Anionic Hydrogen Cluster Ions as a New Form of Condensed Hydrogen

We report the first experimental observation of negatively charged hydrogen and deuterium cluster ions, ${\mathrm{H}}_n^{-}$ and ${\mathrm{D}}_n^{-}$, where $n\ge 5$. These anions are formed by an electron addition to liquid helium nanodroplets doped with molecular hydrogen or deuterium. The ions are stable for at least the lifetime of the experiment, which is several tens of microseconds. Only anions with odd values of $n$ are detected, and some specific ions show anomalously high abundances. The sizes of these “magic number” ions suggest an icosahedral framework of ${\mathrm{H}}_2$ (${\mathrm{D}}_2$) molecules in solvent shells around a central ${\mathrm{H}}^{-}$ (${\mathrm{D}}^{-}$) ion. The first three shells, which contain a total of 44 ${\mathrm{H}}_2$ or ${\mathrm{D}}_2$ molecules, appear to be solidlike, but thereafter a more liquidlike arrangement of the ${\mathrm{H}}_2$ (${\mathrm{D}}_2$) molecules is adopted.

Figure 1

Section of the mass spectrum recorded for hydrogen cluster anions. The dominant series of ions seen here are from ${\mathrm{H}}_n^{-}$, but small quantities of $({\mathrm{H}}_n){\mathrm{OH}}^{-}$ and $({\mathrm{H}}_n){\mathrm{N}}_2^{-}$ are also detected because of trace quantities of water and air in the apparatus. Also shown is a simulation derived from isotopefit [24], which is used to deconvolute overlapping contributions from different sized clusters and from different isotope combinations. The agreement between the fitted and experimental data is essentially perfect in this region.

Figure 2

Plots of the measured ion signal levels for (a) ${\mathrm{H}}_n^{-}$ and (b) ${\mathrm{D}}_n^{-}$ as a function of $n$. The signal levels are expressed as integrated ion counts for a given mass spectral peak.

Figure 3

Calculated equilibrium structures for ${\mathrm{H}}_3^{-}$, ${\mathrm{H}}_5^{-}$, and ${\mathrm{H}}_7^{-}$, which are found to be linear for ${\mathrm{H}}_3^{-}$ and ${\mathrm{H}}_5^{-}$ and trigonal pyramidal for ${\mathrm{H}}_7^{-}$ (${\mathrm{C}}_{3\mathrm{v}}$ point group symmetry). The gray spheres indicate the positions of the nuclei in the hydrogen molecules, whereas the red sphere indicates the position of the nucleus of the atomic hydrogen anion. Detailed information about the structures, including those of the neutral clusters, can be found in Supplemental Material [30], along with other information such as the charge density distributions in the highest occupied molecular orbitals of the anions.