Electron Transfer and Associative Detachment in Low-Temperature Collisions of D(-) with H.

The interaction of D(-) with H was studied experimentally and theoretically at low temperatures. The rate coefficients of associative detachment and electron transfer reactions were measured in the temperature range 10-160 K using a combination of a cryogenic 22-pole trap with a cold effusive beam of atomic hydrogen. Results from quantum-mechanical calculations are in good agreement with the experimental data. The rate coefficient obtained for electron transfer is increasing monotonically with temperature from 1 × 10(-9) cm(3) s(-1) at 10 K to 5 × 10(-9) cm(3) s(-1) at 160 K. The rate coefficient for associative detachment has a flat maximum of 3 × 10(-9) cm(3) s(-1) between 30 and 100 K.

H/D exchange in reactions of OH(-) with D2 and of OD(-) with H2 at low temperatures.

Using a cryogenic linear 22-pole rf ion trap, rate coefficients for H/D exchange reactions of OH(-) with D2 (1) and OD(-) with H2 (2) have been measured at temperatures between 11 K and 300 K with normal hydrogen. Below 60 K, we obtained k1 = 5.5 × 10(-10) cm(3) s(-1) for the exoergic . Upon increasing the temperature above 60 K, the data decrease with a power law, k1(T) ∼T(-2.7), reaching ≈1 × 10(-10) cm(3) s(-1) at 200 K. This observation is tentatively explained with a decrease of the lifetime of the intermediate complex as well as with the assumption that scrambling of the three hydrogen atoms is restricted by the topology of the potential energy surface. The rate coefficient for the endoergic increases with temperature from 12 K up to 300 K, following the Arrhenius equation, k2 = 7.5 × 10(-11) exp(-92 K/T) cm(3) s(-1) over two orders of magnitude. The fitted activation energy, EA-Exp = 7.9 meV, is in perfect accordance with the endothermicity of 24.0 meV, if one accounts for the thermal population of the rotational states of both reactants. The low mean activation energy in comparison with the enthalpy change in the reaction is mainly due to the rotational energy of 14.7 meV contributed by ortho-H2 (J = 1). Nonetheless, one should not ignore the reactivity of pure para-H2 because, according to our model, it already reaches 43% of that of ortho-H2 at 100 K.

Stabilization of H+-H2 collision complexes between 11 and 28 K.

Formation of H(3)(+) via association of H(+) with H(2) has been studied at low temperatures using a 22-pole radiofrequency trap. Operating at hydrogen number densities from 10(11) to 10(14) cm(-3), the contributions of radiative, k(r), and ternary, k(3), association have been extracted from the measured apparent binary rate coefficients, k*=k(r)+k(3)[H(2)]. Surprisingly, k(3) is constant between 11 and 22 K, (2.6±0.8)×10(-29) cm(6) s(-1), while radiative association decreases from k(r)(11 K)=(1.6±0.3)×10(-16) cm(3) s(-1) to k(r)(28 K)=(5±2)×10(-17) cm(3) s(-1). These results are in conflict with simple association models in which formation and stabilization of the complex are treated separately. Tentative explanations are based on the fact that, at low temperatures, only few partial waves contribute to the formation of the collision complex and that ternary association with H(2) may be quite inefficient because of the 'shared proton' structure of H(5)(+).