This experimental research program has two goals. The first is to conduct experiments to understand ionization of highly excited, or Rydberg atoms by strong microwave fields at or above the orbital frequency of the valence electron in the Rydberg atom, and, secondly, manipulate the Rydberg atoms with somewhat weaker microwave fields. The objective in the first part of the project is to link ionization of atoms by low frequency electromagnetic fields to ionization by high frequency fields. In the former case the slowly varying electromagnetic field distorts the atomic potential, allowing the electron to escape. In the latter a photon is absorbed. The objective in the second part of the project is to manipulate the orbital frequency of the electron. The experiments will be carried out using atomic beams of alkali atoms. The atoms are excited to Rydberg states using pulsed dye lasers, and after the laser excitation they are exposed to a microwave pulse. The effect of the microwave pulse is determined by analyzing the final states, either by state selective field ionization of bound atoms or measuring the fraction of atoms ionized. The broader impact of the program involves education as well as applications to other fields.
Atoms have long been understood to be quantum objects, but our intuitive picture of an atom is classical, an electron circulating about an ion core in an elliptical orbit. This intuitive picture is hard to reconcile with the quantum mechanical description of atoms, usually given in terms of time independent wavefunctions. The paradox was resolved by Schrodinger, who pointed out that spatially localized wavepackets wh8ich move in exactly the way a classical electron moves can be constructed of coherent superpositions of stationary wavefunctions of different energies. In an atom the energy differences are only approximately the same, so the spatial localization quickly disperses, so the quasi classical atom is short lived. It has been suggested that adding a small microwave field at the orbital frequency should phase lock the electron’s motion to the field, leading to non dispersing wavepackets. We have demonstrated that nondispersing wavepackets can be made to last more than 50,000 orbits using a linearly polarized microwave field, and we have further shown that it is possible to alter the orbit of the electron’s oscillation by changing the polarization of the microwave field from linear to circular. Changing the microwave polarization in his way changes the electron’s orbit from one which is highly elliptical to one which is circular. Changing the micowave polarization back to linear results in linear orbital motion of the electron. In short, it is possible to manipulate the electron’s orbit. The experimental observations have been compared to both quantum mechanical and classical descriptions. A related project is the demonstration of the classical interpretation of adiabatic rapid passage, a well known quantum mechanical process often used in nuclear magnetic resonance. The frequency of a driving field, a microwave field in our case, is slowly swept through an atomic resonance with the result that 100% of the population is transferred from the initial state to the final state. We have shown theoretically that the dipole moment of the atom oscillates in phase with the microwave field during adiabatic rapid passage. In other words, it is phase locked to the microwave field during adiabatic rapid passage. We have shown experimentally that the phase locking occurs, and we have shown the if the microwave fields is tuned to resonance that the dipole oscillates from leading to lagging the microwave field as the population oscillates between the initial and final states, corresponding to the gain or loss of energy from the microwave field.
