Complementary Characterization of Laser-Induced Plasmas by Optical Emission Spectroscopy and Triple Langmuir Probe (original) (raw)
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Generation and expansion of laser-induced plasma as a spectroscopic emission source
Frontiers of Physics, 2012
Laser-induced plasma represents today a widespread spectroscopic emission source. It can be easily generated using compact and reliable nanosecond pulsed laser on a large variety of materials. Its application for spectrochemical analysis for example with laser-induced breakdown spectroscopy (LIBS) has become so popular that one tends to forget the complex physical and chemical processes leading to its generation and governing its evolution. The purpose of this review article is to summarize the backgrounds necessary to understand and describe the laser-induced plasma from its generation to its expansion into the ambient gas. The objective is not to go into the details of each process; there are numerous specialized papers and books for that in the literature. The goal here is to gather in a same paper the essential understanding elements needed to describe laser-induced plasma as results from a complex process. These elements can be dispersed in several related but independent fields such as laser-matter interaction, laser ablation of material, optical and thermodynamic properties of hot and ionized gas, or plasma propagation in a background gas. We believe that presenting the ensemble of understanding elements of laser-induced plasma in a comprehensive way and in limited pages of this paper will be helpful for further development and optimized use of the LIBS technique. Experimental results obtained in our laboratory are used to illustrate the studied physical processes each time such illustration becomes possible and helpful.
Laser-induced plasma as a function of the laser parameters and the ambient gas
2014
Laser-induced breakdown spectroscopy (LIBS) has been invented for more than 50 years, which analyzes the spectrum of the laser-induced plasma to determine the elemental composition of the ablated sample. Recently, LIBS technique has been well developed and applied in different domains, for example oceanic exploration, pollution monitoring in the environment. LIBS uses the ablation plasma as a light source that contains the elemental composition information of the sample. However, the laser-induced plasma exhibits a transient behavior. Although time-resolved and gated detection can greatly improve the performance of the LIBS technique especially that of calibration-free LIBS (CF-LIBS) with a better determination of plasma temperature, the temporal evolution of the plasma is correlated to its morphology and its spatial inhomogeneity. The determination of the morphology as well as the internal structure of the plasma together with their evolution during plasma expansion into the ambien...
2015
This work discusses time-resolved measurements of atomic and diatomic spectra following laserinduced optical breakdown. Spatiallyand temporallyresolved spectroscopy is employed to characterize micro-plasma generated in laboratory air. Starkbroadened atomic emission profiles for hydrogen alpha and beta are utilized to determine plasma characteristics for specific time delays. The plasma dynamics include: variations in Stark-broadening emission of hydrogen alpha and beta lines, occurrence of molecular spectra due to recombination radiation, and change in line shape and appearance of atomic and molecular spectra due to collision and plasma oscillations. Comparisons of electron densities determined from hydrogen alpha and nitrogen II lines allow one to evaluate hydrogen self-absorption effects within the laser-induced plasma. Of interest are laser ablation measurements of atomic and diatomic emission spectra from aluminium (Al), aluminium monoxide (AlO), titanium (Ti), titanium monoxide...
Characterization of a laser plasma produced from a graphite target
Journal of Physics: Conference Series, 2014
In order to improve the understanding of pulsed laser deposition (PLD) of diamondlike carbon (DLC) films, we have initiated a detailed study of the plasma dynamics of laser produced carbon plasmas. The carbon plasma is produced by focusing a Nd:YAG laser pulse, 380 mJ, 4 ns at 1.06 m, onto a graphite target, at a background pressure of 0.3 mTorr. Time resolved optical emission spectroscopic (OES) observations of the carbon plasma plume are obtained, with time and space resolution, using a SpectraPro 275 spectrograph, with a 15 ns MCP gated OMA. Line emission from CII to CIV carbon ions is identified at different stages of the plasma evolution. Line intensity ratios of successive ionization stages, CIII/CIV, was used to estimate the electron temperature throughout the Saha-Boltzmann equation, under the assumption of local thermodynamic equilibrium (LTE), and Stark broadening of CII lines was used to obtain measurements of the electron density. Characteristic plasma parameters, short after plasma formation, are 3.0 eV and 2⋅10 17 cm −3 , which after 60 ns of plasma expansion decay to 2.7 eV and 5⋅10 16 cm −3 , respectively.
We built a collinear dual-pulse laser-induced breakdown spectroscopy (DP-LIBS) system to study the aluminum (Al) plasma emission by installing a pair of Nd: YAG lasers operating at 266 and 1064 nm. The spectral intensities of selected aluminum doubly-ionized lines were employed to evaluate the optical emission spectra. The influences of the energy ratio of two pulsed lasers on the LIBS intensity for different Al doubly-ionized spectral lines were investigated. The de-excitation rate parameters of the excited ion and the electron impact excitation were computed using the analytical formulas proposed by Smeets and Vriens. The transition probabilities and energy states were computed using Hibbert's configuration interaction, computer package (CIV3). By solving the coupled rate equations including 1s 2 2s 2 2p 6 ns ( 2 S), 1s 2 2s 2 2p 6 np ( 2 P), 1s 2 2s 2 2p 6 nd ( 2 D) (n = 3-5) and 1s 2 2s 2 2p 6 nf ( 2 F) (n = 4, 5) states, the level population densities were computed. We also proposed a theoretical population model in order to investigate the effectiveness of the various processes that might affect the population of the upper levels in Al plasma by using the rate coefficients. In addition, the population densities for the 19 upper levels were also computed. Good compatibility between the experimental and the theoretical model data had been observed. Our results might be significant as reference data for the optimization of the DP-LIBS spectrometry and diagnostics of laser produced plasmas.
2000
A comparative study of ablation plasma plumes originated from single crystal graphite ͑SCG͒ and amorphous carbon (a-C) targets during the preparation of diamond-like carbon ͑DLC͒ films by KrF excimer pulsed laser deposition ͑PLD͒ has been carried out by means of a monochromator equipped with an intensified optical multichannel analyzer. In high vacuum, the emission lines of carbon neutral C and ions of C ϩ , C 2ϩ , and C 3ϩ can be observed from both the SCG and a-C plasma plumes. The emission intensity from C atoms increases with laser energy density (E L) increase for both cases. The C 2 emission intensity from the SCG plasma plume changes drastically with E L , while that from the a-C plasma plume is almost constant. The C 2 /C emission intensity ratio for the a-C case decreases with E L increase. As for the SCG case, the C 2 /C ratio decreases with E L increase up to 3.0 J/cm 2 , and increases slightly with further E L increase. Nanohardness of the deposited films decreases with the increase of the C 2 /C emission intensity ratio. It is suggested that for both the SCG and a-C target cases, the C 2 molecule in the ablated plasma plume may not play an important role in producing high quality DLC films. It is further proposed that the threshold of laser fluence for the formation of diamond-like character film using KrF excimer PLD is 2.1 J/cm 2 (0.84ϫ10 8 W/cm 2) for the a-C target and 3.0 J/cm 2 (1.2ϫ10 8 W/cm 2) for the SCG target. The C 2 vibrational temperature of the SCG and the a-C plasma plumes show different features on both the laser energy density and nitrogen pressure dependencies. Through optical emission spectroscopy and Langmuir probe measurements in vacuum and nitrogen background, it is concluded that there are many particles with higher mass in the SCG plasma plume, especially at relatively lower laser energy density below 3.0 J/cm 2 .
We report measurements of time-and spatially averaged spontaneous-emission spectra following laserinduced breakdown on a solid graphite/ambient gas interface and on solid graphite in vacuum, and also emission spectra from gas-phase optical breakdown in allene C 3 H 4 and helium, and in CO 2 and helium mixtures. These emission spectra were dominated by CII (singly ionized carbon), CIII (doubly ionized carbon), hydrogen Balmer beta ͑H  ͒, and Swan C 2 band features. Using the local thermodynamic equilibrium and thin plasma assumptions, we derived electron number density and electron temperature estimates. The former was in the 10 16 cm Ϫ3 range, while the latter was found to be near 20000 K. In addition, the vibration-rotation temperature of the Swan bands of the C 2 radical was determined to be between 4500 and 7000 K, using an exact theoretical model for simulating diatomic emission spectra. This temperature range is probably caused by the spatial inhomogeneity of the laser-induced plasma plume. Differences are pointed out in the role of ambient CO 2 in a solid graphite target and in gas-phase breakdown plasma.
Time-Resolved Emission Spectroscopy of Atomic and Molecular Species in Laser-Induced Plasma
2018
This work examines atomic and molecular signatures in laser-induced plasma in standard ambient temperature and pressure environments, including background contributions to the spectra that depend on the laser pulse-width. Investigations include solids, gases, and nano-particles. Abel inversions of measured line-of-sight data reveal insight into the radial plasma distribution. For nominal 6 nanosecond laser pulses and for pulse-energies in the range of 100 to 800 milli-Joules, expansion dynamics and turbulence due to shock phenomena are elucidated to address local equilibrium details that are frequently assumed in spatially averaged emission spectroscopy. Chemical equilibrium computations reveal temperature dependence of selected plasma species. Specific interests include atomic hydrogen (H) and cyanide (CN). The atomic H spectra, collected following optical breakdown in ultra-high-pure hydrogen and 9:1 mixtures of ultra-pure hydrogen and nitrogen gases, indicate spherical shel...