It is a long-standing assumption that strong laser pulses cannot induce significant vibrational excitation
and dissociation of a molecule, unless the laser pulse is chirped to account for the anharmonicity of molecular
vibrations. Our results of numerical simulations for the hydrogen molecular ion indicate that this conjecture
may have to be revised. We find an enhancement of dissociation over ionization by many orders of magnitude
for interaction of the molecular ion with an unchirped ultrashort laser pulse at certain wavelengths in the infrared.
The dissociation mechanism is related to an efficient excitation of molecular vibrations via two- and multiphoton
transitions, which we investigated before in view of a control of the internal state of this nonpolar molecule
(see 
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.
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.
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)