Influence of inter-pulse delay on CN molecular emission from femtosecond-nanosecond double-pulse laser-induced breakdown spectroscopy of PMMA

Abstract

An experimental study on molecular emission in laser-induced breakdown spectroscopy (LIBS) was performed using an integrated femtosecond (fs) and nanosecond (ns) double-pulse (DP) laser scheme at the different inter-pulse delay between the fs and ns pulses. This work mainly included the influence of the DP inter-pulse delay on the spectral emission intensity and signal-to-background ratio (SBR). The ablated sample was polymethyl methacrylate (PMMA), and the measured molecular band was CN (Δv = 0) in the B2Σ+ − X2Σ+ transition. The results showed that the negative (ns + fs scheme) and positive (fs + ns scheme) inter-pulse delays were not much different in terms of spectral emission. The SBR excited by the fs + ns DP was higher than that excited by the ns + fs DP, and the maximum value of the SBR in the fs + ns DP-LIBS scheme was about two times that in the ns + fs DP-LIBS scheme. The higher SBR was suitable for spectral analysis, and was easy to reduce the influence of continuous emission on spectral analysis. Also, the vibration temperature of the CN molecule as a function of the inter-pulse delay between the two laser pulses was discussed. This discovery fully demonstrates that the fs + ns DP-LIBS is of great significance for detecting molecular emissions in LIBS.