多电荷态原子团簇发生碎裂相变时的电荷效应研究

Atomic cluster is a finite system comprising several to thousands of atom units. Under high temperature extreme conditions, hot atomic cluster systems can undergo a fragmentation phase transition. Convincing prediction that the transition is driven not only by increasing the excitation energy and temperature of a system but also by increasing its charge, has been obtained from simulations for the caloric curve of finite system. The results suggest the existence of the charge effect on the fragmentation phase transition in finite system. However, due to the absence of experimental study of fragmentation phase transition in charge-selected multiply-charged atomic cluster, clear experimental evidence for the charge effect is still missing till now. In this program, based on the heavy ion accelerator of IMP and ion-imaging techniqe, we plan to ascertain the direct experimental fingerprint of the existence of charge effect. Using the atomic cluster C60/C70 as a model, we plan to induce the fullerenes to undergo a fragmentation phase by heavy ion beam. The initial charge state,exciation energy and temperature of the parent ion before fragmentation are extracted by counting the number of emitted electrons, by analyzing the ionic fragment size distribution, and by analyzing the momentum distribution of ionic fragments reusting from evaporation fragmentation process, respectively. And then, based on the measured relation between excitation energy and temperature of perent, we will firstly investigate the fragmentation phase transition occuring in charge-selected multiply-charged atomic cluster, and provide direct experimental proof for the existence of the charge efect on the phase transition.

原子团簇是由多个原子单元组成的有限体系，在高温极端条件下它会发生碎裂相变，实验研究该相变过程对加深理解有限体系的状态方程和状态演化有重要意义。理论计算的热量曲线预言，驱动多电荷态原子团簇发生碎裂相变的物理量不只是沉积在系统内的激发能和温度，还有系统的初始电荷。本项目拟基于中科院近代物理研究所建成的重离子加速器，采用离子动量成像技术和符合测量方法，选择不同种类和不同入射能量的离子炮弹诱导原子团簇（以富勒烯团簇为例）发生碎裂相变。通过散射粒子电荷态和出射电子数目的符合测量实现母体电荷态的标定；通过分析蒸发碎裂通道对应碎片的初始动量分布实现母体温度的标定；通过分析离子碎片的尺寸分布实现母体激发能的标定。在以上三物理量被标定的前提下，首次开展多电荷态原子团簇的碎裂相变实验研究，测定不同初始电荷态的母体发生碎裂相变时对应的温度和激发能区，从而获得支持原子团簇发生碎裂相变时存在电荷效应的实验依据。

依托本基金项目，将已建成的研究离子、光子诱导原子团簇碎裂的实验装置进行升级改造，在实验技术上实现了电子和离子信息的符合测量。利用改进的实验装置，以富勒烯分子为原子团簇模型，开展了多电荷态原子团簇的碎裂相变实验研究。实验直接测量了高电荷态富勒烯离子发生碎裂前的激发能，根据母体发生碎裂时笼型结构的损坏程度与碎裂现象之间的对应关系创建了一个新型相变指示器，发现了原子团簇发生碎裂相变时存在多步相变过程和电荷效应两大重要特征。通过直接测量具有不同激发能的富勒烯离子发生碎裂后的离子碎片产额，得到了离子碎片随母体激发能的尺寸分布特征，结合先前我们创建的新型的分子温度计，首次构建了富勒烯分子在发生碎裂相变时可能具有了液气相变特征的物理图像。