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

## TL;DR

Cryogenic ion-trap spectroscopy allows precise study of molecular ions in a controlled environment, revealing their electronic and structural properties.

## Contribution

The paper reviews recent advances in cryogenic ion-trap spectroscopy and its application to diverse nonvolatile ions.

## Key findings

- Cryogenic ion-trap spectroscopy provides accurate vibronic and bonding information for a wide range of ions.
- The method enables the study of excited-state dynamics and conformations of molecular ions.
- Recent instrumentation developments have expanded the scope to include complex and nonvolatile species.

## Abstract

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.

Recent development of cryogenic ion‐trap‐based instrumentation has extended the scope of gas‐phase laser spectroscopy to a wide variety of nonvolatile ions, including carbocations and protonated hydrocarbons, organic dyes, host‐guest complexes, micro‐hydrated ions, hypervalent ions, metal clusters, and ionic intermediates formed in solution.