The role of molecular vibration in non-equilibrium plasma modelling is nowadays well recognized in the literature [1]. The internal energy of molecules acts as a sort of energy bank, which can store and release energy with specific processes (collisions with other molecules and with electrons, radiation) and characteristic times, and these processes have strong influence on the overall kinetics. A state-to- state kinetic model includes not simply the chemical species participating in a phenomenon, but also their behaviour as a function of their internal energy. In order to perform this kind of accurate modelling there is the need of detailed data, concerning in particular molecular collision (reactive, inelastic and dissociation/recombination processes), in the form of rate coefficients or cross sections depending on initial and final molecular vibration. Experimental data with this level of detail are scarce and generally limited to very few vibrational states. In the past the input data of vibrational kinetics were obtained by using simple models of vibrational energy transfer, based essentially on a forced harmonic oscillator model. However, this model makes sense for low vibrational energy transitions, and for "purely" inelastic processes [2]. When the collisional system includes one or more reactive channels, the vibrational kinetics of inelastic processes can be significantly different if the reaction threshold is approached or not [3]. Molecular dynamics calculations can give valuable contributions to the solution of these issues. Different methods are available, with different levels of accuracy and requirements of computational resources. However, when dealing with modelling, complete sets of rate coefficients (in the sense of including the whole vibrational ladders of both reagents and products) are required, with ranges of collision energy normally quite large. As a consequence, the only possible strategy should be to merge results from different methods. This has been done in [4], where reaction of light species as a function of initial and final vibration is very accurately reproduced using quasiclassical trajectory method in comparison with accurate time independent quantum mechanical calculations. Also vibration-dependent dissociation/recombination of light species can be studied using quasiclassical trajectories, provided the results of direct three- body and of orbiting resonance theory are correctly merged [5]. Also in this case good comparisons are obtained with quantum mechanical results. What emerges from all these comparisons of quite different vibrational processes is the importance of studying the correct and convenient limits of application of each dynamical method, in order to obtain the best performance by each method and to merge all the results into a consistent collection of data for modelling.
Reactivity, relaxation and dissociation of molecules in plasma modelling
Esposito F
2017
Abstract
The role of molecular vibration in non-equilibrium plasma modelling is nowadays well recognized in the literature [1]. The internal energy of molecules acts as a sort of energy bank, which can store and release energy with specific processes (collisions with other molecules and with electrons, radiation) and characteristic times, and these processes have strong influence on the overall kinetics. A state-to- state kinetic model includes not simply the chemical species participating in a phenomenon, but also their behaviour as a function of their internal energy. In order to perform this kind of accurate modelling there is the need of detailed data, concerning in particular molecular collision (reactive, inelastic and dissociation/recombination processes), in the form of rate coefficients or cross sections depending on initial and final molecular vibration. Experimental data with this level of detail are scarce and generally limited to very few vibrational states. In the past the input data of vibrational kinetics were obtained by using simple models of vibrational energy transfer, based essentially on a forced harmonic oscillator model. However, this model makes sense for low vibrational energy transitions, and for "purely" inelastic processes [2]. When the collisional system includes one or more reactive channels, the vibrational kinetics of inelastic processes can be significantly different if the reaction threshold is approached or not [3]. Molecular dynamics calculations can give valuable contributions to the solution of these issues. Different methods are available, with different levels of accuracy and requirements of computational resources. However, when dealing with modelling, complete sets of rate coefficients (in the sense of including the whole vibrational ladders of both reagents and products) are required, with ranges of collision energy normally quite large. As a consequence, the only possible strategy should be to merge results from different methods. This has been done in [4], where reaction of light species as a function of initial and final vibration is very accurately reproduced using quasiclassical trajectory method in comparison with accurate time independent quantum mechanical calculations. Also vibration-dependent dissociation/recombination of light species can be studied using quasiclassical trajectories, provided the results of direct three- body and of orbiting resonance theory are correctly merged [5]. Also in this case good comparisons are obtained with quantum mechanical results. What emerges from all these comparisons of quite different vibrational processes is the importance of studying the correct and convenient limits of application of each dynamical method, in order to obtain the best performance by each method and to merge all the results into a consistent collection of data for modelling.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
