Rydberg Atom-Microwave Interactions

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The authors are conducting a research program in which they are studying highly excited, or Rydberg, atoms in strong microwave fields. They have two specific goals. The first is to understand novel states of the combined atom-field system which are remarkably stable, even at energies above the ionization limit of the atom, the energy above which the valence electron is normally not bound to the atom. The atom is stabilized by the combination of the Coulomb field and an oscillating field with a frequency far in excess of the intrinsic frequency of the valence electron's orbit. The second goal of the project is to manipulate the Rydberg atoms with weak microwave fields oscillating at frequencies close to the orbital frequencies of the atoms. In classical terms the motion of the valence electron is phase locked to a microwave field, which is then varied to alter the electron's orbit in the same way.These experiments are carried out using an atomic beam of lithium atoms, which are excited to energies near the ionization limit, either to Rydberg states just below the limit or to energies just above it. The excitation is done using a 1 kiloHertz repetition rate laser system, and the excited atoms are exposed to the microwave pulse either during or after laser excitation. The effect of the microwave field is analyzed in one of three ways. After the end of the microwave pulse the final states are analyzed by selective field ionization, which reveals the energy distribution of bound final states. During the microwave pulse the atoms are exposed to a temporally short, 1 picosecond, field pulse which gives the electrons a momentum kick and ionizes the atoms if the electron is moving in the direction of the momentum kick. Finally, during the microwave pulse the atoms are exposed to a phase locked harmonic of the microwave field, which, with the proper phase, ionizes atoms with orbits phase locked to the microwave field. The latter two techniques enable the authors to determine the electron's time resolved velocity.The intellectual merit of the proposed activity is twofold. First, it promises to lead to new insights into the importance of the often ignored Coulomb potential in the presence of strong radiation fields. Specifically, it will clarify how the combination of the Coulomb potential and a strong radiation field can together produce unexpected effects. Second, the Rydberg atom-microwave system is one in which the presence of multiple levels on coherent population transfer can be studied in a quantitative fashion.The authors expect their work to have broad impact beyond their own interests. Insights into strong field physics are immediately transferrable to high intensity laser physics. The techniques used in the manipulation of Rydberg atoms, such as coherent population transfer, can be generalized and used in other contexts and are of particular interest in physical chemistry. More generally, manipulation of Rydberg atoms is of interest for the production of low lying states of antihydrogen, quantum computing, axion searches, and long wavelength photon detection.