metastable ion

in mass spectrometry
An ion which is formed with sufficient excitation to dissociate spontaneously during its flight from the ion source to the detector.
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]