsity (or plasma electron density). On the other hand, the self-absorption effect [20], in which some of the radiation emitted by a material is absorbed by the material itself, also takes place in the radiation fr om laser-induced plasmas. Dual-pulse LIBS (DP-LIBS) originated in research performed more than 20 years ago, in which spatially overlapping laser-induced plasmas formed in bulk aqueous solution could improve the detection limits by orders of magnitude over those seen in nanosecond single-pulse LIBS [6]. Collinear and orthogonal reheating multipulse LIBS of solids were examined in air [7,8]. Orthogonal pre-ablative spark dual-pulse configuration was also characterized [9-14]. The