Evolution And Spectrum Of Underwater Laser-induced Plasmas

With the development of industrialization and urbanization,the earth’s environment has been destroyed and the water resources have been seriously polluted,which has threatened the survival of human beings.Therefore,necessary measures must be taken to find and control the polluted waters in time.According to statistics,the proportion of heavy metal pollution in rivers,lakes and seas in China is as high as 80%.Therefore,an analytical method which can quickly detect heavy metal pollution elements in water is urgently needed.Laser-induced breakdown spectroscopy(LIBS)is a developing method of material element detection,which has advantages of rapid,real-time,online and multi-element detection,and has a good prospect for applications in the rapid heavy metal element detections in water.In this paper,the combination of laser plasma spectral analysis and the dynamic evolution process of cavitation bubbles induced by laser plasma is used to study the underwater orthogonal double-pulse LIBS technique.The effects of the relative position of the focus of two pulse lasers,the time delay between the double pulse lasers and the laser energies on the spectrum intensity and the evolution of the cavitation bubble were studied in detail,which deepened the understanding of the signal enhancement mechanism of the orthogonal double-pulse LIBS technology in water.The main contents of this paper are as follows:1.The relationship between LIBS parameters and the formation and evolution of cavitation bubbles in water under single pulse LIBS was studied.Firstly,the effects of different focusing modes,such as single lens with different focal lengths,combined lens composed of two lenses,on the size,evolution and stability of laser-induced cavitation bubbles were studied.The effect of d the distance between laser focus points and liquid surface on laser-induced cavitation bubbles characteristics was investigated as well.It is found that the largest size and the best stability of the laser-induced cavitation bubbles in water was formed when the laser focus point is 10 mm lower under the liquid surface and combined two lens to focus the laser.The influence of the laser energy on cavitation bubbles is studied as well.The results show that when the laser energy in the range of 60-150 m J,bubble size increases and the stability is better with the increase of laser energy.But too much laser energy will cause severe shaking of the liquid surface.Therefore,the first laser energy of 120 m J can be selected in DP-LIBS,which is beneficial to provide a stable excitation gas environment for the second laser.2.The effects of the relative position of the two laser focuses,interpulse time delay and the laser energy on the spectrum line intensity,stability and cavitation bubbles in the orthogonal double-pulse LIBS of water are studied.The results show that the maximum spectral intensity can be obtained when the two laser focal points are-1.35 mm apart,the second laser channel is1.35 mm to the left of the first laser channel(the second laser is incident from the right).However,compared with other relative position of the double pulse laser focus,the size of cavitation bubbles generated by the double-pulse laser at this position is smaller.When the relative position of the double pulse laser focus is different,the inter pulse time delay to obtain the maximum spectral intensity is different.If only the inter pulse time delay is changed but the laser focus position is not changed,the spectral signal intensity is better when the cavitation bubble size generated by the double pulse laser breakdown is small.It is also found that when the energy of the second laser increases gradually,the spectral line intensity of the orthogonal DP-LIBS in the water decreases first and then increases,which is completely different from the variation trend of the DP-LIBS spectral intensity commonly seen in the air.It can be seen that the existence and evolution of cavitation bubbles induced by the first laser has a dynamic effect on the breakdown environment of the second laser,which directly affects the spectral intensity and stability,as well as the relationship between LIBS parameters and spectral intensity.The results obtained here are helpful to better understand the process and enhancement mechanism of orthogonal DP-LIBS in water.