High-Temperature and Nonequilibrium Partition Function and Thermodynamic Data of Diatomic Molecules

Abstract

Accurate molecular partition functions are required to determine both thermodynamic properties for equilibrium and nonequilibrium flow field calculations, and energy level populations for radiative heat transfer in high enthalpy flows, for instance. Thermodynamic functions of diatomic molecules are computed in this study at high temperatures and for nonequilibrium media in the framework of multitemperature models. Partition functions, average energies, and specific heats of N 2 , N + 2 , NO, O 2 , CN, C 2 , CO, and CO + are calculated up to 50,000 K by direct summation over energy levels using recent and accurate spectroscopic data and dissociation energy values. Estimates are made for the error introduced by neglecting the highest considered electronic states. For nonequilibrium media, the sensitivity of the multitemperature thermodynamic functions to different schemes for energy partitioning is discussed. In particular, the effects of vibration-rotation coupling are investigated. The tabulated results are available upon request.

Partition functions and equilibrium constants for diatomic molecules and atoms of astrophysical interest

“…Thus, we expect that the calculations presented here for these cases are of similar accuracies to these studies. For example, in the important case of CO, we note that the spectroscopic constants employed by us and those employed by Babou et al (2009) for the ground state agree in all cases to better than four significant figures, and it was seen in Fig. 3 that the partition functions agree almost exactly.…”

RADIS: A nonequilibrium line-by-line radiative code for CO2 and HITRAN-like database species

“…In a nonequilibrium model where T vib ̸ = T rot the inclusion of coupling terms raises another question: shall they be assigned to the vibrational or to the rotational energy? For diatomic molecules, this question has been addressed by Babou et al [40], who wrote the total rovibrational energy as the sum of the uncoupled energies for each energy mode E vib , E rot , and an interaction terms E coupling :…”

The High Temperature Vibrational Partition Function of Stretching of Triatomic Systems

“…All those applications may require very high temperatures (for example in hypersonic flows temperatures exceeding 10,000 K are considered) which is the perfect situation for a fully classical approach. At high temperatures the ro-vibrational partition functions are known to disagree between various studies [19], this happens even for the simplest H + 2 molecule [20]. The same can be expected for vibrational partition functions too.…”