Unexpected decoherence in ultrafast photoionization

Attosecond science holds the promise of controlling electron motion to manipulate physical processes at the atomic level. One way of inducing electron motion is photoionization using an attosecond laser pulse. The focus of our recent paper was to answer an open fundamental question about electron control via attosecond photoionization: Can the nonstationary state of the parent ion be described by a Schrödinger wave function, i.e., is the state coherent?

Probability densities of the photoelectron originating from the 4d0 and 5s shells of xenon are shown for different times after the ionizing attosecond pulse. The pulse has a duration of 10 as, a photon energy of 136 eV, and a peak electric field strength of 25 GV/m.

 

Pulses with sufficiently broad coherent bandwidth can now bridge the energy splitting between valence and inner-shell atomic orbitals. 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 showed 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 pointed out strategies for controlling the decoherence, offering new opportunities for x-ray free-electron lasers.

Phys. Rev. Lett. 106 (2011) 053002
  http://prl.aps.org/abstract/PRL/v106/i5/e053003