Quantitative analysis of single and double-pulsed laser-induced breakdown spectroscopy
An investigation was conducted into the effects of double-pulsed laser excitation on the spectra emitted by steel targets in air at atmosphere pressure. Double-pulsed excitation was found to produce an average factor of seven increase in spectral line intensity over single-pulsed excitation of the same total laser energy. The emitted spectral line intensity and plasma electron temperature were strong functions of the laser pulse energy ratio, with both values maximised when the metal was excited by a low-energy first pulse, followed by a high-energy second pulse. The variation of laser pulse separation over the range 7.6s to 30s was found to produce no measurable variation in electron temperature. A model is proposed to explain the effects of double-pulsed laser excitation, based on the re-generation of the primary plasma by UV radiation, energetic ions and shock wave produced by the secondary plasma within the atomised remnants of the primary plasma. The resolution of chromium in a range of steels using single-pulsed and double-pulsed Laser-Induced Breakdown Spectroscopy (LIBS) was investigated. Optical and laser parameters were optimised for each excitation regime in order to maximise the electron temperature of the laser-induced plasma. The slope of the calibration curve produced by double-pulsed LIBS was in the order of 0.55 (depending on line-pair combination), compared with approximately 0.75 for single-pulsed LIBS. The improvement in the correlation coefficient of the calibration curves from an average of 0.984 for single-pulse to 0.996 for double-pulse was attributed to the increase in S/N ratio produced by double-pulsed excitation. Sorting of the spectra into electron temperature-defined subsets resulted in a minimum average standard deviation in measured intensity ratio for narrowest temperature band. The correlation coefficients of the calibration curves were not improved by this process, through the resultant severe reduction in sample size.