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

Abstract

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.

Dynamics of the O(3P) + C2H2 reaction from crossed molecular beam experiments with soft electron ionization detection

“…This suggests that the H 2 elimination pathway is closed at these E c s. Recent QCT calculations for reaction (1) at very high (hyperthermal) collision energies have pointed out that the H 2 -elimination channel can give a small contribution only starting from a collision energy of about 190 kJ mol À1 , thus corroborating our experimental findings. 41 For quantitative and physically meaningful information on the reaction dynamics, the product angular and velocity distributions A coupling may exist between the product angular and velocity distribution in the CM frame. Such a coupling may be subtle and usually high S/N is needed to disentangle it.…”

“…More recently, the reaction was also studied using Fourier transform infrared (FTIR) emission spectroscopy in a flow system at room temperature, where the vibrational population of CO was determined, 40 and in CMB experiments at hyperthermal collision energies, where other highly endothermic channels become open. 41 Considering the importance of the reaction of acetylene with atomic oxygen in hydrocarbon combustion and flames, we have re-investigated the dynamics and the branching ratio of this reaction using an improved CMB technique which exploits (i) an increased instrumental sensitivity, (ii) an improved (with respect to previous CMB studies) resolution for measuring product angular and time-of-flight (TOF) distributions, and (iii) the new ability to detect cleanly the CH 2 radical by using soft electronionization (EI). 42,43 A preliminary account of this work was reported in a Communication 44 showing some data at one collision energy.…”

“…The dynamic results are discussed in light of the most recent electronic structure calculations of the relevant PESs. 7,41,45…”

Dissection of the Multichannel Reaction O(3P) + C2H2: Differential Cross-Sections and Product Energy Distributions

“…Due to its vital significance in combustion chemistry, this reaction has attracted great attention. The kinetic and dynamic properties for this reaction have been measured and calculated over a wide temperature range using various techniques [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ]. Until recently, the experimental rate constants and branching ratios (BRs) have been confirmed by quasi-classical trajectory calculations based on the global potential energy surface (PES) at UCCSD(T)-F12b/VTZ-F12 level developed by our group [ 29 ].…”

“…The crossed molecular beam (CMB) experiments of the O( 3 P) + C 2 H 2 reaction have been investigated by several groups [ 10 , 14 , 19 , 21 , 22 ]. Clemo et al [ 10 ] determined the translational-energy dependence of the total cross-section by using a supersonic beam of O atoms seeded in He.…”