Experimental characterization of microscopic events and behaviors of SO₂–H₂O interactions is crucial to understanding SO₂ atmospheric chemistry but has been proven to be very challenging due to the difficulty in size selection. Here, size-dependent development of SO₂ hydrate structure and cluster growth in the SO₂(H₂O)_n (n = 1–16) complexes was probed by infrared spectroscopy based on threshold photoionization using a tunable vacuum ultraviolet free electron laser. Spectral changes with cluster size demonstrate that the sandwich structure initially formed at n = 1 develops into cycle structures with the sulfur and oxygen atoms in a two-dimensional plane (n = 2 and 3) and then into three-dimensional cage structures (n ≥ 4). SO₂ is favorably bound to the surface of larger water clusters. These stepwise features of SO₂ hydration on various sized water clusters contribute to understanding the reactive sites and electrophilicity of SO₂ on cloud droplets, which may have important atmospheric implications for studying the SO₂-containing aerosol systems.

中文翻译：

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二氧化硫逐步水合基序的红外光谱

_{SO 2} -H ₂ O 相互作用的微观事件和行为的实验表征对于理解 SO ₂大气化学至关重要，但由于尺寸选择的困难已被证明非常具有挑战性。在这里，使用可调谐真空紫外自由电子激光器基于阈值光电离的红外光谱法探测了 SO ₂ (H ₂ O) _n ( n = 1-16) 配合物中 SO ₂水合物结构的尺寸依赖性发展和簇生长。光谱随簇大小的变化表明三明治结构最初形成于n= 1 发展成具有硫和氧原子在二维平面中的循环结构（n = 2 和 3），然后发展成三维笼状结构（n ≥ 4）。SO ₂有利地结合到较大水簇的表面。_{SO 2}在各种大小的水簇上水合的逐步特征有助于了解 SO ₂在云滴上的反应位点和亲电性，这可能对研究含 SO ₂的气溶胶系统具有重要的大气影响。