要旨
Summary
In this thesis the work of my group on the manipulation of the motion of large molecules using electric fields is described. It is demonstrated that neutral molecules can be deflected, focused, and decelerated using inhomogeneous electric fields. The force on the molecule exerted by the fields is due to the Stark interaction; i. e., for the molecules investigated here due to the interaction of the electric dipole moment of the molecule with an external electric field. Since the dipole moment is a property of the individual quantum states, these forces can be used to spatially (and temporary) separate molecules in different quantum states.This has been demonstrated throughout the thesis. Most importantly, we have exploited this effect for the separation of individual conformers(structural isomers) of 3-aminophenol and for the preparation of ensembles ofiodobenzenemolecules in only the lowest rotational quantum states, which are ideally suited for laser alignment and orientation experiments.
For the large molecules investigated in this thesis all states are practically highfield-seeking. Therefore, they cannot be confined by static electric fields. Instead, dynamic focusing schemes need to be employed. We have demonstrated that alternating-gradient focusing can be used to focus large molecules in high-field-seeking states in a quadrupole setup and also using short individual electrode pairs in an alternating-gradient decelerator setup. Using the decelerator we have also demonstrated the slowing of the prototypical large molecule benzonitrile. Due to the complicated Stark manifold with many real and avoided crossings the applicability of these techniques to large molecules was not a priori clear, but could be convincingly demonstrated here. Moreover, we have also demonstrated the possibility to trap
molecules in high-field-seeking states for para-ammonia molecules in the lower inversion level of the rotational ground state.
Overall, a number of different approaches to the manipulation of the motion of large neutral molecules are demonstrated, i. e., the deflection, focusing, and deceleration of large molecules. Three different approaches to the manipulation of the translational motion are pursued. In ascending order of complexity, we have set up an electric field deflector (the electric analog of a Stern-Gerlach deflector), a dynamic focusing selector (the equivalent of a quadrupole ion guide), and an alternating-gradient decelerator (the equivalent of a LINAC). We have also
demonstrated the optimization of the normal Stark decelerator beamline using an automated learning-loop approach using evolutionary algorithms. In addition, experiments on the manipulation of the rotational motion using off-resonant ac laser fields were performed.
Moreover, it is described that these clean samples of large molecules can be extremely advantageous for further experiments in chemistry and physics. They allowed the observation of strongly increased degrees of laser alignment and mixed field orientation compared to thermal ensembles at temperatures as low as 1 K, and they are foreseen to be crucial for benchmark coherent-diffractive-imaging experiments of oriented gas-phase molecules.
