See: metastable state in nuclear chemistry, metastable state in spectrochemistry, metastable ion in mass spectrometry, transient (chemical) species
PAC, 1972, 31, 577 (Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry) on page 609
PAC, 1982, 54, 1533 (Glossary of terms used in nuclear analytical chemistry (Provisional)) on page 1546
PAC, 1985, 57, 1453 (Nomenclature, symbols, units and their usage in spectrochemical analysis - V: Radiation sources (Recommendations 1985)) on page 1463
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1139
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