Crossed-beam studies on the dynamics of the C + C2H2 interstellar reaction leading to linear and cyclic C3H + H and C3 + H2

Abstract

The dynamics of the C + C2H2 reaction has been investigated using two crossed molecular beam apparatus of different concepts. Differential cross sections have been obtained for the C(3PJ) + C2H2(X1sigmag+) --> l/c-C3H + H(2S1/2) reaction in experiments conducted with pulsed supersonic beams and variable beam crossing angle configuration at two relative translational energies ET = 0.80 and 3.5 kJ mol(-1). H(2S1/2) atoms were detected by time-of-flight mass spectrometry with sequential excitation to the 2PJ(o) state using a laser beam tuned at the Lyman-alpha transition around 121.567 nm and ionisation by a second laser beam at 364.7 nm. Doppler-Fizeau spectra of the H atoms were recorded with the Lyman-alpha laser beam parallel to the relative velocity vector of the reagents. These spectra could be fitted using a forward convolution process including two contributions. The recoil energy distribution functions of both contributions were taken as statistical, with total energies corresponding to a reaction exoergicity deltaH0(o) = -11 kJ mol(-1) for the major one, assigned to the c-C3H + H path, and -1.5 kJ mol(-1) for the minor one, assigned to the l-C3H + H path. The angular distribution was taken as also statistical (uniform) for the minor contribution but somewhat backward peaked for the major one. Differential cross sections have been obtained for the three energetically allowed and competitive C(3PJ) + C2H2(X1sigmag+) --> l/c-C3H + H(2S1/2) and C(3PJ) + C2H2(X1sigmag+) --> C3(X1sigmag+) + H2(X1sigmag+) reaction channels in experiments conducted with supersonic continuous beams under 45 degrees crossing angle configuration using "soft" electron-ionisation mass spectrometry time-of-flight detection at ET = 3.5 and 18.5 kJ mol(-1). From measurements of angular and time-of-flight distributions at the mass-to-charge ratios m/z = 37 and 36, product angular and translational energy distributions have been determined in the centre-of-mass system for both linear- and cyclic-C3H isomer formation as well as for C3 production. The variations of the dynamics and product branching ratios with collision energy have been characterized. The ratios c-C3H/l-C3H and C3/C3H from the C(3P) reactions have been both found to decrease with increasing ET. Formation of C3(X1sigmag+) from the C(3P) reaction has been rationalized in terms of intersystem crossing between triplet and singlet C3H2 potential energy surfaces. There is good agreement between the results at ET = 3.5 kJ mol(-1) obtained with the two different crossed molecular beam techniques for the C(3PJ) + C2H2(X1sigmag+) --> l/c-C3H + H(2S1/2) channels. An estimate of the exoergicity of the C(3PJ) + C2H2(X1sigmag+) --> c-C3H + H (2S1/2) pathway from the extent of the translational energy release corroborates the value of deltaH0(o) = -11 kJ mol(-1) obtained from the Doppler-Fizeau measurements. The overall results have been discussed in the light of the available theoretical information on the relevant triplet and singlet C3H2 potential energy surfaces, and compared with the ...

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

“…The background pressure remained at <10 −4 Pa with both beams in operation and the attenuation of one beam by another was <4%. Experiments were performed at selected relative translational energies under beam conditions satisfying the relationship v C = v C2H2 cosχ , hence providing a relative velocity vector of reagents, v r , perpendicular to the C beam and along the axis of one of the laser beams (Costes et al 2006).…”

“…Any slight potential barrier would be of paramount importance in low-temperature environments. Second, in cold interstellar clouds, typical temperatures of 10-30 K correspond to mean translational energies for collisions, E T = 3/2 RT, of 0.12-0.4 kJ mol −1 , respectively, and branching ratios of the integral cross sections R1 = σ (1b)/σ (1a+1b) and R2 = σ (1c)/σ (1) have only been determined above a relative translational energy (collision energy) of E T = 3.5 kJ mol −1 except for a preliminary measurement of R1 at E T = 0.8 kJ mol −1 (Costes et al 2006). In this paper, we report high-level ab initio electronic structure calculations of c-C 3 H and l-C 3 H performed with the coupledcluster method and high-resolution, differential cross-section measurements in crossed-beam experiments at low energies that allow, with the aid of previous low-temperature kinetic results, determination of the branching ratios R1 and R2 as a function of collision energy.…”

“…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: …”

Untangling the chemical evolution of Titan's atmosphere and surface–from homogeneous to heterogeneous chemistry

“…104 The apparatus was described previously. 105 Compared to the original design, the ablation source was modified to incorporate an external motor assembly as described below ( Figure 11). Briefly, the ablation source consists of an aluminum block and a pulsed piezoelectric valve.…”

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

“…The reaction between C-atoms and acetylene, C 2 H 2 , is one of the most well studied gas-phase reactions of atomic carbon with three possible product channels at low temperature and pressure. A wide range of experimental studies of the kinetics [3,4] and dynamics [5][6][7][8] of reaction (1) exist, with numerous theoretical investigations attempting to explain the various experimental observations [9][10][11][12][13][14]. Rate constants for reaction (1) have been measured over a wide temperature range [3,4,15], proving that this process occurs over a barrierless potential energy surface for at least one of the three thermodynamically accessible pathways.…”