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Laser-induced plasmas are widely used in many areas of science and technology; examples include spectrochemical analysis, thin film deposition, and material processing. Several topics will be addressed. First, general phenomenology of laser-induced plasmas will be discussed. Then, a chemical model will be presented based on a coupled solution of Navier-Stokes, state, radiative transfer, material transport, and chemical equations. Results of computer simulations for several chemical systems will be shown and compared to experimental observations obtained by optical imaging, spectroscopy, and tomography. The latter diagnostic tools will also be briefly discussed. Finally, a prospective application of laser-induced plasma and plasma modeling will be illustrated on the example of chemical vapor deposition of molybdenum borides and micro processing and coating of titanium dental implants.
The paper describes past and present efforts in modeling of laser-induced plasma and overviews plasma diagnostics carried out by pump-probe techniques. Besides general information on existing plasma models, the emphasis is given to models relevant to spectrochemical analysis, i.e. models of radiating plasma. Special attention is paid to collisional-radiative (CR) and collisional-dominated (CD) plasma models where radiative processes play an important role. Also, calibration-free (CF) models are considered which may endow with the possibility for standardless spectroscopic analysis. In the diagnostic part, only methods based on the use of additional diagnostic tools (auxiliary lasers, optics, and probes) are described omitting those based on plasma own radiation. A short review is provided on image-based diagnostics (shadowgraphy, schlieren, and interferometry), absorption and fluorescence, Langmuir probe, and less frequently used cavity ringdown and Thomson scattering methods.
Two topics will be addressed: (1) calibration-free LIBS based on Monte Carlo spectral optimization and (2) insight into the possibility of simultaneous elemental and molecular analysis by LIBS. Both topics heavily rely on modeling of laser induced plasma and are closely connected to experiment. In Monte Carlo LIBS (MC LIBS), concentrations are found by fitting model-generated synthetic spectra to experimental spectra. The model of a static uniform isothermal plasma in local thermodynamic equilibrium is employed. Many configurations of plasma parameters and their corresponding spectra are simultaneously generated using a graphic processing unit (GPU). Using the GPU allows for the reduction of computational time down to several minutes for one experimental spectrum that presents the significant progress in comparison with earlier versions of MC LIBS. The method is tested by analyzing industrial oxides containing various concentrations of CaO, Fe2O3, MgO, and TiO2. The agreement within several percent between found and certified concentrations is achieved. Next, a newly developed collisional-dominated model of a laser induced plasma is introduced. The model includes the coupled Navier-Stokes, state, radiative transfer, and material transport equations and incorporates plasma chemistry through the equilibrium approach based on the use of atomic and molecular partition functions. Simple chemical systems are modeled including ablation of Si and C in N2 and Ar atmospheres.The model is used to study evolution of number densities of atomic and molecular species in the expanding plasma plume. The distribution is compared to experimental observations obtained by optical imaging and tomography. To further verify the model, dynamic plasma
A collisional-dominated model of a laser induced plasma will be presented which includes the coupled Navier-Stokes, state, radiative transfer, and material transport equations and incorporates plasma chemistry through the equilibrium approach based on the use of atomic and molecular partition functions. Simple chemical systems are modeled including ablation of Si, C, SiC, CaCl2 in N2 or Ar atmospheres with the formation of molecules N2, C2, Si2, CN, Cl2, SiN, SiC, CaCl, CaCl2 and their corresponding positive and negative ions1,2. The initial conditions are specially chosen to emulate the plasma state on the onset of expansion just after the laser pulse had ended.
The model is used to study evolution of number densities of atomic and molecular species in the expanding plasma plume. The distribution is compared to experimental observations obtained by optical imaging and tomography. The model and experiment serve to elucidate mechanisms of molecular formation in LIPs, the topic which has recently received much attention in the LIBS community.
A collisional-dominated model of a laser induced plasma will be presented which includes the coupled Navier-Stokes, state, radiative transfer, and material transport equations and incorporates plasma chemistry through the equilibrium approach based on the use of atomic and molecular partition functions. Simple chemical systems are modeled including ablation of Si, C, SiC, CaCl2 in N2 or Ar atmospheres with the formation of molecules N2, C2, Si2, CN, Cl2, SiN, SiC, CaCl, CaCl2 and their corresponding positive and negative ions. The model is used to study evolution of number densities of atomic and molecular species in the expanding plasma plume. The distribution is compared to experimental observations obtained by optical imaging and tomography. The model and experiment serve to elucidate mechanisms of molecular formation in LIPs, the topic which has recently received much attention in the LIBS community.
We found cascade IR generation in Al laser induced plasma. This generation includes doublet transitions 3s25s 2S1∕2→ 3s24p 2P1∕2,3∕2 → 3s24s 2S1∕2; corresponding to strong lines at 2110 and 2117 nm, and much weaker lines at 1312–1315 nm. The 3s25s2S 1∕2 starting IR generation level is directly pumped from the 3s23p 2P3∕2 ground level. The starting level for UV generation at 396.2 nm (transitions 3s24s 2S1∕2 → 4p 2P3∕2) is populated due to the fast collisional processes in the plasma plume. These differences led to different time and special dependences on the lasing in the IR and UV spectral range within the aluminum laser induced plasma.
This course will provide an introduction to plasma diagnostic techniques. The major focus of the course will be on the discussions of the practical procedures as well as the underlying physical principles for the measurements of plasma fundamental characteristics (e.g., temperatures, thermodynamic properties, and electron number density). Particular emphasis will be placed on inductively coupled plasma–atomic emission spectrometry, but other analytical plasmas will also be used as examples when appropriate. Selected examples on how one can manipulate the operating conditions of the plasma source, based on the results of plasma diagnostic measurements, to improve its performance used for spectrochemical analysis will also be covered. Topics to be covered include thermal equilibrium, line profiles, temperatures, electron densities, excitation processes, microreactions, pump and probe diagnostics, tomography, temporal and spatial resolution. Basis of plasma computer modeling will be presented.
Laser induced plasma (LIP) is a dynamic, short living event which presents significant difficulty for modeling. In this report, a collisional-dominated chemical model developed earlier* is expanded by the inclusion of a new method for calculation of chemical reactions. The model consists of the coupled Navier-Stokes, state, radiative transfer, material transport, and chemical equations. The latter are written in terms of atomic and molecular partition functions rather than reaction rates. Typically, a solution of such the system of chemical equations is difficult for the entire range of plasma temperatures and densities because reaction constants may vary by hundreds orders of magnitude owing to extreme plasma conditions. No numerical solver of non-linear systems of equations handles this situation with ease. We resolve the problem by using a hierarchical approach. First, we rank the reactions according to their ascendancy. Second, we exploit either the contraction or Newton-Raphson algorithms to solve the system of chemical equations. We illustrate the approach by performing a series of calculations for reacting species Si, C, N, Ca, Cl and their molecules in laser induced plasmas.