Ionization and fragmentation of polyatomic molecules in strongfields: spotlight on nuclear motion
When a molecule interacts with a strong laser field a wealth of processes can take place. In this talk I
will discuss the results of several recent experiments on ionization and fragmentation dynamics of
polyatomic molecules subject to ultrashort, intense laser pulses. In particular, I will highlight the
crucial influence of nuclear dynamics and molecular restructuring during the laser-interaction on the
ionization and fragmentation behavior.
As a first example I will show that C-H bond-stretching during triple ionization of ethylene in between
the second and third ionization steps largely determines whether the molecule fragments into two or
three moieties [1]. As a result, it becomes possible to control the outcome of the fragmentation
reaction dynamics with a very high confidence using the pulse duration as a control parameter.
For slower molecular dynamics a very similar control scheme can be implemented using a doublepulse sequence. We show for CO2 that two relatively delayed pulses can be used for controlling and
visualizing the molecular pathway towards the dication and fragments of it [2].
Nuclear dynamics during laser-pulse interaction is also behind ionization of polyatomic molecules to
high charge states, which results in very fast and complete decomposition of the molecular ion [3, 4].
A short review of the current understanding of this process in polyatomic molecules, where multiple
bonds are involved in the ionization dynamics, will be given.
I will contrast these results with those of very recent experiments on ethylene and acetylene which
revealed that protons during molecular fragmentation can also be ejected on surprisingly long timescales up to microseconds [4]. I will discuss these experiments and offer an explanation for the
surprisingly slow fragmentation dynamics based on population of high-lying vibrational levels due to
molecular restructuring dynamics.
Population of high-lying states is actually a quite common strong-field process: Electrons are not only
emitted, but the strong-field can also induce trapping of electrons in high-lying states – a process
that is also known as frustrated ionization [5]. I will show that this process driven in argon dimers
with few-cycle laser pulses shows a distinct dependence on the carrier-envelope offset phase.
[1]
[2]
[3]
[4]
[4]
[5]
X. Xie et al., Sci. Rep. 5, 12877 (2015).
S. Erattupuzha et al., J. Chem. Phys. submitted, (2015).
S. Roither et al., Phys. Rev. Lett. 106, 163001 (2011).
X. Xie et al., Phys. Rev. A. 89, 023429 (2014).
S. Larimian et al., in preparation (2015).
T. Nubbemeyer et al., Phys. Rev. Lett. 101, 233001 (2008).