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