Direct Measurement of 4f–4f Transitions and Electronic Hot Bands in Lanthanoid-Antenna Complexes by Helium-Tagging Spectroscopy: Toward Molecular-Scale Trapped Ion Qubits

Lanthanoid-antenna complexes are promising building blocks for quantum technologies, yet their potential in the condensed phase is often obscured by phonon-induced decoherence. This article reports the first one-color helium-tagging spectroscopic measurements of isolated [Ho(enpypa)]⁺ and [Yb(enpypa)]⁺ complexes (enpypa = ethylenediamine-pyridine-picolinic acid) at cryogenic temperatures, directly resolving their elusive 4f-4f excitations in the gas phase. For Ho³⁺, at least eight Stark-split multiplets are identified across the visible spectrum, while the Yb³⁺ complex exhibits a sharp ²F_7/2 → ²F_5/2 near-infrared manifold together with a hot band consistent with thermal equilibrium at 4 K. The UV-absorbing ligand shows depletion cross sections over two orders of magnitude larger than the lanthanoid-centered transitions, highlighting the sensitivity and dynamic range of our next-generation apparatus. These results establish cryogenic ion trap spectroscopy as a powerful tool for probing lanthanoid photophysics and pave the way for multicolor spectroscopic investigations of lanthanoid systems tailored for quantum information science.