Study of the reactions of H and D atoms with solid C2H2, C2H4, and C2H6 at cryogenic temperatures

The reactions of the solid thin films of C2H2, C2H4, and C2H6 deposited on the silicon substrate at 10 K with cold H and D atoms (similar to 27 K) sprayed over the solid films were studied systematically by thermal desorption mass spectrometry and Fourier transform (FT)-IR spectroscopy. In the reaction of H with C2H2, C2H6 was found to be the major product but C2H4 could not be detected. This is because the first-step addition reaction H + C2H2 --> C2H3 is the rate-controlling process, and the following reactions to form the final product C2H6 proceed much faster than the initial one. This finding is in accord with the observation of comets Hyakutake and Hale-Bopp, i.e., C2H2 and C2H6 but no C2H4 were detected in the comas of these comets. In all the reactions investigated, the product yields increased drastically with decrease of temperature from 50 to 10 K. This is most likely to be due to the increase of the sticking probability of H and D atoms on the solid films at lower temperatures. This finding led us to conclude that the chemical evolution taking place on the dust grains via II-atom tunneling reactions becomes efficient only at cryogenic temperatures. The abstraction reaction of D with C2H6, D + C2H6 --> DH + C2H5, was found to proceed much slower than the addition reactions of D with C2H4 and C2H2. The rates of reactions of solid C-2 hydrocarbons with H atoms sprayed over the solid films were in the order of C2H4 much greater than C2H2 much greater than C2H6. The reactivity of D with C2H2 was considerably lower than that of H. Such a remarkable isotope effect was not found for C2H4.