It is the science that studies ions and molecules in the gas phase, most often enabled by some form of mass spectrometry.
Ions with small solvation spheres of 1, 2, 3... solvent molecules can be studied in the gas phase and then extrapolated to bulk solution.
[3] RRKM theory is used to compute simple estimates of the unimolecular ion decomposition reaction rates from a few characteristics of the potential energy surface.
Associative ionization is a gas phase reaction in which two atoms or molecules interact to form a single product ion.
CID (also called collisionally activated dissociation - CAD) is a method used to fragment molecular ions in the gas phase.