Phase control of complex autoionizing resonances

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In recent years, advances in short pulse extreme ultra-violet (XUV) sources have brought special attention to autoionizing resonances in atoms and small molecules. These resonances arise due to autoionizing states, which are often viewed as the combination of a weakly bound (Rydberg) electron and an excited ion core. The interaction of the Rydberg electron and the core leads to energy exchange, taking the energy from the core and liberating the weakly bound electron. This simple picture becomes a lot more complex when several such states overlap. Mixing of resonances results in complex spectra challenging their analysis and assignment. However, such complex resonances also provide a unique opportunity to investigate the evolution of electron-electron correlations, potentially with attosecond time resolution. Couplings between weakly bound Rydberg states carry tremendous potential for control of correlated dynamics with laser fields. When autoionization involves several core states, the initial ultrafast excitation creates a coherence between them and could lead to ultrafast hole migration in the cationic core. Laser control of the Rydberg electrons enables control of this charge migration in the ion core. In this proposal we will investigate this concept of control via Rydberg states. We will venture into the domain of strongly mixed autoionizing series of nitrogen molecules, which encode rich dynamics both in the ionic core and in the Rydberg states. We will employ coherent excitation of several overlapping autoionizing states in molecules with an ultrashort XUV pulse, using a weak NIR-vis laser pulse as a knob to control excitation, and using time-resolved photoelectron spectroscopy to follow populations of the core states. The molecular autoionizing resonances of nitrogen investigated in our preliminary work are excellent candidates for this control concept. These resonances are composed of two Rydberg states and decay towards two continua with two different ionic states (X and A states of the nitrogen ion). Accordingly, in this proposal we will develop theoretically and will test experimentally control of the relative phases of the two Rydberg states. We will test two distinct phase control schemes: through an auxiliary discrete state and through the ionization continuum. Owing to the mixed character of these resonances the control of the phase will influence the mixing of the autoionization channels and thus will modify the branching ratio for the ionic states. By controlling the branching ratio in autoionization our method enables shaping of the electronic wavepacket formed in the ion core by autoionization and thus opens the route to manipulation of ultrafast charge migration.