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 (a3Πu) of the C2 molecule is only 2.1 kcal/mol, two reactions, O(3P) + C 2(X1Σg+) and O(3P) + C 2(a3Πu), 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(a3Πu) 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(a3Πu) 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(a3Πu), about 50% of the available energy ends up as electronic excitation. The partitioning for each electronic state in the C2(a3Πu) 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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films