Inelastic Energy Loss of Cluster ions in Solids

本研究致力于研究簇离子与固体之间的相互作用。作为一个基本量，我们研究了电子能量损失率（停止功率）。固体电子由传导电子和核心电子组成，其介电响应用波包模型来描述。在此分析中，我们考虑了以下因素：1)各组成离子的电子分布；2)假设团簇在进入时瞬间电离，电荷态达到速度相关的平均值；3)用标准分子动力学方法估计的库仑爆炸过程；如果是这样，4)团簇取向与入射方向的相关性。我们给出了电子能量损失中簇效应的结果。考虑了碳簇Cn（n=1-8）入射到碳箔上的情况。为方便起见，我们定义了R=DELTAE(Cn)/(nDELTAE(C))和R'=DELTAE(Cn)/n-DELTAE(C)。集群效应是指R'> 1或R'>0。这里DELTAE（Cn）是簇Cn（n=1-8）的能量损失。对于厚度为250*的碳，计算得到的R′与n的比值与实验数据吻合较好，显示出Baudin等人得到的簇效应。重要的结果是清楚地发现了阈值速度的存在，并且阈值速度与簇大小n无关。接下来，作为与停止功率相关的量，我们研究了簇碰撞下动力学过程引起的二次电子产率。在这里，通过统计方式考虑到簇中组成离子的大小，并假设发生瞬时电离。在此条件下，可以发现聚类效应对SE产率的影响。

This research is devoted to the study of interaction between a cluster-ion and a solid. As a basic quantity, we investigated the electronic energy-loss rate (stopping power). The solid electrons consist of conduction electrons and core electrons, whose dielectric response are described by the wave-packet model. In this analysis, we took into account the followings : 1)the electron distribution of each constituent ions, 2)the assumption that on entrance the cluster is instantly ionized and the charge state attains to the velocity-dependent average value, 3)the coulomb explosion process estimated by a standard molecular dynamics method, and if so, 4)the correlation between the cluster orientation and the direction of the incidence. We present the results that show the cluster effect in the electronic energy loss. The case is considered where a carbon cluster Cn(n=1-8) with a few MeV/atom is incident on a carbon foil. For convenience, we defined the quantities R=DELTAE(Cn)/(nDELTAE(C)) and R'=DELTAE(Cn)/n-DELTAE(C). The cluster effect means R>1 or R'>0.Here DELTAE(Cn) is the energy loss of a cluster Cn(n=1-8). The calculated R' values vs n for a carbon of 250* thickness shows good agreement with the experimental data, showing the cluster effect, obtained by Baudin et al. The important results are that the existence of the threshold velocity was clearly found, and that the threshold velocity is independent of cluster size n. Next, as a quantity related to the stopping power, we investigated the secondary electron yield due to the kinetic process under cluster impact. Here the size of constituent ions in a cluster is taken into account by a statistical manner and the instantaneous ionization is assumed to take place. Under these conditions, the cluster effect on the SE yield could be found.