Excitation and Relaxation in Atom-Cluster Collisions

Abstract

Electronic and vibrational degrees of freedom in atom-cluster collisions are treated simultaneously and self-consistently by combining time-dependent density functional theory with classical molecular dynamics. The gradual change of the excitation mechanisms (electronic and vibrational) as well as the related relaxation phenomena (phase transitions and fragmentation) are studied in a common framework as a function of the impact energy (eV. . . MeV). Cluster "transparency" characterized by practically undisturbed atom-cluster penetration is predicted to be an important reaction mechanism within a particular window of impact energies.PACS numbers: 31.70. Hq, 31.15.Ew, 34.50.Bw, 36.40.Ei Collisions with atomic clusters represent a relatively new branch of collision physics as compared to the well established fields of ion-atom collisions [1] and ion-solid interaction [2]. The study of cluster collisions is of particular interest and importance because it offers the possibility to tackle bridge-building questions (like the continuous transition from individual excitations in the elementary ion-atom collision to the macroscopic stopping power in solids) as well as fundamental problems (like phase transitions in finite systems). It is also an extremely challenging and complicated field as the comprehensive understanding of these collisions still requires the development of basically new techniques for both, large-scale (multi-parametric) experiment and manybody (quantum-mechanical) theory. In this request, the present situation resembles very much that of nuclear physics at the beginning of the 80's [3].Experimentally, large progress has been made, meanwhile, in the investigation of adiabatic cluster collisions where the reaction mechanism is determined by vibrational excitations only. Typical examples are the study of the vibrational energy transfer [4], the fusion between clusters [5], the formation of endohedral complexes [6], and the collision induced dissociation (CID) [7]. There is also a lasting interest to study non-adiabatic cluster collisions where electronic transitions occur. Experiments in this field concern the measurements of the charge transfer [8][9][10], ionization and electronic excitation [11], as well as the selective observation of vibrational and electronic excitations [12].Theoretically, adiabatic cluster collisions can be well described by quantum molecular dynamics (QMD) or molecular dynamics (MD) [13][14][15][16][17]. Also, isomeric [18] and solid-liquid phase transitions [19,20] [25,26] approaches where the atomic structure, and thus the vibrational degrees of freedom, are not taken into account. Recently, a general theory has been developed [27] which is able to describe simultaneously adiabatic and non-adiabatic collisions and, in particular, also the still completely unknown transition regime where both -electronic and vibrational -excitations occur. This so-called non-adiabatic quantum molecular dynamics (NA-QMD) [27] can also be used to study, for the first time, phase transitions i...

Non-adiabatic quantum molecular dynamics: ionization of many-electron systems

“…It has been successfully applied so far to very different non-adiabatic processes, like atom-cluster collisions [60], ion-fullerene collisions [61], laser induced excitation and fragmentation of molecules [54] or fragmentation and isomerization of organic molecules in laser fields [62]. However, a realistic description of ionization with the NA-QMD theory 2 is still an open problem.…”

Time-Dependent Density Functional Theory in Atomic Collisions

“…The relevant KS equations are either solved on a grid (for a review see [41]) or in a finite-basis expansion of the time-dependent KS orbitals [42]. The latter formalism has been successfully applied to collisions between ions and sodium clusters followed by electron transfer [43] and fragmentation processes [44].…”

Scattering of a proton with the Li4 cluster: Non-adiabatic molecular dynamics description based on time-dependent density-functional theory

“…[8]. It has later been applied to various collision problems: ion-fullerene collisions [9], atom-sodium cluster collisions [10], charge transfer in atom-cluster collisions [11,12], the stopping power of protons or antiprotons in clusters [13,14] or insulators [15], the excitation and ionization of molecules such as ethylene due to proton collisions [16], or the interaction of protons or heavier ions with carbon nanostructures or graphitic sheets [17,18]. It may also be used to study laser-induced molecular or cluster dynamics in the high-field (but still not relativistic) regime; some examples are Refs.…”