Reactions between Cold CH x + and Slow H and H2

Abstract

Abstract Using the combination of a linear 22-pole ion trap (22 PT) and a coaxial beam of hydrogen atoms, H abstraction from CH x + (x=1, 4 and 5) has been studied. The temperature of the trap, T 22 PT, can be varied between 10 K and 300 K. The velocity distribution of the neutral target beam can be changed by cooling the accommodator (T ACC=10–300 K) and using the focusing features of one or two hexapole magnets. The resulting velocity distributions are characterized by time-of-flight measurements. With the same setup, reactions of mass selected stored ions with a cold effusive beam of H2 molecules have been measured with the discharge turned off. At temperatures of interstellar space, H-abstraction from CH+ is efficient. CH4 + reacts five times faster with H than with H2 at 50 K. In contradiction to ab initio calculations and to the established proton affinity of methane, CH5 + is slowly destroyed in collisions with H in our trap, even at 10 K. Some first results for collisions between CH x + and D atoms are reported. For x=1, H–D exchange is quite efficient, even though it is in competition with the exothermic abstraction reaction. For x=4, H-abstraction, i.e. formation of HD molecules, dominates. Deuteration of CH5 + is measured to be very slow.