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In this work, we investigated the \u03bd3 stretching bands of the light cationic rotors NH3+ and H3O+ in helium nanodroplets. The \u03bd3 band of NH3+ displays a well-resolved rotational structure, whereas that of H3O+ is only partially resolved. The rotational constants associated with rotation about the C3 symmetry axis remain close to their gas-phase values. In contrast, the rotational constants corresponding to tumbling motion are reduced in helium to \u223c60% of their free-molecule values. A substantially larger reduction was reported previously for the CH3+ ion. This difference is attributed to differences in the electronic structures of the ions, which govern their interactions with helium atoms. In addition, the NH3+ spectrum exhibits anomalies in both line positions and intensities, suggesting that the rotational dynamics of ions in superfluid helium may not be adequately described by the conventional free-molecule rotational Hamiltonian.<\/jats:p>","DOI":"10.1063\/5.0348985","type":"journal-article","created":{"date-parts":[[2026,9,28]],"date-time":"2026-09-28T11:31:56Z","timestamp":1790595116000},"update-policy":"https:\/\/doi.org\/10.1063\/aip-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Rotation of NH3+ and H3O+ ions in superfluid helium droplets"],"prefix":"10.1063","volume":"165","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-1568-2043","authenticated-orcid":false,"given":"Amandeep","family":"Singh","sequence":"first","affiliation":[{"name":"Department of Chemistry, University of Southern California 1 , Los Angeles, California 90007,","place":["USA"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0005-4529-2558","authenticated-orcid":false,"given":"Arisa","family":"Iguchi","sequence":"additional","affiliation":[{"name":"Institute of Laser Science, University of Electro- Communications 2 , Tokyo 182-8585,","place":["Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9498-7587","authenticated-orcid":false,"given":"Boris G.","family":"Sartakov","sequence":"additional","affiliation":[{"name":"Fritz-Haber-Institut der Max-Planck-Gesellschaft 3 , Faradayweg 4-6, 14195 Berlin,","place":["Germany"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3120-2572","authenticated-orcid":false,"given":"Susumu","family":"Kuma","sequence":"additional","affiliation":[{"name":"Department of Physics, Rikkyo University 4 , Tokyo 171-8501,","place":["Japan"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8412-817X","authenticated-orcid":false,"given":"Andrey F.","family":"Vilesov","sequence":"additional","affiliation":[{"name":"Department of Chemistry, University of Southern California 1 , Los Angeles, California 90007,","place":["USA"]},{"name":"Department of Physics and Astronomy, University of Southern California 5 , Los Angeles, California 90007,","place":["USA"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"317","published-online":{"date-parts":[[2026,9,28]]},"reference":[{"issue":"2","key":"2026092807315246400_c1","doi-asserted-by":"publisher","first-page":"31","DOI":"10.1063\/1.1359707","article-title":"Superfluid helium droplets: An ultracold nanolaboratory","volume":"54","year":"2001","journal-title":"Phys. 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