自电场作用下离子喷射现象被发现以来，人们一直保持着对其的研究兴趣。如今，离子喷射在材料处理与分析等工程中已有着广泛应用，如聚焦离子束、空间推进器等，这些技术的核心都是对电场作用下液体表面离子喷射的控制。室温离子液体（room-temperature ionic liquids）是一类新型液态电解质材料，由于其热稳定、可定制等卓越特性，有望替代传统液体工质材料，推动新一代离子源（ion source）的开发。鉴于目前实验很难达到离子液体的纯离子喷射工况，也没有足够高的时空分辨率研究液体表面的离子喷射过程，本文运用分子动力学模拟方法，研究了在外加电场作用下，室温离子液体-真空界面处的离子喷射过程。

本文首先使离子液体在控温器作用下维持热平衡1纳秒，在系统足够稳定后，再施加电压驱动离子喷射。我们计算了离子喷射速率随离子液体表面法向电场的变化规律，发现其符合离子喷射理论。分别比较了Langevin、耗散粒子动力学（DPD）和Berendsen控温器作用下的计算结果。研究发现，Langevin控温器作用下的离子喷射速率总小于其他控温器的计算结果，并且控温器参数——阻尼系数对离子喷射速率有显著影响。DPD控温器作用下的模拟结果与摩擦系数的关系不大，且其结果与Berendsen控温器作用下的模拟结果基本一致。基于控温原理分析，我们认为DPD控温器更适合模拟离子喷射现象。还分析了温度对离子喷射速率的影响，发现在300-450K温度范围内，随着温度增加，近似线性增加。

本文还比较了三种施加电场的方法（恒电势法、恒电荷法和恒电场法）对计算结果的影响，发现三种施加电场方法得到的喷射速率-液面法向电场曲线基本一致，其主要原因是模拟系统是准一维系统，当不同方法中液面电场相同时，对应的离子喷射速率也相同。通过统计分析程序运行的时间，发现恒电场方法的计算量最少，其次是恒电势法，恒电荷法消耗的计算资源较大。此外，还发现电场会改变离子液体内部的结构，在使阴阳离子分层的同时，略微增加了离子液体的体积。

我们进一步分析了真空中电场随时间的变化规律，发现真空中的电场存在比较明显的波动，电场的每一次突变都代表着喷射离子穿越真空中的检测面或者有离子从模拟系统中删除。电场随时间的变化主要由喷射离子与离子液体薄膜之间形成的感应电场所造成。

本论文中的工作可以为纯离子态下电喷雾动力学模拟方法的选择提供参考，也能为使用分子动力学模拟研究更复杂的电喷雾现象奠定基础。

Since the phenomenon of electric-field-induced ion emission was discovered, research interests have been devoted to its fundamental mechanism and applications. Nowadays, electric-field-induced ion emission has been widely used in materials processing/analysis and engineering, such as focused ion beam, space thruster, etc. The core of these technologies is the control of ion emission from the liquid surface under electric fields. Room-temperature ionic liquid (RTIL) is a new type of liquid electrolyte materials. Due to its unique properties such as excellent thermal stability and customizability, it is expected to replace traditional liquid materials used in ion sources and promote the development of a new generation of ion source. In view of the fact that current experiments are difficult to achieve the condition of pure ion emission, and the spatio-temporal resolution is not high enough to study the ion emission process on the liquid surface, this paper uses the molecular dynamics simulation method to study the ion emission process at the interface between the RTIL and vacuum under the action of applied electric fields.

In this paper, the RTIL is first equilibrated to reach desired temperatures for 1 nanosecond by using thermostats. After the systems were stable enough, electrical potential difference between electrodes were applied to drive ion emission. We studied ion emission from a planar interface between the RTIL and vacuum under external electric fields by using MD simulations. We calculated the ion emission rate as a function of the electric field normal to the RTIL surface, and found that it accords with the ion emission theory. The results under Langevin, dissipative particle dynamics (DPD), and Berendsen thermostats were compared. It was found that the ion emission rate under Langevin thermostat is smaller than those under other thermostats, and the damping coefficient in Langevin thermostat greatly affect ion emission rate. However, the simulation results under DPD thermostat are nearly independent on the friction coefficient, the parameter in DPD thermostat. It was also observed that the results under DPD thermostat are similar to those under Berendsen thermostat. Based on the analysis of thermostat mechanisms, we proposed that DPD thermostat is more suitable for simulating ion emission phenomenon. The effect of temperature on ion emission rate was also analyzed, and it was found that, in the range of 300-450K, the increase in ion emission rate is approximately linear with the increase of temperature.

We also compared results under different methods for applying the electric field (constant potential, constant charge, and constant field methods), and found that ion emission rate - electric field normal to the RTIL surface curves from the three methods agree with each other. This is mainly because the simulation system we used is quasi-one-dimensional, and the ion emission rates are the same as long as the electric fields normal to the liquid surface are equal. It was found that computational cost of simulations using constant field method is the least, followed by simulations using constant potential method. Simulations using constant charge method consumes the most computation resources. We found that the electric field can change the internal structure of the RTIL: in addition to the layering of cations and anions, the volume of the RTIL is slightly enlarged.

We further analyzed the temporal evolution of the electric field in the vacuum, and observed fluctuations in the amplitude of the electric field. Every sudden change in the electric field represents the passage of emitted ions across specific detecting planes or the removal of ions from the simulation system. Variations of electric fields over time were traced to the induced electric field between the emitted ion and RTIL film.

This work could guide the selection of proper methods for MD simulations of electrospray in the pure ion regime, and lays the foundation to study more complex electrospray phenomena using MD simulations.