Femtosecond Neutralization Dynamics in Cluster-Solid Surface Collisions

Abstract

Experimental results for the relative electron emission yields g͑N͒ of charged clusters colliding at low energies with different surfaces are presented. For fixed collision energy a remarkable cluster size dependence of g͑N͒ is obtained using highly oriented pyrolytic graphite as target. By studying theoretically the collision process within a microscopic model we find that the nonadiabatic survival probability of the charged clusters shows the same behavior as g͑N͒. Thus, g͑N͒ reflects the femtosecond neutralization dynamics during the collision, which may result in damped Stückelberg oscillations for targets with narrow densities of states. [S0031-9007(97)04139-2] PACS numbers: 79.20. Rf, 34.50.Dy, 36.40.Wa Quantum effects observed when two macroscopic objects interact via a tunneling gap are always exciting as they might be important for future nanoelectronic devices. In addition they sometimes allow intriguing insight into the physics of quantum states induced by the charge confinement in a system of reduced dimensionality. As a matter of fact, however, the dynamics of the electron motion usually remains hidden due to the extremely short time scales involved. Only recently the pumpprobe techniques with femtosecond laser light pulses have started to give some progress in the understanding of such processes.Here we will raise the question of whether the transient character of a collision process of a charged metal atom cluster with a solid surface might elucidate the dynamics of the electron motion between the two partners. Will there be a single electron jump as in a classical picture, or might there be a resonant tunneling process where the charge density-once the collision partners overcome a minimum threshold distance-fluctuates between cluster and surface?So far, this question has not been tackled. A variety of collision experiments between clusters and solid surfaces were performed during the last decades. These can be grouped into investigations involving (i) surface modification by cluster bombardment or deposition [1,2], (ii) scattering of intact species and their fragments [3,4], and (iii) electron emission [5][6][7]. Such studies gave, for instance, insight into the stability of small particles and, to a certain extent, their geometry; energetic cluster impact can modify the formation of thin films. During energetic impact there exist nonequilibrium conditions similar to those in shock tubes, but on a very different time scale, a femtosecond scale. New types of chemical reactions, characterized by extremely high density, pressure, and kinetic temperature, are expected to occur. All these experiments give no hints of possible quantum effects, which are the topic of this Letter.The setup of the experiment for collision induced electron emission measurements will be described in detail elsewhere [8]. In short, the clusters are produced in a source of the PACIS type [9]. After acceleration to their final collision energy E coll , they are mass separated in a Wien velocity filter (Colutron 600 B) which...

Ultra‐fast dynamics in solids: non‐equilibrium behaviour of magnetism and atomic structure

“…This seems to explain the interesting electron-emission yield observed by Meiwes-Broer et al [50]. Equation (125) implies that the electron emission is largest if the negatively charged clusters arrive at the surface unneutralized.…”

“…As discussed by Meiwes-Broer et al [50], the electron emission γ due to negatively charged clusters colliding with the surface is related to the neutralization dynamics of the negatively charged clusters: γ(N ) ∼ P (N, t) .…”

Cluster-surface collisions: Characteristics of Xe55- and C20–Si[111] surface bombardment

“…The subject has developed rapidly, and becomes an important component of modern physical science due to its relevance to issues in both fundamental and applied research. Phenomena related to the collision processes include sputtering, 1-7 soft-landing/deposition, 7-11 epitaxial growth, [12][13][14]60 surface modification, [15][16][17][18][19] implantation, 20,57,58 phase transition, [54][55][56][57] fragmentation, [21][22][23][24][25][26][27][28]56 electronic excitation, [29][30][31][32][33][34] and collision induced chemical reactions. 24,[35][36][37]51,52 These studies enable new areas of research via modern cluster technology for the investigation of novel chemical and physical processes.…”

“…Metals ͑Au, Cu, Pt, Al,..͒, 6,9,34,38,40,46,48,53,54,57,58 ionic crystals, 6,55,56 silicon, 11,14,16,[23][24][25]31,[35][36][37]41,42,47 and graphite/ diamond 18,28,29,[42][43][44]50 are typical materials used as substrates, which are sometimes covered by rare gas matrices ͑Ar, Xe, Ne,..͒. 9,54,55 The chemical compositions of clusters cover an even wider range of systems.…”