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Institute of Optics


Photoexcitation of Near-Threshold Atomic Energy Levels


David A. Cardimona

With the recent advent of narrow bandwidth tunable lasers, the atomic energy states lying very near the ionization limit have become accessible to high resolution investigation. These high "Rydberg states" can be easily mixed by perturbing fields because of their small energy separations. The mixing of these nearly degenerate states gives rise to generalized damping constants and frequency shifts in density-matrix equations of motion. We have shown that the generalized damping constants permit conditions under which the steady-state resonance fluorescence or absorption rate of an atomic system composed of several closely-spaced excited states dipole-coupled to a common ground state can be forced to vanish. We have interpreted the generalized radiative frequency shifts as a manifestation of the mixing of the nearly-degenerate Rydberg states by the vacuum field, and have shown how these terms can be renormalized and included in altered damping constants and shifts of the atomic transition frequencies.

Laser excitation to near-threshold continuum states is found to result in non-exponential decay of the ground state population. The non-exponential time dependence is described as arising from the spreading of the initial wavepacket in a Coulomb field and is shown to be independent of the form of the interaction energy used(p • A vs. d • E). We show that the possibility for observing this deviation from pure exponential decay is considerable.

Near-threshold excitation in neutral atoms mixes an infinite number of Rydberg states with the continuum. The inevitable sum over bound states and integral over continuum states which arise in calculations involving such mixing, are found to individually diverge. A method is developed which allows the sum and integral to be performed simultaneously, yielding a finite result. We argue that there is actually a finite experimental limit to the Rydberg series of bound states due to nearest-neighbor interactions and applied field power broadening.

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Last modified 13 September 2006