Solvation structure of iodide on an aqueous solution surface and relevant heterogeneous chemical reactions

离子与溶液表面溶剂分子的特定相互作用导致离子的特征溶剂化结构。由于卤素离子的溶剂化结构与大气中OH自由基的非均相化学反应有关，因此对卤素离子溶剂化结构的研究具有重要意义。第一性原理分子动力学模拟表明，碘离子倾向于位于表面附近，而氯离子则倾向于位于表面深处。我们测量了水溶液表面附近碘离子的吸收光谱：当紫外光照射NaI水溶液时，I-被激发到CTTS（电荷转移到溶剂）状态。然后，它通过吸收另一个光子进一步被激发，电子从I-中释放出来。由于电子从溶液表面逃逸的深度约为1nm，因此我们可以通过气相法检测到来自表面附近的I-的电子。我们测量了电子的信号强度作为激光波长的函数，发现I-在低NaI浓度下倾向于位于溶液内部，在高NaI浓度下开始出现在表面。我们通过检测表面的离子来检测Na^+的溶剂化结构。结果表明，无论NaI浓度如何，Na^+都倾向于在溶液中。

Specific interaction of ions with solvent molecules on a solution surface causes characteristic solvation structures of ions. As solvation structures of halogen ions relate to the heterogeneous chemical reaction of OH radicals in the atmosphere, it is very important to elucidate the solvation structures. First-principles molecular dynamics simulation suggests that iodide ions tend to locate near the surface, whereas chloride ions prefer to be located deep inside the surface.We measured absorption spectrum of iodide ions in the vicinity of the aqueous solution surface : When a NaI aqueous solution is irradiated with a UV light, I- is excited to the CTTS (charge transfer to solvent) state. Then, it is further excited by absorbing another photon and the electron is released from I-. As the electron escape depth from the solution surface is in the order of 1nm, we are able to detect electrons originating from I- located in the vicinity of the surface by the gas phase method. We measured a signal intensity of the electrons as a function of the wavelength of the laser, and found out that I- tends to be located inside the solution at the lower NaI concentration, starting to appear on the surface at the higher NaI concentration. We examined the solvation structures of Na^+ by detecting ions from the surface. It was found that Na^+ tends to be inside the solution regardless of the concentration of NaI.