The Study Of Time-resolved Spectroscopy Of Aluminum Film Plasma Induced By Femtosecond Laser

The use of laser-induced breakdown spectroscopy (LIBS) as an analytical tool has considerably grown over the past ten years. Spectrochemical analysis using LIBS has been proven to be extremely versatile, providing multi-element analysis without sample preparation. LIBS can be regarded as a universal sampling, atomization, excitation and ionization source, since laser-induced plasmas can be produced in gases ionization source. Since laser-induced plasmas can be produced in gases or liquids, as well as from conducting or non-conducting solid samples.Plasma characteristics and analytical performances of plasmas produced by laser strongly depend on the experimental parameters: (1) the laser beam; (2) the effect of the surrounding atmosphere on the laser matter interaction and the plasma emission; (3) the properties of the material analyzed. Most of these parameters have been studied and optimized with the goal of obtaining the best signal-to-noise ratio and a linear correlation between the intensity of the analyte line emission and the bulk composition of the sample. However, the influence of the laser pulse duration on plasma characteristics have just begun to be investigated, investigations being generally performed at long laser pulse duration ( typically a few nanoseconds). In fact, with the recent development of the chirped pulse amplification technique (CPA), ultrashort laser pulse durations can now be achieved quite readily. At such pulse durations, the physical mechanisms involved during the ablation process noticeably differ from those taking place with ns lasers. Because of its very short pulse duration, the laser beam does not interact with the resulting plasma. The part of laser energy absorbed is thus fully deposited into the thermal diffusion while the pulse is on. The shortening of the laser pulse durations thus yields a shrinking of the heat-zone, which prevents an uncontrollable and often undesirable material modification and removal. Therefore, lower ablation thresholds and larger efficiency of material ablation can be obtained with high precision and minimal damage.On the analysis of time-resolved spectroscopy of plasma induced by femtosecond laser, we could have a further understanding of the kinetics process on some particles during the course of evolution in laser-induced plasmas, as well as of the reaction kinetics in which various kinds of particles and the ambient gas interact to produce new particles. According to the analysis, the micro-mechanism of laser ablation materials could be explained further. It is of great significance to the process of laser micro machining, optimizing the experimental condition of laser ion beam sputtering technology and improving the quality of films.In this paper, we present a complete analysis of the plasma initiated on aluminum-film targets with 100fs laser pulses in air at atmospheric pressure. The experiments were carried out with a Ti:Sapphire laser system that outputs 50uJ at a wavelength of 800nm, with a pulse duration of 100fs ( FWHM ) and a repetition rate of 50Hz. The time-resolved spectra were then recorded by a gateable intensified charge-coupled device (ICCD). Trough this method, the temporal evolution of laser-induced plasma can be studied.In the experiment of the temporal evolution of Al plasma, the laser pulse energy was fixed at 50uJ/pulse in air at atmospheric pressure. The results could be summarized as following: the temporal evolution process of Al plasma radiation could be divided into the following stages: at the earliest 40ns after the laser beam arriving at the target surface. In this stage, the radiation was mainly made up of continuum radiation and a series of discrete atom and ion spectra. This emission is attributed to the Bremsstrahlung process collisions of electrons with ions and atoms (free-free emission) and recombination of electrons with ions (free-bound emission). When the delay increased, the continuum drops steeply as a consequence of its recombination into ground-state ions and excited atoms. When delay time was 60ns, the neutral lines from excited Al (I) 394.3nm and 396.1nm were identified, then achieved their maximum intensity step by step. At first, the frequent collisions of excited Al atoms and a great deal of atoms and ions in plasma induced the broadening of the spectral line, so the spectra can not be divided. When the delay increased, the plasma density decrease with the plasma expanded, the collision rate of excited Al atoms decreased, so these two spectra was divided. At last, the plasma spectra lines disappear with the time and spatial scale increasing.We Also obtain the decay curves of Al(I) 394.3nm and Al(I) 396.1nm through the experiment and calculation based on this theory. Compared with the other papers, the decay curve of fs is shorter than the longer pulse duration. In the fs regime, the absorbed laser energy is fully deposited in the matter at the solid density, and no further laser-plasma interaction takes place. Thus, immediately after the laser shot, the plasma can only cool down since no other source of energy was supplied to the plasma. On the other hand, in the long-pulse regime, a significant fraction of the laser energy is absorbed by the plasma due to the inverse Bremsstrahlung. This may also contribute to increase the plasma temperature as well as the electron density. For the micromechanism of fs ablation has not been well known at the present time, we take some approximately methods to estimating the electron temperature and electron density. Supposed the plasma produced in our experiment to be local thermal equilibrium (LTE). On the condition of the LTE, we can estimate the electron temperature through measuring integral intensity of some different spectra for the same ion or atom.When we measure the spectral line width, the whole line profile is mixed with Lorentzian profile by electron collision broadening, Doppler broadening by emission particle motion and Gaussian profile by the modification of quasistatic ionized field. These bring great difficulties to the exact analysis on the collision broadening, if we neglect the influence of the Doppler broadening and Gaussian profile by the modification of quasistatic ionized field, we can approximately consider the Stark-broadening to be Lorentzian profile. Using this simplified method, we can calculate the electron density of the two spectral, Al (I) 394.3nm and Al (I) 396.1nm. The temporal evolution of electron temperature and electron density is of prime importance, since many kinetic reaction rates depend directly or indirectly on these parameters.Establishing the femtosecond time-resolved spectra system, we take a further establishment on the transient absorption spectroscopy system, femtosecond pump-probe measurement system, time resolved fluorescence spectra system and high speed photography system. Moreover, we fully study the micro-mechanism of the interaction between femtosecond laser and matter and various kinds of characteristics of the matter itself.