Untersuchung der Ionen-Molekül-Systeme HBr+ (DBr+) + HBr sowie HBr+ + HCl (DCl) auf der Basis rotationszustandsselektierter Ionen

In this work, ion-molecule reactions in the systems HBr+ (DBr+) + HBr and HBr+ + HCl (DCl) were investigated inside a guided ion beam apparatus under single-collision conditions. Cross sections s were measured as a function of the collision energy E_cm and the rotational energy E_rot of the ion. The rotationally state-selective formation of HBr+ and DBr+ in the electronic and vibrational ground state was realized by resonance enhanced multiphoton ionization (REMPI). Regarding the reaction system HBr+ + HBr, only the proton transfer (PT), which is indistinguishable from hydrogen atom abstraction (HA), is observable: HBr+ + HBr ---> Br + H2Br+ (PT/HA) (enthalpy of reaction dH = -0.31 eV). The cross section s_PT/HA was measured for E_rot-values between 1.4 meV and 83.6 meV. Furthermore, the collision energy E_cm was varied in the range from 0.41 eV to 1.52 eV. Concerning the ion-molecule system DBr+ + HBr, deuteron transfer (DT), which is indistinguishable from hydrogen atom abstraction, is observable. Moreover, charge transfer (CT), which is indistinguishable from H/D-Exchange (HDE), is accessible: DBr+ + HBr ---> Br + HDBr+ (DT/HA) (dH = -0.32 eV), DBr+ + HBr ---> DBr + HBr+ (CT/HDE) (dH = -0.004 eV). Regarding the investigation of these parallel reaction channels, rotational energies in the range from 0.7 meV to 75.9 meV were used, while the collision energy was varied between 0.41 eV and 3.19 eV. Since the PT/HA-channel is exothermic, s_PT/HA decreases with increasing collision energy. The parallel reactions DT/HA and CT/HDE are also exothermic. Consequently, s_DT/HA and s_CT/HDE decrease with increasing collision energy. The relative decrease of s_DT/HA is steeper compared to s_CT/HDE. As a result, the ratio (s_CT/HDE) / (s_DT/HA), which is greater than 1 for all investigated combinations of E_cm and E_rot, increases with increasing collision energy. In the region of lower rotational energies, s_PT/HA decreases with increasing E_rot. In contrast to that, the cross section increases with increasing E_rot in the region of higher rotational energies. Thus, s_PT/HA runs through a minimum. The cross section s_DT/HA also runs through a minimum. However, this minimum is less pronounced. The cross section s_CT/HDE increases with increasing rotational energy. Due to the contrasting behavior of s_DT/HA and s_CT/HDE as a function of E_rot, the ratio (s_CT/HDE) / (s_DT/HA) increases with increasing rotational energy. Regarding the ion-molecule system HBr+ + HCl, the PT-channel (PT_HCl) as well as the CT/HDE-channel are accessible to s-measurements: HBr+ + HCl ---> Br + H2Cl+ (PT_HCl) (dH = -0.51 eV), HBr+ + HCl ---> HBr + HCl+ (CT/HDE) (dH = +0.76 eV). Cross sections for PT_HCl and CT/HDE were measured for E_cm-values between 0.25 eV and 5.85 eV. The rotational energy of the ion was varied in the range from 3.4 meV to 46.8 meV. Concerning the system HBr+ + DCl, the proton transfer (PT_DCl) and the deuterium atom abstraction (DA) are accessible: HBr+ + DCl ---> Br + HDCl+ (PT_DCl) (dH = -0.52 eV), HBr+ + DCl ---> HDBr+ + Cl (DA) (dH = +0.04 eV). Analogous to the undeuterated system, E_rot was varied between 3.4 meV and 46.8 meV during the investigation of the system HBr+ + DCl. The collision energy was varied in the range from 0.12 eV to 2.03 eV. Since PT_HCl and PT_DCl are exothermic, the respective cross sections decrease with increasing collision energy. The slightly endothermic DA-reaction shows an exothermic behavior – s_DA decreases with increasing collision energy. As expected, the efficiency of the considerably endothermic CT/HDE-reaction increases with increasing E_cm. Concerning the reaction system HBr+ + HCl (DCl), the reaction efficiency decreases with increasing enthalpy of reaction dH – s_PT > s_DA > s_CT/HDE. By comparing the E_rot-dependencies of s_PT_HCl, s_PT_DCl, s_DA and s_CT/HDE, an interesting relation becomes obvious: On the path from dH < 0 (PT_HCl, PT_DCl) via dH approx. 0 (DA) to dH > 0 (CT/HDE) the s(E_rot)-trend changes from rotational hindrance via rotation independence to rotational activation. However, this rotational activation only occurs near the thermochemical barrier (dH_CT/HDE = E_cm). Apart from that, s_CT/HDE is independent of E_rot. Within the scope of this doctoral thesis, the experimental results are compared to AIMD-simulations of the system HCl+ + HCl. This comparison makes sense, since the trajectories and reaction mechanisms of the hydrogen halide systems HCl+ (DCl+) + HCl, HBr+ (DBr+) + HBr and HBr+ + HCl (DCl) should be similar. Thus, the simulations provide possible explanations for the experimental s(E_cm)- and s(E_rot)-trends.