Yamamoto Yo-Ichi, Ishiyama Tatsuya, Morita Akihiro, Suzuki Toshinori
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
Department of Applied Chemistry, Graduate School of Science and Engineering, University of Toyama, Toyama 930-8555, Japan.
J Phys Chem B. 2021 Sep 23;125(37):10514-10526. doi: 10.1021/acs.jpcb.1c04765. Epub 2021 Sep 8.
We present a study using extreme UV (EUV) photoemission spectroscopy of the valence electronic structures of aqueous and methanol solutions using a 10 kHz EUV light source based on high-order harmonic generation and a magnetic bottle time-of-flight electron spectrometer. Two aspects of the observed spectra are highlighted in this study. One is variation of the vertical ionization energy (VIE) for liquids as a function of the solute concentration, which is closely related to surface dipoles at the gas-liquid interface. The experimental results show that the VIE of liquid water increases slightly with increasing concentrations of NaCl and NaI and decreases with NaOH. The VIE of liquid methanol was also found to change slightly with NaI. On the other hand, tetrabutylammonium iodide (TBAI) and butylamine (BA) clearly reduce the VIE for liquid water, which is attributed to the formation of an electric double layer (EDL) by segregated solutes at the gas-liquid interface. As evidence for this, when the pH of an aqueous BA solution is reduced to protonate BA, the VIE shift gradually decreases because the protonated BA moves into the bulk to suppress the influence of the EDL. We computed the surface potentials for these solutions using molecular dynamics simulations, and the results supported our interpretation of the experimental results. Another observation is the variation of the relative energy and shape of individual photoelectron bands for solvents, which is related to alteration of the structure and constituents of the first solvation shell of ionized solvent molecules. All of the solutes cause changes in the photoelectron spectra at high concentration, one of the most prominent of which is the degree of splitting of the 3a band for liquid water and the 7a' band for liquid methanol, which are sensitive to hydrogen bonding in the liquids. The 3a splitting decreases with the increasing concentration of NaI, NaCl, and NaOH, indicating that Na penetrates into the hydrogen-bonding network to coordinate to a nonbonding electron of a water molecule. On the other hand, TBAI and BA cause smaller changes in the 3a splitting. Full interpretation of these spectroscopic features awaits extensive quantum chemical calculations and is beyond the scope of this study. However, these results illustrate the strong potential of EUV laser photoemission spectroscopy of liquids for exploration of interfacial and solution chemistry.
我们展示了一项研究，该研究使用基于高次谐波产生的10 kHz极紫外（EUV）光源和磁瓶飞行时间电子能谱仪，对水溶液和甲醇溶液的价电子结构进行EUV光发射光谱分析。本研究突出了观察到的光谱的两个方面。一是液体的垂直电离能（VIE）随溶质浓度的变化，这与气液界面处的表面偶极密切相关。实验结果表明，液态水的VIE随NaCl和NaI浓度的增加而略有增加，随NaOH浓度的增加而降低。液态甲醇的VIE也被发现随NaI浓度的变化而略有变化。另一方面，四丁基碘化铵（TBAI）和丁胺（BA）明显降低了液态水的VIE，这归因于气液界面处溶质分离形成的双电层（EDL）。作为对此的证据，当BA水溶液的pH值降低以使BA质子化时，VIE位移逐渐减小，因为质子化的BA移入本体以抑制EDL的影响。我们使用分子动力学模拟计算了这些溶液的表面电位，结果支持了我们对实验结果的解释。另一个观察结果是溶剂的各个光电子能带的相对能量和形状的变化，这与电离溶剂分子的第一溶剂化层的结构和成分的改变有关。所有溶质在高浓度下都会导致光电子能谱发生变化，其中最显著的变化之一是液态水的3a能带和液态甲醇的7a'能带的分裂程度，它们对液体中的氢键敏感。随着NaI、NaCl和NaOH浓度的增加，3a分裂减小，表明Na渗透到氢键网络中与水分子的一个非键电子配位。另一方面，TBAI和BA对3a分裂的影响较小。对这些光谱特征的全面解释有待广泛的量子化学计算，且超出了本研究的范围。然而，这些结果说明了EUV激光光发射光谱技术在探索界面和溶液化学方面的强大潜力。
