Role of laser pre-pulse wavelength and inter-pulse delay on signal enhancement in collinear double-pulse laser-induced breakdown spectroscopy

a r t i c l e i n f o Keywords: LIBS Double pulse LIBS Plasma diagnostics LPP Dual-pulse (DP) laser-induced breakdown spectroscopy (LIBS) provides significant improvement in signal intensity as compared to conventional single-pulse LIBS. We investigated collinear DPLIBS experimental performance using various laser wavelength combinations employing 1064 nm, 532 nm, and 266 nm Nd:YAG lasers. In particular, the role of the pre-pulse laser wavelength, inter-pulse delay times, and energies of the reheating pulses on LIBS sensitivity improvements is studied. Wavelengths of 1064 nm, 532 nm, and 266 nm pulses were used for generating pre-pulse plasma while 1064 nm pulse was used for reheating the pre-formed plasma generated by the pre-pulse. Significant emission intensity enhancement is noticed for all reheated plasma regardless of the pre-pulse excitation beam wavelength compared to single pulse LIBS. A dual peak in signal enhancement was observed for different inter-pulse delays, especially for 1064:1064 nm combinations, which is explained based on temperature measurement and shockwave expansion phenomenon. Our results also show that 266 nm:1064 nm combination provided maximum absolute signal intensity as compared to 1064 nm:1064 nm or 532 nm:1064 nm. Laser-induced breakdown spectroscopy (LIBS) technique is useful for a variety of applications including elemental analysis, detecting airborne biological agents, quantitative analysis of aerosols etc. [1–3]. Although LIBS is suited for various applications, this technique possesses lower sensitivity and precision than the other forms of elemental analysis. Improvements in the sensitivity of LIBS can be achieved by enhancing the signal strength or reducing the background noise. Recent advancements with femtosecond (fs) laser based LIBS and polarization resolved LIBS showed sharp reduction in background continuum noise, but the technology of a handheld fs laser system is still far away [2,4,5]. Among the various methods proposed for improving the detection sensitivity of LIBS, double-pulse (DP) excitation is more promising [6–11]. The term " double-pulse " can be defined as follows; two laser pulses used for LIBS excitation, which are separated in time by a period of few nanoseconds to several microseconds. Quite recently, several research groups explored applications for DP because of advances in laser technology and better analytical detection systems [3,6,9,11–15]. Double pulses have been found to increase LIBS sensitivity for the analysis of liquids [16], solids immersed in liquid [17], and solids [18,19]. Apart from analytical applications , DP technique has been used in laser-produced plasma (LPP) for x-ray [20,21] and EUV emission enhancement …