Size-Specific Infrared Spectroscopy of Neutral Hydrated Clusters Based on a Vacuum Ultraviolet Free Electron Laser.

Spectroscopic study of neutral hydrated clusters provides microscopic insights into the local structures and dynamics of complex systems but is very challenging because of the difficulty in size discrimination. Recently, we developed infrared (IR) spectroscopy based on threshold ionization using a tunable vacuum ultraviolet free electron laser (VUV-FEL). This experimental method allows for size-specific IR spectroscopic measurement of numerous neutral clusters without confinement, such as messenger tags, ultraviolet chromophores, and host matrix. This Perspective aims to highlight the latest advances in VUV-FEL-based IR spectroscopic studies on the confinement-free neutral amine-water, sulfur dioxide-water, and metal-water clusters. Fruitful collaborations with high-level quantum chemical calculations are reviewed. Future research directions with relevance to the atmosphere, biology, and catalysis are proposed.