Attosecond decoherence dynamics

Pulses with sufficiently broad coherent bandwidth can now bridge the energy splitting between valence and inner-shell atomic orbitals (see Figure 1). One might expect that by ionizing these orbitals using an attosecond pulse, a coherent superposition of the corresponding ionic eigenstates is formed. While the entire system—ion plus photoelectron—is described by a wave function, the ion alone however must be described by a density matrix. This opens up the possibility that the state of the ion is not perfectly coherent. We show in Ref.1 that the Coulomb interaction between the photoelectron and the parent ion triggers complex many-body effects, which unexpectedly enhance the entanglement between photoelectron and ion—leading subsequently to decoherence within the ion. We point out strategies for controlling the decoherence, offering new opportunities for x-ray free-electron lasers.

ionization dynamics
Figure 1: Snapshots of electron wavepacket after ultrafast ionization. ©2010 Stefan Pabst.

Here, we analyze the creation of hole states via single-photon ionization in atoms using a single extreme-ultraviolet attosecond pulse (see Fig. 1). We investigate the creation of hole states via attosecond photoionization using the implementation of the time-dependent configuration-interaction singles (TDCIS) approach described in Ref.2. TDCIS allows us to study ionization dynamics beyond the common single-channel approximation and to understand systematically the relevance of multiple channels in the hole creation process.

Furthermore, we study the impact of the freed photoelectron on the remaining ion and demonstrate that the interaction between the photoelectron and the ion cannot be neglected for currently available state-of-the-art attosecond pulses. In particular, the interchannel coupling of the initially coherently excited hole states greatly enhances the entanglement between the photoelectron and the ionic states. Interchannel coupling is mediated by the photoelectron and mixes different ionization channels, i.e., hole configurations, with each other. Consequently, the degree of coherence among the ionic states is strongly reduced, making it impossible to describe the subsequent electronic motion in the ion with a pure quantum mechanical state (see Figure 2).

ionization dynamics
Figure 2: Time dependence of the degree of coherence for different pulse durations. Illustration is taken from Ref.1. ©2011 American Physical Society.

Our results have far-reaching consequences beyond the atomic case. Molecules will be even more strongly affected by interchannel coupling due to the reduced symmetry and smaller energy splittings between the cation many-electron eigenstates. Interchannel coupling is also likely to be significant for inner-valence hole configurations in molecules, which show strong mixing to configurations outside the TDCIS model space.

The present study suggests that interchannel coupling accompanying the hole creation process will affect attosecond experiments investigating charge transfer processes in photoionized systems. The control of decoherence requires widely tunable attosecond sources, thus offering a new opportunity for x-ray free-electron lasers.