Collision Induced Fragmentation of Molecules and Small Na⁺ _n Clusters: Competition Between Impulsive and Electronic Mechanisms

Abstract

The dynamics of interaction between particles and matter involves of two basic mechanisms. Momentum transfer in collisions between atomic cores, a mechanism dominant at low collision velocity (v ≪ 1 a.u.), is primarily relevant for pure elastic atomic collisions. This first mechanism, referred to as the impulsive mechanism (IM), is primarily responsible for vibro-rotational excitation and reactive processes in molecular collisions. On the other hand, processes induced by excitation of the electron cloud, hereafter referred to as the electronic mechanism (EM), usually operated at much larger velocity (v≈l a.u.). These two mechanisms which are nothing else but the nuclear and electronic contributions of the stopping power of particles in matter [1] are also expected to drive Collision Induced Dissociation[2] (CID) of a simple molecule or a cluster. Let us consider for example the CID of a simple molecule at keV energies. In a first step the molecule is brought into an electronically excited state that can be an unbound or predissociated state. Then in a second step the molecule dissociates far from the collision partner. In contrast at low collision energy, typically below a few eV, dissociation occurs if a large momentum transfer during a close collision to one atomic core of the molecule or cluster is large. As a consequence a stretching of the vibrationnal bond occurs leading to dissociation of the molecule. At intermediate energies (few eV to few 10 keV) both EM and