dimeric ion

in mass spectrometry
https://doi.org/10.1351/goldbook.D01743
An ion formed either when a chemical species exists in the vapour as a dimer and can be detected as such, or when a molecular ion can attach to a neutral molecule within the ion source to form an ion such as [M2]+. where M represents the molecule.
Source:
PAC, 1991, 63, 1541. (Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)) on page 1549 [Terms] [Paper]