Electron excitation and bombardment effect induced by polyatomic ions

研究了快速多原子离子在固体中的电子能量损失及其轰击下的二次电子发射。首先，我们建立了簇离子的自洽平均电荷理论。我们的理论预测，在同等速度下，每离子的簇电荷平均值小于孤立离子的簇电荷平均值。对组成原子数、箔厚度和空间结构的依赖性被发现得到了很好的解释。通过考虑库仑爆炸和簇平均电荷，以MeV/原子能量计算了碳簇离子的电子能量损失。尽管每个离子的平均电荷减少了，但在相同的速度下，簇中每个离子的能量损失大于单个离子的能量损失（这被称为“正”簇效应）。在我们的研究中，“负”簇效应也被预测为较低的离子速度。我们发现团簇效应的阈值速度位于碳团簇玻尔速度的两倍左右，并且它不太依赖于箔的厚度。接下来，我们研究了MeV/原子碳簇在碳箔上的二次电子发射。基于三步模型计算的每离子相对SE能谱显示出亚线性效应。这一结果与Kudo等人最近的实验数据一致。相对SE产率还与团簇停止功率和团簇平均电荷有关。最后利用“能量传递模型”中的电离概率对多重电离过程进行了研究。计算的横截面与稀有气体原子的数据一致，甚至对快速高电荷离子的入射也是如此，没有任何拟合参数。

We studied the electronic energy-loss of swift polyatomic ions in solids and the secondary electron emission under their bombardment. First, we developed the self-consistent average charge theory for cluster ions. Our theory predicts the cluster charge average per ion is less than that of the isolated ion at an equivalent speed. The dependences on the number of constituent atoms, on the foil thickness, and on the spatial structure were found to be well explained. By taking into the coulomb explosion and the cluster average charge, we calculated the electronic energy loss of carbon cluster ions with the MeV/atom energies. In spite of the reduction of the average charge per ion, the energy loss per ion of the cluster is greater than that of the single ion at an equivalent speed(this is called the 'positive' cluster effect). In our treatment, the 'negative' cluster effect is also predicted for lower ion speeds. We found that the threshold speed in this cluster effect is located around two times Bohr velocity for the carbon clusters, and it does not so strongly depend on the foil thickness. Next, we investigated the secondary electron(SE) emission from carbon foil by the MeV/atom carbon clusters. The relative SE energy spectra per ion calculated on the basis of the three-step model showed the sublinear effect. This result is consistent with the recent experimental data by Kudo et al. The relative SE yield is also related with the cluster stopping power and with the cluster average charge. Finally we studied the multiple ionization process by the ionization probability in the 'energy transfer model'. The calculated cross sections agree with the data for rare gas atoms even for the incidence of swift highly charged ions, without any fitting parameters.