Vibrational spectroscopy of size-selected neutral and cationic clusters combined with vacuum-ultraviolet one-photon ionization detection.

Various vibrational spectroscopic techniques combined with vacuum-ultraviolet one-photon ionization mass spectrometry have recently been developed for jet-cooled molecules and clusters. These techniques open general applications of size-selected infrared and also Raman spectroscopies to neutral clusters by overcoming difficulties in the conventional vibrational spectroscopic methods for jet-cooled clusters. Their spectroscopic principles have been demonstrated by investigations for neutral clusters of simple protic molecules without an ultraviolet chromophore, which is necessary for the conventional vibrational spectroscopic methods of gas phase clusters based on the fluorescence or multiphoton ionization detection. In addition, the vacuum-ultraviolet photoionization detection scheme enables us to perform infrared predissociation spectroscopy of unstable cluster cations such as unprotonated cluster cations of protic molecules. In this Perspective, the spectroscopic principles and their applications for neutral and cationic clusters are reviewed.