Fragmentation of molecules or clusters by making them collide with atoms is a way of studying the dynamics of energy deposition and its subsequent distribution in polyatomic objects. Schematically, there are two main mechanisms by which collision-induced fragmentation can occur. The first mechanism involves energy or momentum transfer from the relative collisional motion to internal vibrational and rotational degrees of freedom of the molecule. If the transferred energy lies above the binding energy of an atom or a group of atoms in the molecule the latter may be freed and move apart. A particularly simple example of this mechanism is when the atomic projectile hits, in a close binary encounter, a specific atom or group of atoms in the molecule and kicks it out. This mechanism is called an impulsive mechanism. The second electronic mechanism involves excitation of a dissociative or predissociative electronic state of the molecule, which automatically entails its fragmentation.
Figure 7: Two mechanisms of collision induced fragmentation of Na2+. The impulsive mechanism occurs within the ground electronic state (red arrow), while the electronic mechanism proceeds through excitation of one or several dissociative electronic states (vertical arrows) with following fragmentation (green, purple and blue).
A step forward in the experimental investigation of collision-induced fragmentation has been made by combined time-of-flight and multiparametric coincidence techniques. Applied to the study of the following process
He + Na2+ → He + Na + Na+
this experimental technique has made possible the complete measurement of all observables characterizing the collision-induced dissociation process, including the scattering angle x and the relative kinetic energy e of the fragments (see Fig. 8a). Although such achievements are highly challenging for theory, we were able to reproduce and explain every feature (see Fig. 8b) in the experimental “map” of doubly differential cross section σ(ε,x) for fragmentation. The electronic and impulsive mechanisms of fragmentation manifest themselves very clearly in our results. Our theoretical approach involved quasiclassical trajectory approximation for the relative motion of colliding partners and the quantum wavepacket treatment of the internal (vibrational) degree of freedom.
Figure 8: a) Center of mass scheme for the collision-induced fragmentation of Na2+ in 1 keV collision with He target. b) Theoretically calculated doubly differential cross section σ(ε,χ) for fragmentation. Four structures on the σ(ε,χ) map correspond to four electronic states in Fig. 7 and correlate very closely with available experimental data.
Results of this work allowed detailed characterization of the fragmentation pathways for the multi-channel fragmentation of sodium triatomic:
He + Na3+ → He + Na + Na2+
→ He + Na2 + Na2
→ He + Na + Na + Na+
and for several larger sodium clusters as well.