Diagnosis Of Femtosecond Laser-Induced Plasma And The Investigations Of The Plasma Spectral Lines

When the laser pulse exceeds the damage threshold of the material,the surface of the material will be melted,ablated or ionized,and there will be ionized gas,which is so-called laser-induced plasma?LIP?.The related parameters of plasma,such as electron temperature,electron density and expanding velocity,will be studied and calculated by analyzing laserinduced plasma emitting spectral lines.The diagnosis of plasma can be achieved and the component of the target can also be determined.The LIP has advantages of fast detection,lesstarget damage and remote detection.The laser induced breakdown spectroscopy?LIBS?is based on the analyzing of plasma spectral lines.Therefore,the analyzing of plasma spectral lines is of great significance on understanding the broadening mechanism and the characteristics of plasma.Laser induced breakdown spectroscopy?LIBS?has a highvalue in the aspects of material processing,diagnostic tests and space applications.Femtosecond laser induced breakdown spectroscopy?fs-LIBS?has advantages of low breakdown threshold and atmospheric interference.With the progress of aerotechnics,its application in the universe exploration is becoming one of the hot spots.Ordinarily,the Origin program is used to fit plasma spectral lines.The Voigt function is the convolution of the Lorentz and Gauss function.The Lorentz function is often used to fit lines.In our experiment,the proper fitting function is chosen by comparing with experimental lines.The collision broadening and the natural broadening can be depicted by the Lorentz function,and the Doppler broadening and the instrumental broadening can be depicted by the Gauss function.So finding the precise fitting function can help us understand the broadening mechanism of plasma.Because the plasma is inhomogeneous,the center temperature is higher than that of the edge.When the emitting photons go through from center to the edge of plasma,they will be absorbed by the same atoms or ions.So the spectral lines intensity will be decrease,which is called selfabsorption phenomenon.The self-absorption can result in error when diagnosing plasma.Hence it is important to analyze and understand the self-absorption phenomenon.In this paper,the laser-induced Zn and Cu plasma were produced respectively by using the femtosecond laser beam at 800 nm wavelength,1000 Hz repetition rate and 100 fs pulse duration.The spectral lines were fitted by the Origin program.The plasma was diagnosed by time-andspace-resolved spectral technology.The influence of defocusing distance on self-absorption was also discussed.The main work is as follows:1.Under the background pressures 10 Pa,10³Pa and 10⁵ Pa,the femtosecond laserinducedplasma were analyzed.As plasma expanding,it is not proper to fit the lines by Lorentz function all along.The broadening mechanism of plasma is not all the same as the plasma expanding.At the earlier time of plasma formation,the lines fit well with Lorentz and Voigt shapes.At the later time,the lines fit well with Gauss and Voigt shapes.It is precise to fit spectral lines by Voigt function.By analyzing the Gauss portion and Lorentz portion of the full width at half maximum?FWHM?from Voigt fitting,it is clear that the collision broadening dominates at the earlier time,and the instrumental broadening dominates at the later time.Therefore,the Stark broadening obtained only by Lorentz fitting at the later time may not be proper when diagnosing plasma.In our opinion,the Lorentz portion from Voigt fitting makes the physical scene clearer.The electron temperature and electron density of femtosecond laserinduced plasma were calculated.They decrease as time.At the earlier time,plasma collide acutely,and the collision broadening dominates.As electron temperature and density decreasing,the instrumental broadening becomes obvious.2.Under the background pressure of 10 Pa,the femtosecond laser-induced Cu plasma was diagnosed.The time-resolved spectral lines were analyzed.The expansion velocities of the plasma at different moments were derived from the time of flight?TOF?curves.The plasma should show ellipsoid from analyzing the space-resolved spectra.The ratio of intensity I₅₂₁/I₅₁₀ from wavelengths 521.82 nm and 510.55 nm decreases as time delay.It can be deduced that the relative population of energy level²D_5/2 decreases faster than²P_3/2.3.Under the pressure of 10⁵ Pa,the influence of defocusing distance on the self-absorption of femtosecond laser-induced Cu plasma spectrum was studied.When the target is on the focus and time delay at 120 ns,520 ns and 920 ns,the self-absorption phenomena of wavelengths324.75 nm,327.40 nm,510.55 nm,515.32 nm and 521.82 nmwere discussed.It indicates that the self-absorption at 510.55 nm,515.32 nm and 521.82 nm are not obvious.When the defocusing distance increases,the self-absorption of324.75 nm and 327.40 nm lines becomes obvious,however the self-absorption of 510.55 nm,515.32 nm and 521.82 nm lines fades down.