| Summary: | <p>Combination of a cryogenic ion-trap machine, operated at 4.7 K, with the free-electron-laser FELIX allows the first experimental characterization of the unusually bright antisymmetric stretch (ν<sub>3</sub>) and π-bending (ν<sub>2</sub>) fundamentals of the He–X<sup>+</sup>–He (X = H, D) chromophore of the in situ prepared HHe<sub><em>n</em></sub><sup>+</sup> and DHe<sub><em>n</em></sub><sup>+</sup> (<em>n</em> = 3–6) complexes. The band origins obtained are fully supported by first-principles quantum-chemical computations, performed at the MP2, the CCSD(T), and occasionally the CCSDTQ levels employing extended basis sets. Both the experiments and the computations are consistent with structures for the species with <em>n</em> = 3 and 6 being of T-shaped <em>C</em><sub>2<em>v</em></sub> and of <em>D</em><sub>4<em>h</em></sub> symmetry, respectively, while the species with <em>n</em> = 4 are suggested to exhibit interesting dynamical phenomena related to large-amplitude motions.</p>
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