Shaffer Research Group
Atomic, Molecular, and Optical Physics at the University of Oklahoma
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Stark Slowing of Asymmetric RotorsWeak Field Seeking States and Nonadiabatic TransitionsThis project's goal is to ultimately create a trapped sample of cold molecules and perform experiments on them. The laser-cooling and trapping methods that work for atoms can not be applied to molecules, since molecules have more degrees of freedom - they can vibrate and rotate. Stark slowing is one of the few known methods to create cold molecules. With this method, samples of cold molecules have been created, but only symmetric molecules have been slowed and trapped to date. Our project focuses on asymmetric molecules. Asymmetric molecules are the most common type in nature and their properties are of interest to both industry and academic research.With trapped cold asymmetric molecules, a number of interesting experiments can be performed. Dissociation energies (the energy needed to break a molecule into pieces) can be measured with unprecedented precision, of interest both to industry and academic research. The electric dipole moment of the electron (EDM), a fundamental constant, can also be measured to very high accuracy, allowing for a test of the currently accepted physical theory of matter. Experiments on prospects of quantum computing with cold molecules can also be done. A beam of cold molecules can be created relatively easily by the method of supersonic expansion through a pulsed valve. However, while those molecules are cold (several degrees above absolute zero), they are contained in a beam that moves at high velocities of ~300 m/s in the lab. Before these cold molecules can be trapped, they need to be slowed down to ~20 m/s. We want to build a Stark slower to slow a molecular beam down and prepare molecules for trapping by laser light. A Stark slower consists of ~100 electric field stages, each providing a large spatially increasing electric field (~1000 kV/cm). A molecule with a large permanent electric dipole moment (>2 Debye) has a field dependent rotational energy (Stark Effect: technical and non-technical descriptions), and thus the field will exert forces on such a molecule. These forces change the molecules' orientation in space. If the molecule is in the right rotational quantum state when it enters the slower (weak field seeking state), it will gain rotational ("potential") energy and loose translational (kinetic) energy as it moves through increasing fields - it slows down. Fields have to be switched on and off in just the right time to slow the same bunch of molecules more and more, as it passes through stage after stage of the Stark slower. Currently, we are investigating the theory of asymmetric molecules in a Stark slower. We do so by numerically calculating the Stark energies, orientational probability distribution functions and nonadiabatic transition probabilities for several small asymmetric rotor molecules. The goal of these calculations is to find good candidates for Stark slowing. We also want to see if the quantum mechanical phenomenon of nonadiabatic transitions could be used to enhance the slowing efficiency. First results of this study were presented at the DAMOP 2005 conference as a poster titled "Stark slowing of asymmetric rotors: Weak field seeking states and nonadiabatic transitions". |
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