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The adsorption of helium or hydrogen on cationic triphenylene (TPL, C₁₈H₁₂), a planar polycyclic aromatic hydrocarbon (PAH) molecule, and of helium on cationic 1,3,5-triphenylbenzene (TPB, C₂₄H₁₈), a propeller-shaped PAH, is studied by a combination of high-resolution mass spectrometry and classical and quantum computational methods. Mass spectra indicate that He<i>_n</i>TPL⁺ complexes are particularly stable if <i>n</i> = 2 or 6, in good agreement with the quantum calculations that show that for these sizes, the helium atoms are strongly localized on either side of the central carbon ring for <i>n</i> = 2 and on either side of the three outer rings for <i>n</i> = 6. Theory suggests that He₁₄TPL⁺ is also particularly stable, with the helium atoms strongly localized on either side of the central and outer rings plus the vacancies between the outer rings. For He<i>_n</i>TPB⁺, the mass spectra hint at enhanced stability for <i>n</i> = 2, 4 and, possibly, 11. Here, the agreement with theory is less satisfactory, probably because TPB⁺ is a highly fluxional molecule. In the global energy minimum, the phenyl groups are rotated in the same direction, but when the zero-point harmonic correction is included, a structure with one phenyl group being rotated opposite to the other two becomes lower in energy. The energy barrier between the two isomers is very small, and TPB⁺ could be in a mixture of symmetric and antisymmetric states, or possibly even vibrationally delocalized.