Experimental Methods in Cation Spectroscopy

The spectroscopic study of molecular cations in the gas phase poses a range of challenges which are generally associated with their high chemical reactivity, the difficulties in producing sufficiently high number densities of the desired ion species in a spectrometer, and the limits on the particle densities imposed by the space‐charge effect. The present article provides an overview of different experimental techniques that have been developed over the past years to overcome these problems. The topics discussed include highly sensitive laser‐based spectroscopic methods (cavity ring‐down spectroscopy, frequency‐, velocity‐ and concentration‐modulation schemes, and various types of action spectroscopy), the production of internally cold samples of ions and the use of mass‐spectrometric methods to select the species of interest out of a mixed sample. Owing to these developments, the range of molecular cations amenable to a spectroscopic characterization has been extended considerably over the past years and now includes various types of cluster ions, protonated biomolecules, ionic carbon chains, and the ions of polycyclic aromatic hydrocarbons.