Rotational dynamics of molecules in external fields

Zusammenfassung

In this thesis, we theoretically investigate the spectrum and the rotational dynamics of different systems formed by polar linear molecules interacting with electric fields created by external sources or by a Rydberg atom. The first part presents the rotational motion of a polar linear molecule in a time-dependent electric field using the rigid rotor description. We consider an electric field that rotates having constant field strength, which mimics the experimental configurations to guide molecules. We perform a general description for any polar linear molecule, by including only the electric dipole moment interaction with the electric field, which is dominant. The field-dressed rotational dynamics is studied for different time scales, and the experimental field configurations that ensure an adiabatic dynamics are found. Secondly, we consider a polar molecule in a two-color non-resonant cw laser field linearly polarized. Its laser frequency is chosen so that it cannot drive any electronic, vibrational or rotational transitions, and in a regime where the time-average approximation does not hold, and whose validity is also investigated. We study the field-dressed dynamics by including systematically the interactions with the electric dipole moment, polarizability and hyperpolarizability, showing that the latter terms can not be neglected. A complete analysis of the symmetries and the identities satisfied by several expectation values are provided. We prove that the orientation and alignment can be expressed as analytic functions in terms of the laser parameters, and the need of a two-color field with a specific relation of the frequencies to orient the molecules. In the second part, we consider two identical polar linear molecules, which are coupled via electric dipole-dipole interaction, and interacts with a time-dependent electric field through its permanent dipole moment. A full study of the symmetries allows us to present the identities satisfied by the quantities that characterize the system and to simplify the description. We consider small inter-molecular distances, not realistic experimentally, to reach the regime where the electric and dipole-dipole interactions are comparable. The spectrum, entanglement and field-dressed dynamics are analyzed for different spatial configurations of the molecules, and we identify some regions with large molecular orientations and moderate entanglement. Finally, we investigate the electronic structure and main properties of a triatomic ultra-long range Rydberg molecule formed by a cesium Rydberg atom and a RbCs molecule. Our description treats the Rydberg atom as a single-electron system, the diatomic molecule as rigid rotor being in the electronic and vibrational ground state, and includes the charge-dipole and S-wave scattering interactions. For two different excitations of the Rydberg atom, we analyze the electronic spectrum, the avoided crossings between near adiabatic potential curves, and find the conditions for adiabatic transitions through these crossings. In the avoided crossing regions, we use a beyond Born-Oppenheimer description to obtain the vibrational bound states. We identify the best states for the photoassociation of these Rydberg molecules in the laboratory, and the pairs of Rydberg states that could be used for quantum computing processing combined with any alkali dimer including Cs.