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For interpretation, we performed theoretical computations using post-Hartree\u2013Fock ab\u00a0initio computations to determine the neutral and ionic species formed during these experiments and the corresponding dissociation channels. We also mapped the potential energy surfaces of FA+ of doublet and quartet electronic states and its isomers along the reactive coordinates. Computations reveal that the major products are HCO+ or COH+ and HOCO+ or HCO2+ isomeric forms, depending on the initially populated parent cationic electronic state. Dissociation proceeds either directly or after intramolecular isomerization processes, via the HOCOH+ or H2OCO+ isomers. We also highlighted the importance of vibronic and spin\u2013orbit couplings during these processes.<\/jats:p>","DOI":"10.1063\/5.0325779","type":"journal-article","created":{"date-parts":[[2026,5,6]],"date-time":"2026-05-06T11:44:16Z","timestamp":1778067856000},"update-policy":"https:\/\/doi.org\/10.1063\/aip-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Unimolecular decomposition of formic acid cation (HCOOH+): Theory and experiment"],"prefix":"10.1063","volume":"164","author":[{"given":"A.","family":"Gloriod","sequence":"first","affiliation":[{"name":"Universit\u00e9 Gustave Eiffel, COSYS\/IMSE 1 , 5 Bd Descartes, 77454 Champs sur Marne,","place":["France"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3541-8872","authenticated-orcid":false,"given":"A. 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