Electron, Where Are You?

Attosecond laser technology is expected to provide movies of the ultrafast quantum world of electrons in atoms, molecules and other materials. We studied how a strong laser field drives the electron in between the two protons of the hydrogen molecular ion. As a surprise, we found that the electron dynamics is more complex than previously assumed and sometimes even counterintuitive. This leads to the unexpected result that the electron may not leave the molecule through the tunnel exit when the field of the laser is strongest, in contrast to the predictions of popular quasistatic ionization pictures. Indeed, our numerical results show that there are multiple bursts of electron ejection at low field strengths in the oscillating electric field of a laser (see Figure). In collaboration with the experimental group of R. Dörner we further showed how the attosecond intramolecular dynamics can be mapped onto the momenta of the electron in the continuum where it becomes observable.

References:

F. He et al., Phys. Rev. Lett. 101, 213002 (2008)
N. Takemoto and A. Becker, Phys. Rev. Lett. 105, 203004 (2010)
M. Odenweller et al., Phys. Rev. Lett. 107, 143004 (2011)

Collaborations:

R. Dörner (University Frankfurt, Germany)
F. He (now, Shanghai Jiao Tong University, China)
U. Thumm (Kansas State University, USA)

Discussion of our work with the group of R. Dörner in NewScientist: Attoclock turns electrons into movie stars

JILA research highlight: Quantum Bod Swapping.

Towards Imaging a Molecular Reaction

The ability to observe and control, in real time, the dynamics of electrons in a chemical bond is one of the goals of ultrafast intense laser science. We have shown that electron rearrangement in the whole valence shell of a dissociating bromine molecule can be visualized by ionization of the molecule during its transition from molecule to atoms. Theoretical results for the total (see Figure) and alignment dependent ionization signals are in good agreement with experimental data, obtained in the groups of M. Murnane and H. Kapteyn. Characteristic changes in the signals can be attributed to dominant contributions from different orbitals during the dissociation. Our results show a molecular-like response of the system for long times after the initiation of the dissociation and, hence, up to rather large internuclear distances.

Results of numerical simulations for the dissociating hydrogen molecular ion show that ultrashort laser pulse technology should make it possible to image a nuclear wave packet during the dissociation of a diatomic molecule. We propose to make use of two-center interference effects in the ionization yields. From the interference patterns dynamic information about the wavepacket, namely its velocity, mean internuclear distance and spreading, can be retrieved.

References:

W. Li et al., Proc. Nat. Acad. Sciences U.S.A. 107, 20219 (2010)
A. Jaron, IEEE Journ. Select. Topics Quant. Electron. (in press)
A. Picon et al., Phys. Rev. A 83, 013414 (2011)

Collaboration:

M.M. Murnane and H.C. Kapteyn (JILA)

JILA research highlight (Br2): The Long Goodbye.

Presentation of the Br2 work received the Best Poster Presenter Award at the International Symposium Ultrafast Intense Laser Science 9, Dec 2010

Suppressed Molecular Ionization

Ion yields of molecules measured in the past with CO2 lasers were almost identical to those of atoms with comparable ionization potentials. This similarity has been thought to be a consequence of the tunnel ionization picture, in which the ionization probability depends on the ionization potential and the field strength. Recent experiments with Ti:sapphire lasers have shown that this interpretation does not hold. Diatomics and complex molecules do reach, with a few exceptions, saturated ionization at higher intensities than the companion atoms. S-matrix results are in agreement with experimental data for a series of molecules, as exemplified for C60. The suppression is found to be due to interference effects between partial waves emitted from different nuclear centers in the molecule.

References:

J. Muth-Böhm et al., Phys. Rev. Lett. 85, 2280 (2000)
A. Jaron-Becker et al., Phys. Rev. Lett. 96, 143006 (2006)

Collaboration:

F.H.M. Faisal (Bielefeld University, Germany)