H/D accretion, especially onto ionized fullerenes, is expected to be very efficient in space. In this work, we study hydrogenated and deuterated fullerene cations and their photodissociation behavior in the gas phase. The experimental results show that hydrogenated fullerene cations (i.e., [CH] and [CH], n up to 30) and deuterated fullerene cations (i.e., [CD] and [CD], n up to 21) are formed efficiently through the ion-atom collision reaction pathway. Upon irradiation, the hydrogenated and deuterated fullerene cations dissociate into fullerene cations and H/H or D/D species. The structures of the newly formed hydrogenated and deuterated fullerene cations (C and C) and the bonding energies for these reaction pathways are investigated by means of quantum chemical calculations. The competition between hydrogenation and dehydrogenation is confirmed, and the hydrogenation-to-dehydrogenation ratio in the accretion processes in the gas phase is determined. We infer that the proportion of accreted hydrogen and deuterium atoms on the surface of fullerenes is similar to that of hydrogen and deuterium atoms in the interstellar environment where these fullerenes are located, especially when the interstellar environments are similar to our experimental conditions, i.e., the hot environment.
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