Theoretical studies of the O(³P) + C₂ reaction at hyperthermal energies

The O + C₂ 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 (X¹Σg₊) and the first excited triplet state (a³Π_u) of the C₂ molecule is only 2.1 kcal/mol, two reactions, O(³P) + C ₂(X¹Σg₊) and O(³P) + C ₂(a³Π_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( ³P) + C₂(a³Π_u) reaction can occur on singlet, triplet, and quintet surfaces to give the spin-allowed electronically excited CO(¹Σ) + C(¹D), CO( ³Π) + C(³P), and CO(³Π) + C( ¹D) products as well as the ground state product CO( ¹Σ) + C(³P), with CO(³Π) + C( ³P) being the most important, while O(³P) + C ₂(X¹Σg₊) reacts on triplet surfaces to give primarily the CO(¹Σ) + C(³P) 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(³P) reacting with C₂(a³Π_u) is three times smaller than with C₂(X¹Σg₊). 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(³P) + C₂(X¹Σg₊) 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(³P) + C ₂(a³Π_u), about 50% of the available energy ends up as electronic excitation. The partitioning for each electronic state in the C₂(a³Π_u) reaction is strongly state dependent, but for CO(³Π) + C(³P) product vibrational excitation accounts for 60% of the energy in excess of electronic energy.