Radiation Mechanism for Chemically Active Plasma

Customer Goals and Requirements

One of the leading suppliers of fluorescent lightings in the Unite States and all over the world has initiated investigations aimed at improving lamp efficiency and removing mercury from lamps. Such investigations require the evaluation of numerous candidates in their ability to provide visible light with a high yield and good color rendering. This cannot be accomplished without a deep understanding of physical and chemical processes in plasma. The analysis of light radiation efficiency needs detailed primary information on the cross sections and probabilities of elementary processes and advanced tools for modeling the kinetics of non-equilibrium discharges in molecular gases.

Kintech Solution

Kintech Lab has developed a multiscale multiphysics approach to the prediction of plasma composition, emission spectra, and low-pressure gas-discharge power and performed an extended screening of promising radiation sources for luminescent lamps using the family of Kintech programs, Chemical WorkBench and Khimera. This approach serves an independent predictive research tool complementary to experimental techniques. It proved to be very useful for selecting the most efficient radiators and the best plasma conditions.

Integrated Procedure of the Predictive Modeling of Plasma Kinetics

Ab initio calculations of unknown parameters for atoms, molecules, and their interactions
Evaluation of cross sections, rate constants, and radiation parameters
Construction of a physicochemical mechanism
Kinetic modeling, mechanism reduction, determination of key kinetic stages

Below, the procedure is illustrated for the case of Ar–GaI₃ low-pressure glow discharge.

Step 1. Calculation of the unknown parameters of atoms, molecules, and their interactions.
State-of-art ab initio calculations give sufficiently accurate nonempirical estimates of electronic energies, structures, multipole momentums, transition probabilities and other parameters for diatomic and polyatomic molecules both in the ground and electronically excited states. With these data, one can estimate cross sections of electronic processes with an accuracy 5-20%.

: Electronic states and excitations in GaI Ga excitation

Steps 2&3. Evaluation of rate parameters and mechanism construction.
Currently available methods of electronic structure calculations for molecules in the ground state and scanning potential energy surfaces were used to predict heats of reactions and activation barriers to within a few kcal/mol. Rate constants of direct thermal molecular reactions were accurately calculated using the standard transition state theory. Kinetic mechanism including neutral and ion-molecular reactions and also electronic processes was constructed.

: Reaction profile for GaI₂+ GaI₃^– -> GaI+GaI₄^–

Step 4. Kinetic modeling and approach validation.
A numerical model comprising a plasma chemistry module involving balance equations for charged and neutral species coupled with an electric circuit equation allows the description of the principal characteristic of the discharge. Sensitivity analysis indicates the influence of various reactions on the emission efficiency. Finally, optimum conditions in term of radiation efficiency are determined.

Step 5. Comparison with the experiment: emission spectra.
A comparison of the experimentally observed and calculated emission spectrum of atoms and molecules demonstrates the predictive character of our approach.

Mechanism details see in M.Deminsky at al., J. Phys. D: Appl. Phys. 40 (2007) 3857–3881