Objective Orthogonal double-pulse (DP) laser-induced breakdown spectroscopy (LIBS) in reheating mode provides intrinsic advantages, such as reduced self-absorption, capability of quantitative elemental analysis using very low ablative laser pulse energy, and relatively large signal enhancement factor under low ablative laser pulse energy. Conventional reheating orthogonal DP-LIBS allows realizing sensitive elemental analysis under microablation or elemental mapping analysis with high spatial resolution. However, as the reheating laser is focused on sample plasma with low particle density, only part of the reheating laser can be absorbed, reducing the utilization efficiency of laser energy. Therefore, under high ablative laser pulse energy, the signal of orthogonal DP-LIBS cannot be suitably enhanced compared with single-pulse LIBS using the same ablation laser. To improve the energy utilization efficiency in reheating laser and increase the signal enhancement factor in orthogonal DP-LIBS, target-enhanced orthogonal DP-LIBS is proposed, and its signal enhancement mechanism is studied.
Methods A solid target with low spectral interference was added in reheating orthogonal DP-LIBS. In addition, a rotating solid aluminum target was placed perpendicular to the target sample. Two electro-optically Q-switched Nd∶YAG lasers with 5 Hz pulse repetition rate and 1064 nm wavelength were used as excitation sources. The beam of the first laser was perpendicularly focused on the sample surface to produce sample plasma, and the beam of the second laser was focused on the surface of the aluminum target to produce target plasma. Moreover, the focal spot of the second laser beam was located in front of the sample surface, and the distance from the focal spot to the sample surface was adjusted to avoid sample ablation by the second laser. For evaluation, a compact fiber-optic spectrometer coupled with a nonintensified charge-coupled device was used to record the spectra. In addition, a monochromator and a photomultiplier tube were used to record the temporal profile of the light emission, and a brass standard sample was analyzed.
Results and Discussions The signal intensities of the sample elements were significantly enhanced by the interaction between the target and sample plasmas. The temporal profiles of plasma emission from the brass sample were recorded at 324.75 and 324.00 nm using orthogonal DP-LIBS and the proposed target-enhanced orthogonal DP-LIBS. The persistence time of the copper atomic emission observed at 324.75 nm was prolonged to 18.0 μs using the target-enhanced orthogonal DP-LIBS (Fig. 2). The effect of the interpulse delay on the atomic emission intensities was experimentally determined. The best delay time for the two laser pulses was approximately 4.0 μs. In the target-enhanced orthogonal DP-LIBS, multiple emission lines of different elements could be simultaneously enhanced (Fig. 3). Under the optimal experimental conditions, the signal intensity was enhanced by a factor of 24-146 compared with single-pulse LIBS and by a factor of 6-10 compared with conventional orthogonal DP-LIBS. Thus, a substantial improvement in the intensity was obtained from aluminum target enhancement (Table 1). To clarify the signal enhancement mechanism of target enhancement, the plasma temperature variations in orthogonal DP-LIBS and target-enhanced orthogonal DP-LIBS were determined based on Boltzmann plots using various copper atomic lines. Plasma temperature was 6874 and 9752 K for orthogonal DP-LIBS and target-enhanced orthogonal DP-LIBS, respectively (Fig. 5), corresponding to a temperature increase of 2878 K. The averaged electron density in orthogonal DP-LIBS and the target-enhanced orthogonal DP-LIBS was evaluated according to the Stark broadening of selected atomic emission lines. Under the considered experimental conditions, the difference in Stark broadening of the selected atomic emission lines could not be distinguished. This may be owing to the change in electron density being negligible in both cases. Overall, the experimental results indicated that the proposed reheating target-enhanced orthogonal DP-LIBS could improve the signal for a given sample ablation condition compared with the conventional orthogonal DP-LIBS.
Conclusions A target-enhanced orthogonal DP-LIBS technique considering reheating is proposed. Using a solid target in conventional orthogonal DP-LIBS, plasma emission can be substantially enhanced. The signal enhancement results from the increase in plasma temperature and collision mechanism. The energy utilization efficiency of the reheating laser is higher than that in conventional orthogonal DP-LIBS when the reheating laser points to the surface of a solid aluminum target. The additional particles from the solid target are ablated by the reheating laser. Under the assistance of the solid target, collisions during plasma interactions promote re-excitation and ionization of species, which can lead to a considerable signal enhancement. The proposed target-enhanced orthogonal DP-LIBS allows the use of low ablative laser pulse energy and can improve analytical sensitivity with microablation and high spatial resolution. Moreover, the proposed technique can drastically improve the signal intensity of orthogonal DP-LIBS, and it can further improve the analytical sensitivity of orthogonal DP-LIBS and deliver superior analytical results. The proposed target-enhanced orthogonal DP-LIBS can find applicability in elemental analysis of precious samples and small samples in fields such as jewelry identification, art, archaeology, and materials science.
