Influence of distance between focusing lens and sample surface on femtosecond laser-induced Cu plasma

It is important to optimize emission intensity of laser-induced breakdown spectroscopy (LIBS) for improving detection sensitivity. This paper investigates the influence of the distance between focusing lens and sample surface (DBFS) on the emission line intensity and electron temperature of Cu plasma induced by femtosecond pulse laser. The results indicate that the line intensities first increase, and then drop with an increase in the DBFS. The position for obtaining highest line intensity is not the geometrical focus of the converging lens, but a short distance near the geometrical focus towards the converging lens. And the strongest emission position moves toward the converging lens as femtosecond laser energy increases. In addition, the electron temperature is calculated by using the Cu (I) lines for different DBFSs and energies. It is found that the change in the electron temperature with the DBFS is similar to that in the spectral intensity. Therefore, changing the distance is an effective way to optimize the spectral intensity in femtosecond-LIBS.