Fragmentation phase transition in atomic clusters II

Abstract

We present a statistical fragmentation study of doubly charged alkali (Li, Na, K) and antimony clusters. The evaporation of one charged trimer is the most dominant decay channel (asymmetric fission) at low excitation energies. For small sodium clusters this was quite early found in molecular dynamical calculations by Landman et al. [1]. For doubly charged lithium clusters, we predict Li + 9 to be the preferential dissociation channel. As already seen experimentally a more symmetric fission is found for doubly charged antimony clusters. This different behavior compared to the alkali metal clusters is in our model essentially due to a larger fissility of antimony. This is checked by repeating the calculations for Na ++ 52 with a bulk fissility parameter set artificially equal to the value of Sb.

Multistage transformation and charge effect during the fragmentation phase transition in atomic clusters

“…Typically, the upper limits of the NF-AD stage and the lower limits of the AD-MF stage are found to be 58 ± 3 and 55 ± 3 eV, respectively, indicating the absence of a threefold coexistence. The behavior is in qualitative agreement with the typical features of the FPT predicted by Gross's group [13][14][15][16]. Actually, the experimental observations of the multistage transformation is similar to that of the melting phase transition occurring in an atomic cluster, which was proved continually to be a multistage process initiated by a "premelting" stage where the surface of a cluster melts before the core [23,24].…”

“…Unlike the macrosystem case, theoretical treatment of the fragmentation phase transition (FPT) requires a microcanonical description. In a series of theoretical works from Gross's group [13][14][15][16], the calculated caloric curve for a cluster system was found to be divided into four parts: after an initially rising, a plateau and a backbending are present before the curve rises again. The results indicate a multistage FPT corresponding to a hierarchical opening of the cluster configuration with increasing energy, which was found also related to a stepwise appearance of typical fragmentation phenomena from no fragmentation (NF) through asymmetrical dissociation (AD) and multifragmentation (MF) to complete fragmentation (CF).…”

Fragmentation phase transition in atomic clusters I

“…It leads to a predominantly symmetric nuclear fission whereas for example doubly charged alkali clusters fission highly asymmetrically until strong shell effects of the daughters favor a more symmetrical split. The conditions for symmetric versus asymmetric fission will be discussed in the next paper of this series [28]. A further peculiarity of clusters plays an important role in fragmentation: Atomic clusters have a much higher internal density of states than nuclei.…”

On the fragmentation of multiply charged sodium clusters

“…So, one has to correct for this transition probability difference in order to keep detailed balance between the initial and final states. The correct inclusion of detailed balance for charged clusters is an improvement over the earlier work [10,11,12] where detailed balance was included only approximately for charged clusters.…”

“…In the present work the fragmentation of charged atomic Na clusters of mass 200, with cluster charges ranging from 0 to 9, is investigated assuming that statistical equilibration is achieved at some point before the fragmentation process takes place. The statistical method used is the Micro-canonical Metropolis Monte Carlo (MMMC) technique, described in detail in [7,8,9,10,11,12]. The fundamental assumption is that, at equilibrium, all micro-canonically accessible phase-space cells ξ i corresponding to a fixed energy (and any other conserved quantities) are equally probable.…”