Reactions of Cn (n = 1-3) with ions stored in a temperature-variable radio-frequency trap

A new experimental set-up has been developed for studying astrochemically relevant collisions between small neutral carbon molecules Cn (n = 1-3) and stored ions. The ions are confined for seconds in a ring electrode trap (RET) the temperature of which can be varied over a wide range (presently 80-600 K). There they interact with an effusive carbon beam, which is produced via high-temperature vaporization of a carbon rod. Due to the accessible temperature range and other features of the set-up, rate coefficients can be measured, which are of importance for understanding the chemistry occurring in the outflow of stars, the formation of hydrocarbons in stellar atmospheres, and the interaction of the stored product ions with radiation. Results are reported for the interaction of stored D3+ with hot Cn. The D+ transfer dominates over all other exothermic product channels for n = 1-3. The reaction rate coefficients measured for forming CnD+ are almost a factor two smaller than values presently used in astrophysical models. Another important class of reactions concerns the growth of pure carbon chains via associative Cm+ + Cn collisions. First results indicate that the rate coefficients are slower than generally assumed in models. Due to the weak signal, only rough limits can be reported. For future studies, the number density of carbon penetrating the trap must be increased. This and the planned extension of the temperature range is briefly discussed in the outlook.