SPECTROSCOPIC SIGNATURES OF HHen+ (n=2−6)

The combination of cryogenic ion trap machines, operated close to 4~K, with different laser sources allowed the first experimental characterization of the antisymmetric stretch ($\nu_3$) and bending ($\nu_2$) fundamentals of the linear He--H$^+$--He core of the HHe$_n^+$ complexes for $n=3-6$. The found band origins, at around 1290~cm$^{-1}$ for $\nu_3$ and around 850~cm$^{-1}$ for $\nu_2$, are fully supported by first-principles quantum-chemical computations. These results are consistent with the structure for HHe$_3^+$ being of T-shaped C$_{2v}$ symmetry and HHe$_6^+$ being of D$_{4h}$ symmetry, while HHe$_4^+$ is suggested to exhibit interesting dynamical phenomena related to large-amplitude motion. Very recently, using a quantum cascade laser, the linear HHe$_2^+$ has been probed in high resolution, yielding molecular parameters for this fundamental three-nucleus-four-electron system for the first time