Electronic Population Inversion in HCCO/DCCO Products from Hyperthermal Collisions of O((3)P) with HCCH/DCCD

The dynamics of hyperthermal O((3)P) reactions with acetylene have been investigated with the use of crossed molecular beams techniques, employing both mass spectrometric and optical detection of products. With collision energies of 40-150 kcal mol(-1), O((3)P) + HCCH/DCCD → HCCO/DCCO + H/D may follow multiple pathways to form the ketenyl radical (HCCO or DCCO) in ground doublet states or in electronically excited quartet and doublet states. Theoretical calculations support the assignment of the various reaction pathways. The fraction of electronic excitation is substantial. At the highest collision energy studied, ∼65% of the ketenyl radical products that survive are electronically excited, with the majority of the excited products in a quartet state. In this case, a population inversion exists between the electronically excited quartet and ground doublet states of the ketenyl product. Such significant electronic excitation in products is unusual in bimolecular reactions, especially when ground-state products are accessible by spin-allowed pathways.