Abstract
This paper investigated the effect of lens focusing distance (LFD) on optical emission from femtosecond laser-induced breakdown spectroscopy (LIBS) of Al in air. The measurements showed that the emission intensities of Al (I) line and AlO molecular band strongly depended on the LFD. Along with the change in the LFD, two extreme emission intensities appeared in Al (I) line and AlO (0-0). It was due to self-focusing, defocusing, refocusing effects during femtosecond laser propagation in air. That is to say, there were two convergent points. The spectral intensity of Al (I) line at first focus point was higher than that at second focus point. However, the spectral intensity of AlO (0-0) at second focus point was higher compared with that at first focus point. For molecular emission of femtosecond LIBS, the second focus point is more suitable for generation of AlO molecule. In addition, the plasma temperature was also calculated. The temperature at first focus point was higher than that at second focus point. Therefore, the mechanism of weaker molecular emission at first focus point is due to higher plasma temperature. This research will be beneficial to optimize the spectral intensity of molecular emission in the future femtosecond LIBS.
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