Abstract
Recent developments in terahertz technologies provide new tools for analysis, inspection, and nondestructive sensing. If a heavy atom is encapsulated in a cage of a porous material, the atom should vibrate slowly and resonate with a low-frequency terahertz light. From this perspective, a cyanide-bridged metal framework is a suitable system because it contains many cages that can adsorb Cs ions. Herein we show the vibration mode of a Cs ion in a cage of a cyanide-bridged metal framework. First-principles phonon mode calculations and terahertz time-domain spectroscopy (THz-TDS) measurements indicate that the vibration mode of a Cs ion in a cyanide-bridged manganese-iron framework is at 1.5 THz, which is significantly apart from other lattice vibrations. Taking advantage of this feature, we develop a THz-light detection method for Cs ions, which is useful for non-contact sensing of Cs ions in dangerous environments or harmful circumstances.