Critical Sizes against Coulomb Dissociation of Highly Charged Sodium Clusters Obtained by Ion Impact

Chandezon, F.; Guet, C.; Huber, B. A.; Jalabert, D.; Maurel, Marie‐Christine; Monnand, E.; Ristori, C.; Rocco, J.C.

doi:10.1103/physrevlett.74.3784

Cited by 125 publications

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“…To achieve better fragmentation statistics, at the expense of some loss of first-principles accuracy, we employ a soft, phenomenological pseudopotential [21], which permits a relatively large real-space grid step ∆ = 1.35 a.u. We do not expect our choice of functional E xc [ρ] or pseudopotential to affect significantly the main results for barrierless fission.To study the fragmentation of a single species Na N Q+ at an "initial temperature" T in , we run M dynamical simulations arising from an ensemble of M initial conditions constructed as follows: (i) We optimize the geometry of the neutral cluster Na N at 0 K; (ii) we perform an MD run of ≥ 20 ps for Na N at 400 K (which is roughly the temperature of the clusters Na N output by the cluster source in the collision experiments [8][9][10][11]); (iii) we take M ionic (nuclear) configurations {R (n) …”

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Charge-Induced Fragmentation of Sodium Clusters

et al. 2001

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The fission of highly charged sodium clusters with fissilities X > 1 is studied by ab initio molecular dynamics. Na244+ is found to undergo predominantly sequential Na3 + emission on a time scale of 1 ps, while Na24 Q+ (5 ≤ Q ≤ 8) undergoes multifragmentation on a time scale ≥ 0.1 ps, with Na + increasingly the dominant fragment as Q increases. All singly-charged fragments Nan + up to size n = 6 are observed. The observed fragment spectrum is, within statistical error, independent of the temperature T of the parent cluster for T ≤ 1500 K. These findings are consistent with and explain recent trends observed experimentally. 36.40.Qv, 36.40.Wa, 31.15.Qg, 31.15.Ew Starting with the work of Sattler et al.[1] on van der Waal's clusters, the study of the fragmentation (fission) of charged atomic or molecular clusters has proved a valuable experimental tool for investigating the intrinsic stability and binding forces of these objects. These studies and others yield important insights into the behavior of matter at the small size limit and the size-dependent evolution toward bulk properties. Much recent work on fission has been devoted to metallic clusters, both experimentally [2][3][4][5][6][7][8][9][10][11] and theoretically [4,9,[12][13][14][15][16]. Fission of metallic clusters is particularly interesting on account of the similarities and differences with the nuclear fission process [12]. Over a century ago, Lord Rayleigh [17] studied the problem of the time development of the Coulomb instability of a surface-charged liquid drop, and conjectured that the excess charge would be removed by the emission of jets, rather than by fission into two parts of more or less equal size (as occurs for nuclei, where the charge is distributed uniformly through the volume).According to the Rayleigh criterion [17], a charged liquid drop is unstable against Coulomb forces when its fissility X > 1, where X = E Coul /(2E surf ) is proportional to the ratio of the Coulomb to surface energy of the drop. For a metallic cluster M N Q+ , assumed spherical with radius r s N 1/3 (r s is the Wigner-Seitz radius for the metal), this gives X = (16πr 3 s σ) −1 Q 2 /N , where σ is the surface tension, or X ≈ 2.5Q 2 /N for Na N Q+ . Now, fission experiments on metallic clusters where the cluster is charged by laser ionization [2][3][4][5][6][7] have so far produced only clusters with X < 1, for which an energy barrier exists against fission. Thermally activated fission may be observed, however, and the dominant charged fragment is found to be Na 3 + , which has a closed electronic shell and is particularly stable; Na + has not so far been observed. In a different type of experiment [8][9][10][11], sodium clusters are ionized by collision with a beam of highly charged ions, a technique that is expected to allow study of a much wider range of X and cluster temperatures. Coincidence measurements reveal multifragmentation processes in some cases, often with Na + as the dominant fragment. In this Letter, we offer the first systematic dynamical study ...

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Charge-Induced Fragmentation of Sodium Clusters

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“…For X ∼ 1 clusters are found in metastable states and must overcome a potential barrier in order to fission. As in many other recent studies, 5,6,7,8,9,10,11 of both experimental and theoretical character, we focus on the latter situation and present results of our all-electrons density functional calculations which help to achieve better understanding of various aspects of fission of metastable charged metal clusters. We use sodium clusters, well studied experimentally, as a sample revealing general properties of univalent metal clusters.…”

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Geometrical and statistical factors in fission of small metal clusters

et al. 2005

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Fission of metastable charged univalent metal clusters has been studied on example of N a 2+ 10 and N a 2+ 18 clusters by means of density functional theory methods. Energetics of the process, i.e. dissociation energies and fission barriers, as well as its dynamics, i.e. fission pathways, have been analyzed. The dissociation energies and fission barriers have been calculated for the full range of fission channels for the N a 2+ 10 cluster. Our data confirms the viewpoint that there is some correlation between these two quantities, which can often be explained by electronic shell effects. However, there is no quantitative correspondence between the dissociation energies and heights of fission barriers and the former can not serve as a quick estimate for the latter. The impact of cluster structure on the fission process has been elucidated. The calculations show that the geometry of the smaller fragment and geometry of its immediate neighborhood in the larger fragment play a leading role in defining the fission barrier height: energy barriers for removing different combinations of atoms from the same parent cluster can vary greatly, while energy barriers for removing similar groups of atoms from similar places in different isomers of the parent cluster are usually similar. The present study demonstrates importance of rearrangement of the cluster structure during fission. The rearrangment can lower the fission barriers significantly. It may include forming a neck between the two fragments or fissioning via another isomer state of the parent cluster; examples of such processes are given. For several low-lying isomers of N a 2+ 10 cluster the potential barriers for transitions between these isomer states are calculated and compared with the corresponding fission barriers. These data suggest that there is a competition between "direct" fission and fission going via intermediate isomer states of the parent cluster. An impact of the cluster geometry on the change of the system's entropy due to fission is also discussed.

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“…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.…”

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Excitation and Relaxation in Atom-Cluster Collisions

Saalmann

Schmidt

1998

Phys. Rev. Lett.

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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...

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Critical Sizes against Coulomb Dissociation of Highly Charged Sodium Clusters Obtained by Ion Impact

Cited by 125 publications

References 16 publications

Charge-Induced Fragmentation of Sodium Clusters

Charge-Induced Fragmentation of Sodium Clusters

Geometrical and statistical factors in fission of small metal clusters

Excitation and Relaxation in Atom-Cluster Collisions

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