Liquid-jet photoelectron spectroscopy (LJ-PES) enabled a breakthrough in the experimental study of the electronic structure of liquid water, aqueous solutions, and volatile liquids more generally. The novelty of this technique, dating back over 25 years, lies in stabilizing a continuous, micron-diameter LJ in a vacuum environment to enable PES studies. A key quantity in PES is the most probable energy associated with vertical promotion of an electron into vacuum: the vertical ionization energy, VIE, for neutrals and cations, or vertical detachment energy, VDE, for anions. These quantities can be used to identify species, their chemical states and bonding environments, and their structural properties in solution. The ability to accurately measure VIEs and VDEs is correspondingly crucial. An associated principal challenge is the determination of these quantities with respect to well-defined energy references. Only with recently developed methods are such measurements routinely and generally viable for liquids. Practically, these methods involve the application of condensed-matter concepts to the acquisition of photoelectron (PE) spectra from liquid samples, rather than solely relying on molecular-physics treatments that have been commonly implemented since the first LJ-PES experiments. This includes explicit consideration of the traversal of electrons to and through the liquid’s surface, prior to free-electron detection. Our approach to measuring VIEs and VDEs with respect to the liquid vacuum level specifically involves detecting the lowest-energy electrons emitted from the sample, which have barely enough energy to surmount the surface potential and accumulate in the low-energy tail of the liquid-phase spectrum. By applying a sufficient bias potential to the liquid sample, this low-energy spectral tail can generally be exposed, with its sharp, low-energy cutoff revealing the genuine kinetic-energy-zero in a measured spectrum, independent of any perturbing intrinsic or extrinsic potentials in the experiment. Together with a precisely known ionizing photon energy, this feature enables the straightforward determination of VIEs or VDEs, with respect to the liquid-phase vacuum level, from any PE feature of interest. Furthermore, by additionally determining solution-phase VIEs and VDEs with respect to the common equilibrated energy level in condensed matter, the Fermi level─the generally implemented reference energy in solid-state PES─solution work functions, eΦ, and liquid-vacuum surface dipole effects can be quantified. With LJs, the Fermi level can only be properly accessed by controlling unwanted surface charging and all other extrinsic potentials, which lead to energy shifts of all PE features and preclude access to accurate electronic energetics. More specifically, conditions must be engineered to minimize all undesirable potentials, while maintaining the equilibrated, intrinsic (contact) potential difference between the sample and apparatus. The establishment of these liquid-phase, accurate energy-referencing protocols importantly enables VIE and VDE determinations from near-arbitrary solutions and the quantitative distinction between bulk electronic structure and interfacial effects. We will review and exemplify these protocols for liquid water and several exemplary aqueous solutions here, with a focus on the lowest-ionization- or lowest-detachment-energy PE peaks, which importantly relate to the oxidative stabilities of aqueous-phase species.

中文翻译：

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来自光电子能谱的液体的绝对电子能量学和定量功函数

液体喷射光电子能谱 (LJ-PES) 使液态水、水溶液和更普遍的挥发性液体的电子结构实验研究取得突破。这项技术的新颖性可追溯到 25 年前，其新颖之处在于在真空环境中稳定连续的微米直径 LJ，以进行 PES 研究。PES 中的一个关键量是与垂直提升电子进入真空相关的最可能的能量：垂直电离能 VIE，用于中性和阳离子，或垂直分离能，VDE，用于阴离子。这些量可用于识别物种、它们的化学状态和键合环境，以及它们在溶液中的结构特性。准确衡量 VIE 和 VDE 的能力相应地至关重要。一个相关的主要挑战是根据明确定义的能量参考来确定这些量。只有最近开发的方法才能对液体进行常规和普遍可行的测量。实际上，这些方法涉及将凝聚态概念应用于从液体样品中获取光电子 (PE) 光谱，而不是仅仅依赖于自第一次 LJ-PES 实验以来普遍实施的分子物理学处理。这包括在自由电子检测之前明确考虑电子穿过液体表面的情况。我们针对液体真空水平测量 VIE 和 VDE 的方法特别涉及检测样品发射的最低能量电子，它们几乎没有足够的能量来克服表面势能并积聚在液相光谱的低能尾部。通过对液体样品施加足够的偏置电位，通常可以暴露这种低能量光谱尾部，其尖锐的低能量截止揭示了测量光谱中真正的零动能，与任何扰动的内在或外在因素无关实验中的潜力。结合精确已知的电离光子能量，此功能可以根据任何感兴趣的 PE 特征直接确定与液相真空水平相关的 VIE 或 VDE。此外，通过另外确定关于凝聚态物质中常见平衡能级的溶液相 VIE 和 VDE，费米能级——固态 PES 中通常实现的参考能量——溶液功函数、eΦ 和液体-真空表面偶极子效应可以量化。使用 LJ，费米能级只能通过控制不需要的表面电荷和所有其他外在势能来正确访问，这会导致所有 PE 特征的能量转移并妨碍获得准确的电子能量学。更具体地说，必须设计条件以尽量减少所有不需要的电位，同时保持样品和设备之间的平衡、固有（接触）电位差。这些液相、准确的能量参考协议的建立，重要的是能够从近乎任意的解决方案中确定 VIE 和 VDE，以及体电子结构和界面效应之间的定量区分。