metastable ion

in mass spectrometry
https://doi.org/10.1351/goldbook.M03874
An ion which is formed with sufficient excitation to dissociate spontaneously during its flight from the ion source to the detector.
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]