The dual-pulse LIBS (DP-LIBS) technology can effectively enhance the spectral intensity of LIBS and has received widespread attention in LIBS analysis.

To understand the enhancement mechanism of traditional collinear dual pulse LIBS and long-short collinear dual pulse LIBS spectra, a comparative study was conducted on two DP-LIBS with different laser excitation schemes, i.e. the conventional collinear dual nanosecond pulse excitation scheme, and the long-short collinear dual-pulse excitation scheme which combines a microsecond pulse and a nanosecond pulse.

The enhancement mechanism and variation trend of spectral intensity were investigated by systematically analyzing the laser ablation morphology and LIBS spectra collected under different inter-pulse delays, spectral acquisition delays and laser pulse energy in both DP-LIBS modes.

The results show that, in conventional collinear DP-LIBS, the spectral intensity increases rapidly within a short delay time of 0–2 μs, but remains relatively high in the longer delay range of 2–14 μs. And the optimal inter-pulse delay is around 4 μs in conventional collinear DP-LIBS. In contrast, the optimal inter-pulse delay for the long-short collinear DP-LIBS is approximately 25 μs, which is determined by the peak power timing of the long-pulse laser.

In the conventional DP-LIBS configuration, spectral enhancement is more sensitive to the energy variations of the second pulse than to those of the first pulse. In the long-short pulse scheme, increasing the energy of the long-pulse laser facilitates sample heating and surface modification, thereby enhancing spectral intensity. However, excessive long-pulse laser energy might cause sample melting and material ejection, which in turn diminishes the ablation efficiency of the subsequent short-pulse laser and reduces the overall spectral intensity. Further analysis of the ablation morphology reveals that the conventional collinear DP-LIBS tends to produce deeper ablation craters, whereas the long-short collinear DP-LIBS is more likely to generate larger ablation craters.