Peeking Inside a Dimer

We have studied the energy exchange between electrons in a helium dimer upon photon absorption. Results of numerical simulations for double photoionization are found to be in good agreement with recent experimental data for the angular distribution of the emitted electrons (see Figure). Together with the temporal evolution of the two-electron probability distribution this provides direct evidence for the knockoff mechanism. According to this mechanism the photon energy, which is initially absorbed by an electron at one of the atoms in the dimer, is then shared between the electrons over distances of several Angstroms via a collisional process. Using a Hamiltonian reduction method we have been able to study the role of the interactions between different particles in the process - in collaboration with C. Ruiz (Universidad Salamanca, Spain) and R. Dörner (Universität Frankfurt, Germany).

H. Ni and A. Becker, Phys. Rev. A 89, 033402 (2014)
H. Ni et al., Phys. Rev. A 88, 013407 (2013)

Selection Rules

We have analyzed so-called selection rules for the emission of two electrons from helium atom following the absorption of a few photons in a laser field. The increase of the number of absorbed photons leads to alternating suppression and non-suppression of the back-to-back emission of the two electrons. The Figure shows a snapshot of the two-electron dynamics following single-photon (inner part of distribution) and two-photon double ionization (outer part) - in collaboration with C. Ruiz (Universidad Salamanca, Spain).

H. Ni et al., J. Phys. B 44, 175601 (2011)

featured by J. Phys. B in their LabTalk.

In 2018 our theoretical predictions have been confirmed in an experiment performed by the group of R. Dörner.

Past Projects

Two-Electron Models Beyond 1D
We developed and applied models for the interaction of two-electron atoms and molecules with intense laser pulses.
C. Ruiz et al. Phys. Rev. Lett. 96, 053001 (2006)
A. Staudte et al., Phys. Rev. Lett. 99, 263002 (2007)
S. Baier et al., Phys. Rev. A 78, 013409 (2008)
S. Chen et al., Phys. Rev. A 82, 033426 (2010)
JILA research highlight: Ionize Me.

Mechanism of Non-Sequential Double Ionization
We used S-matrix theory to identify the mechanism of non-sequential double ionization in strong laser fields.
A. Becker and F.H.M. Faisal, J. Phys. B 29, L197 (1996)
A. Becker and F.H.M. Faisal, Phys. Rev. A 59, R1742, R3182 (1999)
A. Becker and F.H.M. Faisal, Phys. Rev. Lett. 84, 3546 (2000)
M. Weckenbrock et al., Phys. Rev. Lett. 91, 123004 (2003)