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

Abstract

The reaction between ground state oxygen atoms, O((3)P), and the acetylene molecule, C2H2, has been investigated in crossed molecular beam experiments with mass-spectrometric detection and time-of-flight analysis at three different collision energies, Ec = 34.4, 41.1 and 54.6 kJ mol(-1). From product angular and velocity distribution measurements of the HCCO and CH2 products in the laboratory frame, product angular and translational energy distributions in the center-of-mass frame were determined. Measurements on the CH2 product were made possible by employing for product detection the recently implemented soft electron-ionization (EI) technique with low-energy, tunable electrons, which has permitted suppressing interference coming from the dissociative ionization of reactants, products and background gases. It has been found that the title reaction leads only to two competing channels: H + HCCO (ketenyl) and CO + CH2 (triplet methylene). The branching ratio of cross sections between the two competing channels has been determined to be σ(HCCO)/[σ(HCCO) + σ(CH2)] = 0.79 ± 0.05, independent of collision energy within the experimental uncertainty. This value is in line with that obtained in the most recent and accurate kinetics determination at room temperature as well as with that predicted from recent theoretical calculations based on statistical rate theory and weak-collision master equation analysis and on dynamics surface-hopping quasiclassical trajectory calculations on-the-fly on coupled triplet/singlet ab initio potential energy surfaces. The firm assessment of the branching ratio as a function of translational energy for this important reaction, besides its fundamental significance, is of considerable relevance for the implementation of theoretical models of hydrocarbon combustion.

Reaction dynamics of oxygen atoms with unsaturated hydrocarbons from crossed molecular beam studies: primary products, branching ratios and role of intersystem crossing

“…The uncertainty of the overall procedure is attributable to three main reasons. Assuming that the overestimate of the σ ion obtained following Fitch and Sauter [96] is similar for HCCO and CH 2 , their ratio should be reliable to within 20-30% [70]. However, it should be noted that the recently developed 'modified additivity rules' appear to give satisfactory results also for polyatomic radicals; furthermore, the total and partial ionisation cross sections of CH 2 are experimentally known and this has permitted to test the additivity rules (see Ref.…”

“…Exploiting other experimental improvements, which have led to an increased sensitivity of the CMB instrument [64,68], we have shown [17,65,66,70] that the simple approach of soft EI permits in many cases to achieve real universal detection in CMB experiments, and thus to identify all primary reaction products of multichannel reactions, determine their BRs, and characterise the reaction dynamics of each channel (see Section 3). As an example of the improvements obtained with soft EI, in Figure 2 we report a mass spectrum of the allene (C 3 H 4 ) reactant beam used in studies of the O ( 3 P) + allene reaction (see Section 3.3) obtained by employing 60 eV (top panel) and 13 eV (bottom panel) electron energy.…”

“…An accurate knowledge of the BRs is certainly of high interest in the modelling of combustion systems. This works very well for the simplest case of two competing channels, as in the O( 3 P) + C 2 H 2 reaction [70]; for more complex cases, when more competing channels are present (and this is the case for most of the systems investigated) when some m/z cannot be explored, we rely on the fragmentation pattern of the stable molecule/radical or of a similar molecule. Unfortunately, these quantities are not easy to determine accurately in a CMB study.…”

“…To solve this problem, soft photoionisation (PI) by tunable VUV synchrotron in CMB machines was first developed at the Advanced Light Source (ALS) of Berkeley (USA) [50][51][52][53] and, later, at the National Synchrotron Radiation Research Centre in Hsinchu (Taiwan) [54][55][56][57][58][59]. Table-top alternatives include VUV laser ionisation [41,[60][61][62][63] or, as pioneered in our laboratory, soft electron-ionisation (EI) by means of an ioniser with tuneable electron energy [16,17,[64][65][66][67][68][69][70][71][72][73]. Soft EI cannot compete with the high selectivity and resolution of the synchrotron or laser radiation.…”

“…Table-top alternatives include VUV laser ionisation [41,[60][61][62][63] or, as pioneered in our laboratory, soft electron-ionisation (EI) by means of an ioniser with tuneable electron energy [16,17,[64][65][66][67][68][69][70][71][72][73]. In addition to the of O( 3 P) + unsaturated hydrocarbons, which are reviewed here [16,17,65,66,[69][70][71][72][73] we have also investigated a series of complex polyatomic reactive systems such as C( 3 P) + C 2 H 2 [67], C( 3 P) + C 2 H 4 [74], C 2 + C 2 H 2 [75], CN + C 2 H 2 [76], CN + C 2 H 4 [77], N( 2 D) + CH 4 [78,79] [85], and N( 2 D) + C 2 H 6 [86]. Nevertheless, the suppression of the interfering signal was good enough to allow us to perform CMB experiments where all the competing pathways of multichannel reactions could be investigated.…”