Vibrational and Rotational Excitation and Dissociation of CO2 Reexamined

Abstract

Quantum chemistry calculations are undertaken to elucidate the potential energy surfaces of the lowest singlet (S0) and triplet (T1) electronic states of CO 2 . The region of the potential energy surfaces where the S0 and T1 states cross is determined and its effect on the CO 2 dissociation rate coefficients is discussed. It is suggested that the singlet-triplet transition is the bottleneck in the CO 2 excitation process that controls the temperature dependence of the dissociation rate. NASA is currently evaluating the requirements for the safe landing of large spacecraft (> 40 MT) on Mars. The most recent NASA aerothermodynamics model for hypersonic Mars entry was devised by Park and co-workers [1] nearly 20 years ago. It is based on a simple model (a modification of the Landau-Teller formula) for vibration excitation and relaxation [2-5] and 20-40 year old chemical kinetics data (see [1]). In this model, separate translation and vibration temperatures (T and T V ) are used to describe the thermal nonequilibrium behavior and rotation is assumed to be equilibrated with translation. The composition of the Martian atmosphere is approximately 95% CO 2 , 3% N 2 and 2% Ar and the average pressure at the surface is 600 Pa [6] (less than 1% of the atmospheric pressure at the Earth’s surface). Thus, aerothermal models for Mars entry must include an accurate treatment of the major molecular species CO 2 , CO and N 2 and, for predicting heat flux, the minor species CN and C 2 . In the original model [1], CO 2 dissociation was assumed to be extremely rapid and little attention was paid to the parameters for its vibration relaxation and dissociation. Furthermore, it was assumed that the CO 2 vibration modes are strongly coupled and that all modes could be described by a single vibrational relaxation time, even though the asymmetric stretch mode is expected to relax more slowly at very low pressures than the symmetric stretch and bending modes. Specifically, recent developments have shown the need for reexamination of the chemistry model for Mars entry. Aerocapture trajectories have been proposed for Mars missions [6] that would involve low-speed entry of very large vehicles, resulting in incomplete CO 2 dissociation and radiative heating from CO 2 IR bands. The characterization of shock heated CO 2 produced in ground-test facilities at NASA Ames and CUBRC [7] is uncertain because CFD simulations using the Park Mars chemistry model do not match observations [8,9]. The possibility of direct formation of