Malerz Sebastian, Trinter Florian, Hergenhahn Uwe, Ghrist Aaron, Ali Hebatallah, Nicolas Christophe, Saak Clara-Magdalena, Richter Clemens, Hartweg Sebastian, Nahon Laurent, Lee Chin, Goy Claudia, Neumark Daniel M, Meijer Gerard, Wilkinson Iain, Winter Bernd, Thürmer Stephan
Molecular Physics Department, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany.
Phys Chem Chem Phys. 2021 Apr 14;23(14):8246-8260. doi: 10.1039/d1cp00430a. Epub 2021 Mar 12.
We report on the effects of electron collision and indirect ionization processes, occurring at photoexcitation and electron kinetic energies well below 30 eV, on the photoemission spectra of liquid water. We show that the nascent photoelectron spectrum and, hence, the inferred electron binding energy can only be accurately determined if electron energies are large enough that cross sections for quasi-elastic scattering processes, such as vibrational excitation, are negligible. Otherwise, quasi-elastic scattering leads to strong, down-to-few-meV kinetic energy scattering losses from the direct photoelectron features, which manifest in severely distorted intrinsic photoelectron peak shapes. The associated cross-over point from predominant (known) electronically inelastic to quasi-elastic scattering seems to arise at surprisingly large electron kinetic energies, of approximately 10-14 eV. Concomitantly, we present evidence for the onset of indirect, autoionization phenomena (occurring via superexcited states) within a few eV of the primary and secondary ionization thresholds. These processes are inferred to compete with the direct ionization channels and primarily produce low-energy photoelectrons at photon and electron impact excitation energies below ∼15 eV. Our results highlight that vibrational inelastic electron scattering processes and neutral photoexcitation and autoionization channels become increasingly important when photon and electron kinetic energies are decreased towards the ionization threshold. Correspondingly, we show that for neat water and aqueous solutions, great care must be taken when quantitatively analyzing photoelectron spectra measured too close to the ionization threshold. Such care is essential for the accurate determination of solvent and solute ionization energies as well as photoelectron branching ratios and peak magnitudes.
我们报告了在光激发和电子动能远低于30电子伏特时发生的电子碰撞和间接电离过程对液态水光发射光谱的影响。我们表明，只有当电子能量足够大，使得诸如振动激发等准弹性散射过程的截面可忽略不计时，才能准确确定初生光电子能谱以及由此推断的电子结合能。否则，准弹性散射会导致直接光电子特征出现强烈的、低至几电子伏特的动能散射损失，这表现为固有光电子峰形状严重扭曲。从主要的（已知的）电子非弹性散射到准弹性散射的相关交叉点似乎出现在令人惊讶的大电子动能处，约为10 - 14电子伏特。同时，我们提供了证据，表明在初级和次级电离阈值的几电子伏特范围内存在间接自电离现象（通过超激发态发生）。据推断，这些过程与直接电离通道竞争，并且在光子和电子碰撞激发能量低于约15电子伏特时主要产生低能光电子。我们的结果突出表明，当光子和电子动能朝着电离阈值降低时，振动非弹性电子散射过程以及中性光激发和自电离通道变得越来越重要。相应地，我们表明，对于纯水和水溶液，在定量分析离电离阈值太近时测量的光电子能谱时必须格外小心。这种小心对于准确确定溶剂和溶质的电离能以及光电子分支比和峰强度至关重要。
