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

Abstract

The C + C 2 H 2 reaction is a key process in interstellar cloud chemistry. In a crossed-beam scattering experiment approaching the low-collision energies that characterize these environments, we determined relative differential cross sections by detecting the H-atom product. High-level ab initio calculations of the reaction energies of two competing pathways, leading to cyclic and linear C 3 H, were also performed. Both channels are clearly distinguishable: the integral cross section of the c-C 3 H + H channel monotonically decreases with increasing relative translational energy whilst the l-C 3 H + H channel exhibits a translational energy threshold. Moreover, a comparison of the H-atom yields from the C + C 2 H 2 and C + C 2 H 4 reactions shows that the C 3 + H 2 nonadiabatic channel dominates. These results are consistent with the calculated enthalpies and corroborate earlier low-temperature kinetic experiments. Branching ratios of the three reaction pathways are given in the T = 15-300 K temperature domain for inclusion in astrochemical databases.

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

“…The H-atom yields listed in Table 2 Costes et al [7] seems to be erroneous, the energy (temperature) dependence of these results is nonetheless similar to the one determined in this work, indicating that the C 3 + H 2 product channel becomes less favourable as the temperature falls over the 300 -50 K range. In contrast, the earlier studies of Leonori et al [6] and Gu et al [8] …”

“…Similarly, the ratio (1a)/(1a)+(1b) was also found to decrease with increasing collision energy. The most recent experimental investigation of this reaction by Costes et al [7], using a pulsed CMB method coupled with resonance enhanced multiphoton ionization to detect H-atoms, also determined the ratios (1a)/(1a)+(1b) over the range 0.44 -4.5 kJ mol -1 and (1c)/(1) at energies of 0.8 and 4.8 kJ mol -1 (corresponding to the mean energies of thermal distributions at 64 and 385 K respectively).…”

“…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. As the translational energies of C 2 H 2 and C 2 H 4 were essentially the same (similar reduced masses and identical relative velocities) giving rise to the same collision energy, they were able to deconvolute the respective H-atom signals in the resulting Doppler-Fizeau spectra (see Figure 7 of Costes et al [7]) given the much lower exoergicity for H-atoms produced by the C + C 2 H 2 reaction. They derived values of 0.82 and 0.87 for the ratio (1c)/(1) at 0.8 and 4.8 kJ mol -1 , considerably larger than the ones obtained by previous work in the same energy range [6,8], or at equivalent temperatures [15].…”

Photochemistry of C3Hphydrocarbons in Titan’s stratosphere revisited

“…As a result, the formation of C( 3 P) + C 2 H 2 products is likely to be very low, whilst isomerization to H + c−C 3 H is also likely to be low with respect to C 3 + H 2 formation given the lower exit barriers. As shown experimentally (Costes et al 2009;Bergeat & Loison 2001) and theoretically (Mebel et al 2007), intersystem crossing is efficient enough to quickly convert 3 C 3 H 2 into 1 C 3 H 2 . The electronic degeneracy factor is then high and the rate constant should be close to the capture rate constant.…”

“…The main exit channel leads to C 3 + H 2 formation on the singlet surface with a notable barrier on the exit channel located only −10 kJ mol −1 below the reactants. Recent calculations coupled with low-temperature experiments (Costes et al 2009) indicate that the exit channel C( 3 P) + C 2 H 2 might be thermoneutral or even slightly exothermic with respect to H + l−C 3 H. Nevertheless, once the cyclic 3 C 3 H 2 intermediate has been formed, both exit channels H + c−C 3 H and C + C 2 H 2 can be reached without barrier; the former being lower in energy. As a result, the formation of C( 3 P) + C 2 H 2 products is likely to be very low, whilst isomerization to H + c−C 3 H is also likely to be low with respect to C 3 + H 2 formation given the lower exit barriers.…”