Effect of laser pulse energy on orthogonal double femtosecond pulse laser-induced breakdown spectroscopy.

In this paper, the effect of laser pulse energy on orthogonal double femtosecond pulse laser induced breakdown spectroscopy (LIBS) in air is studied. In the experiment, the energy of the probe pulse is changeable, while the pump pulse energy is held constant. At the same time, a systematic study of the laser induced breakdown spectroscopy signal dependence on the inter-pulse delay between the two pulses is performed. It is noted that the double pulse orthogonal configuration yields 2-32 times signal enhancement for the ionic and atomic lines as compared to the single pulse LIBS spectra when an optimum temporal separation between the two pulses is used, while there is no significant signal enhancement for the molecular lines in the studied range of the delay. It is also noted that the dependence of the enhancement factor for ionic and atomic lines on the inter-pulse delay can be fitted by Gaussian function. Furthermore, the electron temperature obtained by the relative line-to-continuum intensity ratio method was used to explain the LIBS signal enhancement.