Unraveling the Dynamics of the C(³P,D) + C₂H₂ Reactions by the Crossed Molecular Beam Scattering Technique

Abstract

A detailed investigation of the dynamics of the reactions of ground- and excited-state carbon atoms, C(3P) and C(1D), with acetylene is reported over a wide collision energy range (3.6-49.1 kJ mol-1) using the crossed molecular beam (CMB) scattering technique with electron ionization mass spectrometric detection and time-of-flight (TOF) analysis. We have exploited the capability of (a) generating continuous intense supersonic beams of C(3P, 1D), (b) crossing the two reactant beams at different intersection angles (45, 90, and 135 degrees ) to attain a wide range of collision energies, and (c) tuning the energy of the ionizing electrons to low values (soft ionization) to suppress interferences from dissociative ionization processes. From angular and TOF distribution measurements of products at m/z=37 and 36, the primary reaction products of the C(3P) and C(1D) reactions with C2H2 have been identified to be cyclic (c)-C3H + H, linear (l)-C3H + H, and C3 + H2. From the data analysis, product angular and translational energy distributions in the center-of-mass (CM) system for both the linear and cyclic C3H isomers as well as the C3 product from C(3P) and for l/c-C3H and C3 from C(1D) have been derived as a function of collision energy from 3.6 to 49.1 kJ mol-1. The cyclic/linear C3H ratio and the C3/(C3 + c/l-C3H) branching ratios for the C(3P) reaction have been determined as a function of collision energy. The present findings have been compared with those from previous CMB studies using pulsed beams; here, a marked contrast is noted in the CM angular distributions for both C3H- and C3-forming channels from C(3P) and their trend with collision energy. Consequently, the interpretation of the reaction dynamics derived in the present work contradicts that previously proposed from the pulsed CMB studies. The results have been discussed in the light of the available theoretical information on the relevant triplet and singlet C3H2 ab initio potential energy surfaces (PESs). In particular, the branching ratios for the C(3P) + C2H2 reaction have been compared with the available theoretical predictions (approximate quantum scattering calculations and quasiclassical trajectory calculations on ab initio triplet PESs and, very recent, statistical calculations on ab initio triplet PESs as well as on ab initio triplet/singlet PESs including nonadiabatic effects, that is, intersystem crossing). While the experimental branching ratios have been corroborated by the statistical predictions, strong disagreement has been found with the results of the dynamical calculations. The astrophysical implications of the present results have been noted.

Temperature dependent product yields for the spin forbidden singlet channel of the C(3P) + C2H2 reaction

“…These authors derived values for (1a)/(1a)+(1b) which were mostly consistent with those determined by Leonori et al [6]. To obtain the branching fractions for C 3 production (1c)/(1), Costes et al [7] crossed a beam containing an equimolar mixture of C 2 H 2 and C 2 H 4 with a beam containing C-atoms, effectively using the C + C 2 H 4 reaction as a reference.…”

“…Leonori et al [6] used the crossed molecular beam (CMB) scattering technique coupled with soft electron ionization mass spectrometry to obtain angular and translational energy distributions for products at masses m/z = 36 (C 3 ) and 37 (l-/c-C 3 H) over a range of collision energies from 3.6 to 49.1 kJ mol -1 . Using these results, these authors were able to derive product yields for the C 3 + H 2 channel, (1c)/(1), and the one between the two H-atom production channels, (1a)/(1a)+(1b).…”

NON-THRESHOLD, THRESHOLD, AND NONADIABATIC BEHAVIOR OF THE KEY INTERSTELLAR C + C2H2REACTION

“…Figure 6 emphasizes that the spin-forbidden nonadiabatic C 3 + H 2 channel (1c) is the dominant one. The branching ratio R2 = σ (1c)/σ (1) takes values 0.82 and 0.87 at E T = 0.8 and 4.8 kJ mol −1 , respectively, which are higher than those given in the crossed-beam experiments of Leonori et al (2008) at E T 3.5 kJ mol −1 , the bulk study at 300 K of Bergeat & Loison (2001) and the calculations of Mebel et al (2007), and Park et al (2006). It is in line with the crossed-beam study of Gu et al (2007) since extrapolation of their curve based on data at E T 8.8 kJ mol −1 would result in R2 around 0.9 at low E T .…”

“…It now becomes impossible to fit the experimental spectra with a single contribution from reaction (1a). We introduce center-of-mass functions very similar to those obtained by Leonori et al (2008) at E T = 3.5 kJ mol −1 for the scattered C 3 H product. The values ΔH clear that using the ΔH…”

“…Numerous theoretical and experimental studies have aimed at elucidating the dynamics of the C + C 2 H 2 reaction: determinations of the potential energy surfaces (Takahashi & Yamashita 1996;Ochsenfeld et al 1997;Guadagnini et al 1998;Buonomo & Clary 2001;Clary et al 2002;Takayanagi 2005;Mebel et al 2007) including calculations in the full nine dimensions (Park et al 2006), quantum (Buonomo & Clary 2001;Clary et al 2002;Takayanagi 2006), quasi-classical trajectory (Park et al 2006), and statistical calculations (Mebel et al 2007); measurements of differential cross sections and integral cross sections in crossed molecular beam experiments employing electron-impact ionization mass spectrometric time-of-flight analysis for C 3 H and C 3 detection with pulsed Gu et al 2007) or continuous beams (Clary et al 2002;Cartechini et al 2002;Costes et al 2006;Leonori et al 2008) and spectroscopic probing of the H-atom product (Clary et al 2002;Cartechini et al 2002;Costes et al 2006). Three channels have been identified: two H-elimination channels leading to cyclic and linear forms of the C 3 H radical, the former being slightly exoergic and the latter being almost thermoneutral Mebel et al 2007), and one strongly exoergic H 2 -elimination channel which is forbidden in the Born-Oppenheimer approximation, since it requires a nonadiabatic transition from the ground-state triplet potential energy surface to a singlet one: …”