Influence of the distance between the focused point and Al surface on atomic and molecular emission spectra in air

The focused position of the lens relative to the sample surface affects the density, temperature, and dynamic characteristics of laser-induced plasma, which are important for improving the spectral intensity, minimizing self-absorption, and improving stability of laser-induced breakdown spectroscopy (LIBS). The emission intensities of the Al atomic line and the AlO B²Σ⁺–X²Σ⁺(0,0) band, using nanosecond pulse laser ablation of an aluminum target in air, at three different focused point-to-sample distances were investigated. The Al atomic line was optimal when the focus was 1 cm above the sample surface, which is attributed to the low density and high temperature of the plasma. Conversely, the AlO B²Σ⁺–X²Σ⁺(0,0) band emission spectrum is superior when the focused point is 1 cm below the Al surface, with the spectral intensity enhancing as the number of laser shots increases. A time-resolved pump–probe shadowgraph technique was employed to record dynamic snapshots of the ablation plume at different focused point-to-sample distances to account for the enhancement mechanism of the spectral intensity. The intensity variations in the atomic and molecular spectra are related to the shock wave propagation velocity in the longitudinal and radial directions, providing insights into the enhancement mechanism of the spectral signals. Moreover, the morphology of craters was analyzed by using a scanning electron microscope and a profilometer, revealing that the depth-to-diameter ratio and ablation amount correlated with different spectral intensity variations at focused point-to-sample distances of −1 and 0 cm. These results will assist in choosing optimal strategies for quantifying elements using LIBS atomic or molecular spectrometry.