Formation, Dynamics and Application of Ultracold Molecules

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The essence of modern Physics is quantum mechanics. In particular, the structure of the atomic nucleus and the surrounding electrons, and the corresponding structure of molecules and materials is inherently quantum mechanical. Understanding the quantum structure of atoms, molecules, and materials at room temperature and higher, where thousands or millions or more quantum states are populated, is possible using the selectivity of lasers. However, at ultracold temperatures of less than 0.001 K, only a few quantum states are populated and exotic phenomena such as superfluidity and Bose-Einstein condensation of many particles, each in a single unique quantum state are possible. This research involves understanding the quantum states of simple molecules composed of two atoms at ultracold temperatures, including how the molecules are formed in particular selectable states and how the molecules change when they are exposed to light of specific wavelengths or collide with other molecules or atoms. In response, they may emit light, emit an electron, or split into two atoms. There are a number of interesting possible applications of this behavior such as quantum cryptography, quantum computing and quantum simulation of more complex systems. This area of frontier research provides an uncharted and challenging education for students in Physics and related fields.In our project, we will work with two carefully selected diatomic molecules, Rb2 and KRb. One fascinating electronic quantum state in both molecules is the triplet state. This state can in principle decay, but calculations for Rb2 predict it has a very long lifetime of up to 200 seconds in its lowest rovibrational level, during which it can be studied. Our experiments will define the limits of this "metastability" at ultracold temperatures due to spontaneous emission of light or to collisions with ultracold atoms or molecules. We will also study another fascinating type of electronic quantum state in these molecules: the so-called "trilobite-like" or "butterfly" electronic quantum states, where a completely new kind of chemical bonding takes place at internuclear distances at least ten times larger than ordinary chemical bonds. Their name comes from the complex spatial structures of the square of the electronic wavefunction, which can resemble ancient trilobites or modern butterflies. This novel bonding involves the attraction between the electron of a highly excited "Rydberg" atom with a second ground state atom imbedded within the large Rydberg electronic wavefunction. Spectroscopy will determine the predicted and unusual "interatomic roller coaster" potential energy curves with numerous maxima and minima, as well as the multiple decay processes that determine lifetimes of these states. In particular, the mechanisms of the puzzlingly rapid rates of decay of these states to (Rb2) cation and the negative electron will be determined. We will also investigate the long-range electronic states arising from the ion-pair (Rb cation and Rb anion), some in the same region of energy and internuclear distance as the "butterfly" states. These are "Heavy Rydberg" states, bound by the Coulomb attraction in the same way as in the H atom. The ion pair experiments will yield valuable information on the coupling of shorter range covalent states, where the bonding electrons are shared, to the longer range ionic states, where the two valence electrons clearly reside on one of the atoms.