Study Of Enhancemental Characterization Of Femtosecond-laser-induced-breakdown Spectroscopy

In this paper, a series of femtosecond laser-induced breakdown spectroscopy (LIBS) research work were carried out with a compact setup, which consists of a Ti:Sapphire chirped-pulse amplification laser, fiber optic spectrometer, echelle spectrometer equipped, and so on. Meanwhile, based on the studies of single pulse LIBS, we did the orthogonal double pulse LIBS and collinear double pulse LIBS experiments to explore the possibility of improving the emission intensity of femtosecond LIBS. After a lot of research work, the following achievements are obtained:(1) Single-pulse and double-pulse optical emission spectroscopy (OES) measurements were carried out in air by using ultrashort laser pulses at atmospheric pressure. The aim was to use spectroscopic methods to analyse the early phase of laser-induced plasma after the femtosecond laser pulse. The temporal behavior of emission spectra of air plasma has been characterized. The plasma emission obtained in the double-pulse scheme shows significant differences from that of single-pulse scheme. As only one line is available in the single-pulse scheme, the plasma temperature measurements were performed using only the relative line-to-continuum intensity ratio method, whereas the relative line-to-line intensity ratio method and the relative line-to-continuum intensity ratio method were used simultaneously to estimate the electron temperature in the double-pulse scheme. The results reveal that the temperature values obtained by the two methods in the double-pulse scheme agree with each other. Moreover, this shows that the relative line-to-continuum intensity ratio method is suitable for the early phase diagnostics of laser-induced plasma. The electron number density estimations were made using the Stark broadening method. Within the early phase of laser-induced plasma, the temporal evolution of the electron number density indicates a power law decrease with delay time.(2) The spectra of N2plasma induced by a femtosecond pulsed laser has been studied experimentally at sub-atmospheric pressure. The results show that the spectra of laser-induced plasmas for all sample pressures are composed of continuous spectra and line spectra. As the sample pressure is reduced the intensity of continuous spectra undergoes the change from slow increase to rapid decrease; on the other hand, the intensities of N+spectra increase significantly with the decrease of pressure. The spectra of N++species are observed when the pressure is lower than0.3atm. The behaviors of the femtosecond laser propagation and energy absorption at sub-atmospheric pressure are also given and the feature of the laser-induced plasma channel is briefly discussed.(3) The effect of laser pulse energy and the inter-pulse delay on orthogonal double femtosecond pulse laser induced breakdown spectroscopy (LIBS) in air are 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 yields2--32times 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.(4) An experiment study of collinear geometry double pulse femtosecond LIBS was performed on Ni sample in ambient air in an effort to clarify the contributing processes responsible for signal enhancement observed in comparison with single pulse case. Double-pulse LIBS spectra show a very clear enhancement when an optimum inter-pulse delay was used. The influences of the inter-pulse delay between the two pulses in the LIBS signal intensity, the electron temperature and density were investigated. It is most remarkable that the evolutions of signal enhancement and electron temperature versus the inter-pulse delay showed the same behavior and revealed two main regimes of interaction. We conclude that the observed increase in temperature can explain the LIBS signal enhancement.