Dissociation and internal excitation of molecular nitrogen due to N + N₂collisions using direct molecular simulation

In this work we present a molecular level study of N2+N collisions, focusing on excitation of internal energy modes and non-equilibrium dissociation. The computation technique used here is the direct molecular simulation (DMS) method and the molecular interactions have been modeled using an ab−initio potential energy surface (PES) developed at NASA’s Ames Research Center. We carried out vibrational excitation calculations between 5000K and 30000K and found that the characteristic vibrational excitation time for the N + N₂process was an order of magnitude lower than that predicted by the Millikan and White correlation. It is observed that during vibrational excitation the high energy tail of the vibrational energy distribution gets over populated first and the lower energy levels get populated as the system evolves. It is found that the non-equilibrium dissociation rate coefficients for the N + N₂process are larger than those for the N₂+ N₂process. This is attributed to the non-equilibrium vibrational energy distributions for the N + N₂process being less depleted than that for the N₂+ N₂process. For an isothermal simulation we find that the probability of dissociation goes as 1/T_trfor molecules with internal energy (∈_int) less than ∼ 9.9eV,while for molecules with ∈_int>9.9eV the dissociation probability was weakly dependent on translational temperature of the system. We compared non-equilibrium dissociation rate coefficients and characteristic vibrational excitation times obtained by using the ab − initio PES developed at NASA’s Ames Research Center to those obtained by using an ab − initio PES developed at the University of Minnesota. Good agreement was found between the macroscopic properties and molecular level description of the system obtained by using the two PESs.