Theoretical Studies of the O(3P) + C2 Reaction at Hyperthermal Energies

The O + C-2 reaction has been investigated with the quasiclassical trajectory (QCT) method in conjunction with direct dynamics electronic structure calculations using density functional theory (DFT) forces. Trajectory surface-hopping calculations have also been performed to study spin-forbidden reactions. Calculations were performed at collision energies of 1-5 eV so as to simulate conditions relevant to erosion of carbon-based materials on spacecraft in low Earth orbit (LEO). Since the energy difference between the electronic ground state (X-1 Sigma(+)(g)) and the first excited triplet state (a(3)Pi(u)) of the C-2 molecule is only 2.1 kcal/mol, two reactions, O(P-3) + C-2(X-1 Sigma(+)(g)) and O(P-3) + C-2(a(3)Pi(u)), have been studied. We present here the detailed mechanism, electronic branching, product energy disposal, and angular distribution for these reactions. The calculations show that the O(P-3) + C-2(a(3)Pi(u)) reaction can occur on singlet, triplet, and quintet surfaces to give the spin-allowed electronically excited CO((1)Sigma) + C(D-1), CO((3)Pi) + C(P-3), and CO ((3)Pi) + C(D-1) products as well as the ground state product CO((1)Sigma) + C(P-3), with CO((3)Pi) + C(P-3) being the most important, while O(P-3) + C-2(X-1 Sigma(+)(g)) reacts on triplet surfaces to give primarily the CO((1)Sigma) + C(P-3) product with only minor branching to spin-forbidden excited states. Reactions at 1 eV energy proceed on all surfaces through formation of the collision complex CCO, while the collision complex only forms briefly at 5 eV. The CO + C cross section for O(P-3) reacting with C-2(a(3)Pi(u)) is three times smaller than with C-2(X-1 Sigma(+)(g)). Angular distributions show that the product CO + C is more and more backward scattered as collision energy is increased as can be explained in terms of collision lifetime shortening at higher energies. Product energy disposal shows that for O(P-3) + C-2(X-1 Sigma(+)(g)) about 50% of the total available energy is deposited in relative translation, 10% is in CO rotation, and 40% is in CO vibration. For O(P-3) + C-2(a(3)Pi(u)), about 50% of the available energy ends up as electronic excitation. The partitioning for each electronic state in the C-2(a(3)Pi(u)) reaction is strongly state dependent, but for CO((3)Pi) + C(P-3) product vibrational excitation accounts for 60% of the energy in excess of electronic energy.