Molecular Quantum Control by Coherence Effects

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This project will use laser light to control molecules, atoms that are chemically bound together. Typically it takes at least two electrons to create a chemical bond between atoms. The spins of these electrons can be parallel (triplet states) or antiparallel (singlet states). This project will use laser light to switch the molecule between a singlet and a triplet state, thus realizing a "spin switch" with potential applications to quantum computing. Additionally, this project will study the collisions of atoms and molecules under well-defined initial conditions. In general, molecules are not spherically symmetric objects and as a result most collisional processes involving them strongly depend on the relative alignment of the colliding partners. This project will use lasers to control this alignment. Understanding the basic physics of collision processes between atoms and molecules is of importance for processes such as chemical reactivity. The mixing of the character of quantum states due to coupling depends on the strength of the interaction as well as on the energy separation between the interacting quantum states. Using the AC Stark effect of a strong control laser to shift the energy levels of a spin-orbit coupled pair of ro-vibrational levels of the Lithium dimer into resonance, the population transfer between a singlet and a triplet state would occur in a controlled fashion in the time domain under the influence of an external optical field. When two states are on resonance (have the same energy) the probability function has an oscillatory time dependence with a period inversely proportional to the coupling strength of the two states. Thus, if the population is initially placed in one of the states and the two states are rapidly brought into resonance only for the duration of half integer oscillatory periods, and then rapidly moved off resonance, the population will effectively switch from one of the states to the other one. The electric field amplitude of the control laser is determined by the resonance condition of the two states, and the pulse duration by their oscillatory period. The kinetics and dynamics of collisions between alkali molecules and noble gas atoms will be studied under well-defined initial conditions of molecular alignment created using the AC Stark effect of a control laser. The orientation will be achieved by removing the degeneracy of the magnetic sublevels using the orientation dependence of the transition dipole moment. The magnetic sublevels are projections of the total angular momentum of the molecule on a laboratory fixed axis defined by the polarization direction of the resonant laser field. For sufficiently large control laser Rabi frequencies, the sublevels will be separated well enough so specific quantized orientations of the molecules of one or more rotational quantum states can be probed and the collisional transfer between them studied. An advantage of this method is that it will allow for selective removal of the magnetic sublevel degeneracy in contrast to cases where external magnetic or electric fields are used.