Carbon Clusters: Thermochemistry and Electronic Structure at High Temperatures

This paper studies the thermochemistry and electronic structure of small carbon clusters and hydrocarbons which are major constituents of pyrolysis gases released into the boundary layer of ablating heat shields. Our focus lies on clusters of up to four carbon atoms including C3H and C4H. A study of electronically excited states of these molecules computed using the EOM-CCSD method has revealed C4 and C4H to be potential sources of radiation absorption in the boundary layer. We also study the effect of low-lying excited electronic states on the thermodynamics in the high temperature regime and show that neglecting these states records a difference of upto 12% in the computed Cp values. Finally, comparisons of the equilibrium mole fractions obtained using the thermodynamics computed in this paper with the existing state-of-the-art tables used for hypersonic applications (for example JANAF and Gurvich Tables) show an order of magnitude difference in the mixture compositions.