open access

https://doi.org/10.14943/lowtemsci.78.61

https://hdl.handle.net/2115/77766

Observation of proton ordering in crystalline ice films grown on a low-temperature substrate by sum frequency generation spectroscopy

日本語

Sum-frequency generation spectroscopy

2020-03-24

低温科学第78巻編集委員会

低温科学

78

61-70

Interfacial water is ubiquitous in nature and plays crucial roles in a variety of disciplines. In such a symmetrybreaking system, not only adsorption geometry but also anisotropic molecular orientation (i. e., H-up/H-down configuration) is a key structural parameter that determines the unique physicochemical properties of interfacial water systems. However, orientation of water molecules in interfacial hydrogen bond networks has been extremely difficult to investigate with traditional experimental techniques, such as electron diffraction, grazing X-ray scattering, and even scanning probe microscopy because hydrogen has only a single electron and thus respond extremely weakly to the probes used in these techniques. Therefore, the determination of molecular orientation of interfacial water has been an experimental challenge. We have used recently developed heterodyne-detected sum frequency generation (SFG) spectroscopy in our pioneering investigation of the orientation and structure of water molecules on the metal surfaces of the Pt(111). Then, we succeeded in directly demonstrating that the adsorbed first layer water molecules prefer an H-down configuration. The H-down ordering in the first layer was significantly pinned by the Pt (111) substrate and was subsequently propagated to the overlayer during the growth of a multilayer ice film. Temperature dependence of the SFG spectra revealed that such an exotic proton ordering is thermodynamically stable and has an extremely high critical temperature of ～173 K, which is more than twice as large as that of ferroelectric bulk ice XI (Tc = ～72 K). It was demonstrated that anisotropy at the heterointerface is key for stimulating novel exotic proton ordering in many-body proton systems such as ice.

紀要論文

Version of Record

1880-7593