Reactivity and relaxation of vibrationally excited molecules with open shell atoms

Accurate modelling of low temperature plasmas requires molecular collision input data including the detail over the whole ladder of vibrational states v. This is a big challenge, because it is difficult to obtain these data from experiments. On the theoretical-computational side, these data are obtained with the aid of simple models (for inelastic processes essentially based on forced harmonic oscillator (FHO) mechanism), or with molecular dynamics at various levels (quasiclassical (QCT), semiclassical (SC), approximate (AQ) and exact (EQ) quantum mechanical methods, with computational cost rapidly increasing with accuracy). It is found that low collision energy inelastic probabilities and related thermal and sub-thermal rates could not be properly evaluated without (a) accurately formulating the long range tail of the Potential Energy Surface (PES) and (b) going beyond a pure QCT dynamical treatment. Item (a) was tackled by smoothly connecting the strong interaction region fitted to highly accurate ab initio values using polynomials in bond order like variables to an Improved Lennard Jones ILJ one parameterized to computed ab initio and measured inelastic scattering data. Item (b) was tackled by overtaking the inadequacy of classical trajectory treatments by adopting quantum classical (QC) approximations for diatom diatom systems and exact quantum treatments for atom diatom ones. In this way diatom diatom QC calculations were able to reproduce anti-Arrhenius behaviours of quasiresonant non reactive N2+N2 rate coefficients at threshold [1] while EQ calculations were able to single out pure N+N2 vibrational pumping up mechanisms [2]. However, the most accurate method can become unfeasible when its application is extended to the total energy ranges typically required in plasma modelling. As shown in [3] for H+HeH+(v) reaction, the computational cost of time independent EQ calculations increases very rapidly with collision energy, while the QCT method is extremely efficient and accurate from intermediate to high collision energies, including state-to-state results for this very light collisional system. The best strategy that can be devised is to study the limits of application of less accurate methods in order to use them as a seamless continuation of EQ calculations on the total energy axis. In this sense it is the work, started in [4-5], and currently in development about inelastic processes in O+N2(v) collisions in the gas temperature range from 300 to 20000K. The aspect of special interest in [4-5] is the indication of a criterion for easily separating the accurate QCT contribution to the inelastic process (the quasi-reactive part, indeed [4-5]) from the less reliable one. As a consequence, a SC method can be used with a similar criterion for obtaining the complementary missing information. This procedure allows to limit the use of more expensive SC methods to quite low ranges of total energy, maintaining both a high level of accuracy and a high computational efficiency.