Amplitude Modulation of Atomic Wavefunctions - Final Technical Report

Cooke, William E.

College of William and Mary, Williamsburg, VA (United States)

关键词: Decay;    Amplitudes;    Electron-Electron Interactions;    Strontium;    Barium;   

DOI  :  10.2172/794140
RP-ID  :  NONE
RP-ID  :  FG02-97ER14800
RP-ID  :  794140

美国|英语

来源: UNT Digital Library

【 摘 要 】

This project developed a novel wave function modulation technique. Other modulation techniques use tailored laser pulses to directly excite a time-dependent, modulated wave function from a ground state. Our technique began with one electron already excited to a time independent eigenfunction. Then, by using excitations of a second valence electron, we modulated the other wave function. Our technique had the benefit that it was very efficient, and required low power lasers with no need for precise phase or amplitude control. On the other hand it had the difficulty of being a multi-step laser excitation with a maximum repetition rate of 10 Hz. Over the course of this project, we showed that the technique did work, and work efficiently. However, it was easy to generalize. Since the modulation depended on a difference between electron-electron interactions with the inner electron in a ground or excited state, the efficiency of the modulation was strongly state dependent. For example, we never showed any significant modulation in our tests of barium states, while our strontium measurements did show efficient modulation as long as the state to be modulated was in the 5snd group with n between 30 and 50. We completed some studies of the dependence of the amplitude modulation as we varied the time between the excitation and de-excitation pulses applied to the inner electron. The amplitude of the nearest neighbor states was well described by Multi-Channel Quantum Defect theory, but farther satellites were problematical. This may have simply reflected the low density of measurement points, since the amplitudes of the farther satellites oscillate more quickly with time. As we developed our technique, we showed that we could directly measure autoionization decay rates in the time domain, and that the net effect of a state belonging to a Rydberg series was that exponential decay could not be measured, since any short excitation created a coherent superposition that decayed with significant structure. In addition, we showed that these short-lived states could not be power-broadened in the normal sense. Instead, even at very high power densities, we observed unbroadened, but saturated line shapes. This was a verification of calculations that showed that a when a cosecant-squared pulse shape drives a two level system, that the time and frequency dependencies factor. This means that the entire line shape amplitude varies with pulse power, but that its width and central position are insensitive to driving power.