Whether the surface of neat water is “acidic” or “basic” remains an active and controversial field of research. Most of the experimental evidence supporting the preferential adsorption of H₃O⁺ ions stems from nonlinear optical spectroscopy methods typically carried out at extreme pH conditions (pH < 1). Here, we use vibrational sum frequency spectroscopy (VSFS) to target the “proton continuum”, an unexplored frequency range characteristic of hydrated protons and hydroxide ions. The VSFS spectra of neat water show a broad and nonzero signal intensity between 1700 and 3000 cm^–1 in the three different polarization combinations examined. By comparing the SF response of water with that from dilute HCl and NaOH aqueous solutions, we conclude the intensity does not originate from either adsorbed H₃O⁺ or OH^– ions. Contributions from the nonresonant background are then critically considered by comparing the experimental results with many-body molecular dynamics (MB-MD) simulated spectra.

中文翻译：

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整齐的水蒸气界面：质子连续体和非共振背景

净水表面是“酸性”还是“碱性”仍然是活跃且有争议的研究领域。支持H ₃ O ⁺离子优先吸附的大多数实验证据均来自非线性光学光谱法，通常在极端pH条件（pH <1）下进行。在这里，我们使用振动和频谱（VSFS）来瞄准“质子连续体”，这是水合质子和氢氧根离子的未探索的频率范围特征。净水的VSFS光谱显示1700至3000 cm ^–1之间的信号强度较宽且非零在三种不同的极化组合中进行了研究。通过比较，从稀HCl和NaOH水溶液的水的SF响应，我们得出结论的强度不从任一吸附ħ发起₃ ö ⁺或OH ^-离子。然后，通过将实验结果与多体分子动力学（MB-MD）模拟光谱进行比较，可以严格考虑来自非共振背景的贡献。