Dynamics on Multiple Potential Energy Surfaces: Quantitative Studies of Elementary Processes Relevant to Hypersonics

Abstract

The determination of thermal and vibrational relaxation rates of triatomic systems suitable for application in hypersonic model calculations is discussed. For this, potential energy surfaces for ground and electronically excited state species need to be computed and represented with high accuracy and quasiclassical or quantum nuclear dynamics simulations provide the basis for determining the relevant rates. These include thermal reaction rates, state-to-state cross sections, or vibrational relaxation rates. For exemplary systems -[NNO], [NOO], and [CNO] -all individual steps are described and a literature overview for them is provided. Finally, as some of these quantities involve considerable computational expense, for the example of state-to-state cross sections the construction of an efficient model based on neural networks is discussed. All such data is required and being used in more coarse-grained computational fluid dynamics simulations.

Quantitative Molecular Simulations

“…One recent field which has witnessed quantitative simulations concerns small-molecule reactions relevant to hypersonics and atmospheric re-entry. [158][159][160][161] For this, a simulation strategy involving high-level electronic structure calculations, global and reactive RKHS-based PESs and QCT simulations has been successfully developed and used for a range of atom+diatom reactions. 60,162,163 The focus was primarily on computing thermal reaction rates, final state distributions and vibrational relaxation times over a wide temperature range, up to 20000 K. A typical PES for one electronic state is based on 10 4 energies and the reference energies are typically reproduced within a few cm −1 by the RKHS interpolation.…”

Machine Learning for Observables: Reactant to Product State Distributions for Atom-Diatom Collisions

“…Here, i ∈ [E trans , v, j, E trans , v , j ] labels the degree of freedom, n labels the data set, N ∈ [2,3] is the total number of distributions drawn from Set2, the corresponding distributions P n (i) used for and obtained from QCT simulations and the random weights…”

“…In the following, a single data set for Set3 is generated by randomly specifying the number of distributions N ∈ [2,3] to be combined although larger values for N are possible and will be explored later. The final set of reactant and product state distributions is characterized by N sets of temperatures T and corresponding normalized weights w = (w 1 /w tot , ..., w N /w tot ).…”

“…The realistic description of chemical and reactive systems with a large number of available states, such as explosions, hypersonic gas flow around space vehicles upon re-entry into the atmosphere, or meteorites penetrating deep into the lower parts of Earth's or a planet's atmosphere, requires an understanding of the relevant processes at a molecular level. [1][2][3] Correctly describing the population of the available state space under such nonequilibrium conditions (e.g. high temperatures in atmospheric re-entry with T > 10000 K) from ground-based experiments is extremely challenging.…”

Perspective: Non-conventional Force Fields for Applications in Spectroscopy and Chemical Reaction Dynamics

“…Extensive QCT simulations on these PESs now provide a comprehensive and consistent set of thermal rates and vibrational relaxation times for the most relevant systems involved in hypersonics. 150…”