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 cm3 s−1 for the exoergic reaction (1). Upon increasing the temperature above 60 K, the data decrease with a power law, k1(T) [similar] T−2.7, reaching ≈1 × 10−10 cm3 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 reaction (2) increases with temperature from 12 K up to 300 K, following the Arrhenius equation, k2 = 7.5 × 10−11 exp(−92 K/T) cm3 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.Dieser Beitrag ist aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich

Laboratorní výzkum reakcí iontů s molekulárním i atomárním vodíkem při teplotách relevantních pro astrochemii

V této práci jsou prezentovány výsledky laboratorního výzkumu ion-molekulových reakcí při nízkých teplotách, kde jedním z reaktantů byl atomární nebo molekulární vodík anebo molekulární deuterium. U reakcí atomárních iontů N+ a H+ s molekulárním vodíkem byl studován vliv konfigurace jaderných spinů a rotační excitace na rychlostní koeficienty těchto reakcí a jejich závislost na teplotě. Studium reakcí aniontů bylo spojeno s výzkumem reakcí izotopické výměny a izotopického efektu. Byla provedena měření rychlostních koeficientů tvorby molekul H2O a D2O v reakcích O- s H2 a D2, reakcí izotopické výměny OH- + D2 a OD- + H2 a také byly studovány reakční kanály asociativního odtržení a přenosu náboje v reakci D- + H. Experimenty byly prováděny na aparatuře AB-22PT s iontovou pastí. Tato aparatura zahrnuje systémy pro tvorbu, vedení, záchyt a detekci iontů a také samostatný zdroj atomárního H. Chladící systém umožnil měření teplotních závislostí rychlostních koeficientů při teplotách relevantních pro astrochemii (10 K - 300 K).The results of the laboratory study of reaction rate coefficients of several ion- molecule reactions with atomic and molecular hydrogen and molecular deuterium at low temperatures are presented in the thesis. The reaction rate coefficients of the N+ and H+ reaction with H2 were measured with respect to the nuclear spin configuration and rotational excitation of H2. The reactions of anions were a subject of the isotope exchange and isotope effect study. The measurements of the rate coefficients of H2O and D2O formation in the reaction of O- with H2 and D2, isotope exchange reactions OH- + D2 and OD- + H2, and associative detachment and charge transfer channels of D- + H interaction were performed. Experiments were carried out using an AB-22PT instrument with an ion trap. It has producing, guiding, trapping, and detecting systems for ions and a separate source of atomic H. The cooling system allowed to measure the temperature dependencies of the reaction rate coefficients at temperatures relevant to astrochemistry (10 K - 300 K)Katedra fyziky povrchů a plazmatuDepartment of Surface and Plasma ScienceMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Laboratory studies of ion-molecule reactions with molecular and atomic hydrogen at temperatures relevant to astrochemistry

The results of the laboratory study of reaction rate coefficients of several ion- molecule reactions with atomic and molecular hydrogen and molecular deuterium at low temperatures are presented in the thesis. The reaction rate coefficients of the N+ and H+ reaction with H2 were measured with respect to the nuclear spin configuration and rotational excitation of H2. The reactions of anions were a subject of the isotope exchange and isotope effect study. The measurements of the rate coefficients of H2O and D2O formation in the reaction of O- with H2 and D2, isotope exchange reactions OH- + D2 and OD- + H2, and associative detachment and charge transfer channels of D- + H interaction were performed. Experiments were carried out using an AB-22PT instrument with an ion trap. It has producing, guiding, trapping, and detecting systems for ions and a separate source of atomic H. The cooling system allowed to measure the temperature dependencies of the reaction rate coefficients at temperatures relevant to astrochemistry (10 K - 300 K