从分子动力学推断离子团簇的热解离动力学

中文摘要 AI 辅助

在纯离子模式下运行的电喷雾离子液体离子源发射亚稳态、低溶剂化数的离子团簇，其发射后的解离会改变羽流组成、能量分布和诊断观测量。虽然无场测量支持微秒时间尺度上的一级热激活衰变，但加速区域的特点是快速变化的电场和纳秒级的驻留时间，使得解离变得强烈场增强。在本工作中，使用微正则系综分子动力学（MD）轨迹在受控内能和施加电场下推断EMI-BF4离子团簇的解离动力学。在600至1000 K的温度和10^6至10^9 V/m的均匀电场下，模拟了正、负二聚体、三聚体和四聚体。使用基于连通性的碎裂判据提取解离寿命，并对产物通道进行分类以获得路径分辨的分支概率。在所有溶剂化数下，寿命在弱场极限下与场无关，但在10^8 V/m以上会下降数个数量级。团簇极性依赖性在温度控制区域最为显著，并随着静电功占主导而减弱。MD模拟进一步揭示了场驱动的解离拓扑变化，包括三聚体碎裂从中性对蒸发到带电核心离子喷射的转变，以及涉及单中性损失、双中性发射和核心离子喷射的三种竞争性四聚体路径。MD数据集被简化为{\ au_n}(E,T)的紧凑参数化形式，为多尺度羽流输运模型提供可转移的动力学输入。

英文摘要

Electrospray ionic liquid ion sources operating in the pure ion regime emit metastable, low solvation number ion clusters whose post-emission dissociation modifies plume composition, energy distributions, and diagnostic observables. While field-free measurements support first-order, thermally activated decay on microsecond timescales, the acceleration region is characterized by rapidly varying electric fields and nanosecond residence times such that dissociation becomes strongly field-enhanced. In this work, microcanonical ensemble molecular dynamics (MD) trajectories are used to infer dissociation kinetics of EMI-BF4 ion clusters under controlled internal energy and applied electric field. Positive and negative dimers, trimers, and tetramers are simulated at temperatures between 600 to 1000 K and uniform fields of 10^6 to 10^9 V/m. Dissociation lifetimes are extracted using a connectivity-based fragmentation criterion, and product channels are classified to obtain pathway-resolved branching probabilities. Across all solvation numbers, lifetimes are field independent in the weak field limit, but collapse by orders of magnitude above 10^8 V/m. Cluster polarity dependence is most prominent in the temperature-controlled regime and diminish as electrostatic work dominates. The MD simulations further reveal that field-driven changes in dissociation topology, including a transition in trimer breakup from neutral pair evaporation to charged core ion ejection, and three competing tetramer pathways involving single neutral loss, double neutral emission, and core ion ejection. The MD dataset is reduced to compact parameterizations of {τ_n}(E,T), yielding transferable kinetic inputs for multiscale plume transport models.

发表机构

• Sibley School of Mechanical & Aerospace Engineering, Cornell University（康奈尔大学西布利机械与航空工程学院）

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