Infrared spectroscopy of cold trapped molecular ions using He‐tagging

This review summarizes experimental activities to study the structure of molecular ions via He tagging . The method is based on the attachment of a weakly bound helium atom to a cold ion followed by laser‐induced predissociation (LIP). Since my early involvements (it started in 1977 with a letter from Y.T. Lee), radio frequency (rf) ion traps and ion guides have been important elements in instruments dedicated to ion spectroscopy. Accumulating ions in a ring electrode trap (RET) and confining them together with the laser‐induced photofragments in a long octopole has been demonstrated in 1978 in Berkeley via photodissociation of metastable O 2 + ions. In the early stage of this instrument, as well as in various further developments, supersonic expansions have been used to create weakly bound complexes. An important step forward for ion spectroscopy was to push the conditions of cryogenic ion traps so far that, finally, He atoms could be attached to almost any mass‐selected ion of interest, including multiply charged ions and C 60 + . Currently, modern ion storage instruments reach temperatures below 3 K and can be operated at helium densities above 10 16  cm −3 , opening up many avenues of application in spectroscopy, reaction dynamics, and analytical chemistry. In addition to a personal historical review, I discuss recent progress made with new cryogenic ion traps, especially in the field of He tagging. He‐M + ions have been formed via ternary association for all kind of M + ions ranging from atoms such as He + , N + , or Fe + via molecules N 2 + , VO + , and H 3 + to various polyatomic ions. The in situ synthesis of tagged ions made unique discoveries possible, such as determining the structure of doubly charged benzene, the first identification of a carrier of diffuse interstellar bands, or the characterization of the fundamental 4 electron 4 center system He–H 3 + . In the conclusions, hints to additional applications will be given, emphasizing on the versatility of temperature‐variable ion traps.