Molecular Physics seminar

Molecule and cluster fragmentation dynamics: Isotopic effect and charge mobility

by Amine Cassimi (CIMAP/CIRIL, Caen, France)

FA31 ()


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.