Multielectron Effects

Mollow Sidebands

We have shown that sidebands of the usual odd harmonics occur in high harmonic spectra of open shell molecules at equilibrium geometries. The sidebands are due to a driving laser induced one-photon coupling (Rabi flopping) between one of the inner valence orbitals and the HOMO, a mechanism analogous to Mollow triplets known from quantum optics. The phenomenon is found and illustrated for parallel as well as perpendicular transitions, di-, tri- and polyatomic molecules (see Figure).

Y. Xia and A. Jaron-Becker, Opt. Express 24, 4689 (2016)

Multielectron Contributions in HHG

High harmonic generation (HHG) is an important spectroscopic tool to study the response of molecules to intense laser fields. Usually, in these studies the single-active-electron approximation is applied, in which the dynamics of electrons from different orbitals are considered independently from each other. However, using time-dependent density functional theory we have shown that multielectron effects play an important role for the interpretation of recent experimental data for the ellipticity of higher-order harmonics from nitrogen molecules. In particular, our results contain essential contributions from the HOMO, the HOMO-1 and the HOMO-2 orbitals over the whole HHG spectrum (see Figure).

Y. Xia and A. Jaron-Becker, Opt. Lett. 39, 1463 (2014)

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.

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)

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)