Observation of oscillations between degenerate bound-state configurations in rapidly autoionizing two-electron atoms

The oscillation between bound-state configurations in a rapidly autoionizing three-body Coulomb system has been directly observed. Using a 500-fsec laser pulse, calcium atoms are excited to the pure ${4p}_{3/2}15d$ two-electron configuration at an energy greater than 3 eV above the ionization limit. As a result of configuration interaction, the electrons scatter coherently into multiple bound and continuum configurations. The oscillation between the degenerate ${4p}_{1/2}{n}^{\prime }d$ and ${4p}_{3/2}\mathrm{nd}$ modes as well as autoionization into ${4s}_{1/2}\epsilon \mathcal{l},$ ${3d}_{3/2}\epsilon \mathcal{l},$ and ${3d}_{5/2}\epsilon \mathcal{l}$ continua are observed explicitly using bound-state interferometry. The measured time dependence of the ${4p}_{3/2}15d$ character is in excellent agreement with the Fourier transform of the frequency domain excitation cross section. To our knowledge, this is the first experimental demonstration of the equivalence of time and frequency domain spectra in a multiconfigurational system involving bound and continuum channels.