Electronic Spectroscopy of Cold Gas‐Phase Ions in a Cryogenic Ion Trap: Vibronic States, Bonding Characteristics, and Photochemistry

Laser spectroscopy under cryogenic gas-phase conditions enables the high-precision investigation of intrinsic molecular properties by minimizing perturbations from external environments such as impurities, solvents, and counterions. In particular, cryogenic ion-trap (CIT) spectroscopy has broadened access to a diverse range of molecular and cluster ions. When combined with electronic ultraviolet-visible (UV-Vis) spectroscopy, which is typically performed in action schemes (photofragmentation, evaporation of inert tag molecules, laser-induced fluorescence, and so on), it serves as a sensitive and accurate probe for the vibronic structures, bonding characteristics, conformations, and excited-state dynamics of target ions. In this review, we present a brief overview of typical experimental setups and key techniques for CIT spectroscopy; we then survey recent case studies for various ions, including carbocations and protonated hydrocarbons, organic dyes, host-guest complexes, microhydrated ions, hypervalent ions, metal clusters, and chemical intermediates formed in solution. We highlight the precise and unambiguous information accessible using this method and illustrate the rapidly expanding scope of modern gas-phase chemistry.