Atom cloud expansion movie (mpeg) Caution: Large movie (2 Mb)!

Ultracold Rydberg Gases, Molecules and Collisions

One focus of our research is to study the effects of collisions and interactions between high principle quantum number Rydberg atoms. A Rydberg atom behaves in many ways like a simple Bohr atom. For atoms excited to high n Rydberg states, the electron orbits the nucleus at a large radius. The polarizability of the Rydberg atom (the amount the atom is "distorted" by an electric field) is large because the electron spends most of its time far from the atomic core for example. When the atom is placed in even a small electric field, so that there is a preferred spatial direction, it develops a large dipole moment.

Rydberg atoms in high n states at ultracold temperatures can interact with each other at very large distances. Cold gases made up of these atoms exhibit many interesting properties such as many body effects, extremely large collision cross sections, resonant energy exchange and spontaneous ionization to form ultracold plasmas.

The second focus of our research is to investigate ultracold collisions using photofragment energy and angle resolved product state distributions. Ultimately, these experiments are aimed at determining state to state differential cross sections for 3 body recombination in high density low temperature atom traps. These experiments will help to determine dephasing rates in Bose-Einstein condensates.

These experiments take place in a magneto-optic trap or a far off resonance dipole trap. There are two different forces that can be used to trap atoms using light. The two forces are manifested in the magneto-optic trap (MOT) and the dipole trap, or far off resonance trap, (FORT). The radiation scattering force is primarily used to form the MOT while the dipole or gradient force is used to form the dipole trap.

The gradient force arises when a spatially varying laser beam intensity interacts with the dynamic polarizability of an atom. If the laser field is at a frequency below the atomic resonance frequency, the induced dipole moment of the atom is in phase with the oscillating electric field. In analogy with a driven harmonic oscillator, the interaction is W= - p E. If the dipole is in phase with the electric field, an atom's energy decreases as it moves into a region of high intensity. A region of high electric field intensity such as the focus of a laser beam can be used to make an atom trap using this effect. The MOT uses the momentum imparted to an atom as it scatters laser light to trap atoms. Six laser beams are superposed along the 3 Cartesian axes. Along each axis two beams counter propagate. A magnetic field is used to make the net momemtum transfered to the atoms depend spatially on the distance away from the origin of the coordinate system. In this way a restoring force is formed that pushes slowly moving atoms back to the origin, forming a trap.