Experiments on hydrogenation and deuteration of C2H2 and C2H4 on low temperature surfaces :formation mechanism of C2H6 with astronomical interest
H. Kobayashi,1,2 H. Hidaka,3 T. Hama,3 H. Kawakita,1 and N. Watanabe3
1Koyama Astronomical Observatory, Kyoto Sangyo University, Japan
2Kyoto Nijikoubou, LLP, Japan
3
Institute of Low temperature Science, Hokkaido University, Japan

Simple hydrocarbons, acetylene (C2H2), ethylene (C2H4) and ethane (C2H6) widely exist in various circumstancesin space. Although the gas phase C2H2 and C2H4 are detected in limited objects of interstellar medium (ISM, e.g.,[1,2]), C2H6 was not detected in the ISM yet. Observational bias might be a reason for this difference because thebands of these species in infrared region are greatly affected by severe telluric absorptions. Alternatively theobservational results may reflect the result of chemical evolution in ISM. Thus we performed quantitative
experiments providing a precious clue for understanding the evolution of these hydrocarbons. As previouslyproposed, one of most important processes to form C2H6 is hydrogenation of C2H2 on low temperature surface(C2H2 -> C2H3 -> C2H4 -> C2H5 -> C2H6) [3, 4]. We conducted hydrogenation experiments of pure C2H2 andC2H4 solids (to compare our results with the previous studies [3, 4]) and those on amorphous solid water (ASW,to reproduce the ISM condition) at 10K and 20K, respectively. Deuteration experiments of C2H2 and C2H4 on
ASW were also performed. All experiments were conducted by using laboratory setup for surface reaction ininterstellar environment (LASSIE) at Institute of Low Temperature Science, Hokkaido University [5].
Obtained effective reaction rate constants, which are defined by a reaction rate constant times a number densityof atomic hydrogen/deuterium adsorbed on the surface, are shown in Figure 1 with a result of CO hydrogenation
on ASW reported by Hidaka et al. [6]. At 10K, the difference in the effective hydrogenation reaction rate
constants between C2H2 and C2H4 was found to be a factor of ~3 on both pure solids and ASW samples, whereas
previous experimental studies [3, 4] reported that the hydrogenation rate of C2H4 is larger than that of C2H2 byorders of magnitude. The kinetic isotope effects of hydrogenation/deuteration are found to be very small in both
C2H2 and C2H4 reactions at 10 K. This significantly small kinetic isotope effect for tunneling reactions can beexplained as “diffusive reaction” where the reaction process is dominated by diffusion of H and D atoms on thesurface. At 20 K, the effective rate constants of deuteration became larger than those of the hydrogenation. This
unusual behavior would be attributable to the difference of the number densities of H and D atoms on thesurface.

Figure 1: Temperature dependences of theeffective reaction rate constants for the H
and D atom exposure for each sample(filled square with solid line; C2H2-ASW+ H, filled square with dashed line;
pure-C2H2 solid + H, filled circle withsolid line; C2H4-ASW + H, filled circle
with dashed line; pure C2H4 solid + H,open square with solid line; C2H2-ASW +D, open circle with solid line; C2H4-ASW+ H, triangle; CO-ASW + H).

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