Single ionization of an atom in a strong field has been successfully modeled with the aid of measured ion yields and electron spectra. Double-ionization (DI), however, has presented its own challenges since it cannot be explained by the intuitive extension of the single electron theories. At low intensities, DI ion yields exhibit a knee structure commonly associated with the presence of an additional ionization mechanism. Until three years ago, the electron energy distributions from the double ionization process have remained a mystery. New techniques had to be developed in order to obtain the DI's electron spectrum since its signal is hidden by the much stronger (by orders of magnitude) single ionization events.; The early results from these experiments seemed consistent with the rescattering model of double ionization. This model explains the enhancement in the strong-field limit where the field can be treated as quasi-static, e.g., for helium at high intensities. However, at lower intensities, the double ionization of xenon falls within a regime where rescattering becomes classically forbidden, and the effects of the quantum nature of the interaction become important.; In this thesis, a high-resolution, high statistics electron-ion coincidence spectrometer has been used to measure in detail the double ionization electron energy spectra in xenon and argon. Through the use of the Keldysh parameter the transition from tunneling/rescattering to multiphoton ionization was mapped with these two species. The measurements in xenon show that at low intensities rescattering is not predominant. In argon, a clear transition to the tunneling regime is evident. For the first time, such an evolution has been observed in the DI spectrum.; Since xenon's DI spectrum shows structure akin to multiphoton ionization, a wavelength dependent study was performed. A clear difference between the collected electron spectra and ion yield ratios has been observed for the different wavelengths, which suggests an ion-core excitation as an enhancement mechanism. This study opens the door to understanding other mechanisms that may take place in the double ionization of lighter rare gases, which are overpowered by the rescattering enhancement.
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