原子团簇的液气相变和负热容现象实验研究

The atomic cluster is one of typical representatives of finite small systems comprising several to hundreds of atom units. Convincing prediction has been obtained from the micro-canonical ensemble theory that there are rich non-thermodynamical limit effects in atomic clusters under high temperature extreme conditions, such as liquid-gas phase transition and negative heat capacity. The study of phase transition in atomic clusters has recently attracted a strong interest from theoretical and experimental communities. But up to now, a theoretical thermodynamics framework for finite small systems is still under construction, in which the requirement of thermodynamical limit is not met. In order to accurately constitute the theoretical framework, it is very significative to experimentally study these non-thermodynamical limit effects. The aim of this program is to experimentally study the liquid-gas phase transition and negative heat capacity in an atomic cluster system based on the heavy ion accelerator of IMP. The fullerene molecules (C60/C70) will be selected as model systems because experiments on fullerene molecules are much easier than other cluster systems. We plan to induce the fulleren molecules to undergo excitation, ionization, evaporation, and multi-fragmentation using heavy ions with different energies and different species. The recoil ions, the projectile particles, and the ejected electrons will be measured using an event-by-event mode and an ion-velocity map imaging technique. The informations (excitation energy, excitation temperature, and initial charge state) of parents prior to fragmentation will be extracted by analyzing the recorded data. Based on the measured relation between excitation energy and temperature of parent and the fragment distribution behavior, we will experimentally confirm whether there occur a liquid-gas phase transition and a negative heat capacity in fragmenting atomic cluster systems.

原子团簇是由多个原子组成的有限小系统，理论预言在高温极端条件下它会呈现出诸多非热力学极限效应下的物理现象，例如液气相变和负热容。目前用来描述有限小系统的热力学理论框架仍在构建之中，实验研究这些非热力学极限下的物理现象对正确构建这一新型理论具有重要的指导意义。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开展原子团簇的液气相变和负热容现象的实验研究。计划选取实验室容易操纵的碳富勒烯分子（C60/C70）作为原子团簇的模型系统；采用不同种类和不同入射能量的离子炮弹诱导其发生激发、电离、蒸发碎裂和多重碎裂过程；基于动量谱仪和符合测量技术实现反冲离子、出射电子和散射离子的符合测量；通过数据关联分析获得母体碎裂前的激发能、激发温度和碎裂后的碎片尺寸分布。实验测定的母体激发能和碎片尺寸分布随母体温度的变化关系，将成为检验原子团簇发生碎裂相变时是否具有液气相变特征和是否存在负热容现象的实验依据。

依托本基金项目，完成了实验装置的改进、整体联调和测试。利用改进的实验装置开展了原子团簇的电离和碎裂相变的物理实验研究、以及对影响此类实验精度的各实验技术环节的因素进行了定量评估，具体研究进展如下：1.测量了气相C60分子束从溢出束向定向束的流态转变；发现了分子束在定向流态中平动能无法被冷却的奇特现象，这一现象暗示了流态传输中存在明显的分子内环境效应；这些实验观察促使我们修正广泛应用50年的分子流速度分布方程。2.对气相原子团簇的电离和碎裂实验中的弱电荷效应进行了实验测定，量化了影响速度成像谱仪分辨能力的极限离子密度约为5*10^6 ions/cm3；如此弱的空间电荷效应在传统的飞行时间质谱实验中通常是被忽略的。3.提出了一个准确实验评估速度成像探测器是否饱和的方法，量化了不同的实验条件下探测器可接受的最大离子流密度。4.基于实验测量到的一个新颖的时间切片二维位置影像（影像中每个离子的种类和来源均可以很好的鉴别），并结合我们发展的C60分子竞争衰变模型，提出了一个准确测定母体离子发生碎裂相变前所对应的绝对内能和绝对光吸收截面的新方法；该方法可以扩展应用到与C60分子具有类似衰变行为的其它原子团簇和复杂分子体系。