A FURTHER INVESTIGATION OF THE REACTION OF C2H2-2( WITH H)

The reactions of C2H2+ with H-2 and C2D2+ + D-2 have been studied in a selected ion flow drift tube (SIFDT) over a range of centre-of-mass e nergies E(c) from thermal (300 K) to a few tenths of an electron volt. The reactions proceed along parallel bimolecular and termolecular cha nnels producing C2H3+ (C2D3+) and C2H4+ (C2D4+). The relative importan ce of the bimolecular channel increases and that of the termolecular c hannel decreases with increasing E(c) in accordance with previous data . The rates for the bimolecular reactions increase as the C=C stretchi ng vibrations in the reactant ion are excited in collisions with the h elium carrier gas atoms in the SIFDT at higher E(c) values, consistent with other work in which these vibration modes are selectively excite d using laser ionisation. The question is addressed as to whether the atom abstraction reactions are exothermic or endothermic. Based on the new experimental data, some recent ion trap data and previous data on the reaction of C-2 H-3(+) with H atoms, it is concluded that the rea ctions are slightly endothermic, and it is tentatively suggested that a potential barrier also exists to the reactions. The interstellar imp lications of these results are briefly referred to.