Molecule and cluster fragmentation dynamics: Isotopic effect and charge mobility
by
Amine Cassimi(CIMAP/CIRIL, Caen, France)
→
Europe/Stockholm
FA31
FA31
Description
Our approach, as usual when physicists want to know the inside of a
microscopic object, is to blow it up and see. Indeed, while a molecule
absorbs high enough energy, bonds break and ions are produced. Thus,
electronic processes involved in the interaction may be revealed by the
fragmentation. These processes dominantly contribute to energy loss of
swift projectiles in matter. In the case of swift ions, the energy is
deposited on the electrons and leads to defect production due to
displacement of atoms. One fundamental remaining question in this field
is how electronic energy is transformed into kinetic energy of the
nuclei, and finally ends as tracks in the bulk. Ion-impact-induced
fragmentation of molecules is an example of how this transfer may
proceed. Neutral as well as charged fragments may react with neighboring
atoms or molecules. Both fragmentation and fragment reactivity are
invoked in the case of biological tissues as responsible of the observed
damages (cell death or mutation) and named respectively, direct and
indirect effects. Remains the fundamental question of what exactly
governs molecular dissociation itself. The chemical forces originating
from the remaining electrons are expected to govern this dissociation.
Thus, a refined analysis of the fragmentation patterns (stability,
branching ratios between the different fragmentation pathways and
Kinetic Energy Release (KER) distributions) would play a major role in
assessing the validity of the calculated molecular potential energy
surfaces.
Comparing the collision time to the different molecular characteristic
times (vibration, rotation) leads to the commonly accepted two-step
picture for molecular fragmentation. In a first step, electron removal
takes place on a fixed-in-space molecule with the transient molecular
ion keeping the equilibrium inter-nuclear distance of the neutral
molecule. Then, in a second step, nuclear motion starts driven by the
fragmentation dynamics. Moreover, ions offer the opportunity to change
this interaction duration as well. By changing projectile velocity,
fragmentation in the projectile coulomb field is achievable.
The growing interest in the understanding of molecular fragmentation has
strongly benefited from the recent evolution of ultrafast timing and
imaging techniques implemented in Recoil Ion Momentum Spectroscopy
(RIMS) which allows now multi-hit detection with 4pi solid angle.
Measurement of all fragment momenta in coincidence makes it possible to
determine the complete kinematics of the process.
Our results concerning the general features of the fragmentation
dynamics as well as specific effects such as isotopic effects in water
molecules and charge mobility in Van der Waals clusters will be presented.