Field Evaporation of Tungsten-Helium Molecular Ions

Ions field evaporating from tungsten in the presence of helium are identified by a time-of-flight atom probe of improved resolution. Helium is found to be adsorbed at all imaged surface sites. From the brightly imaged crystal regions the adsorbate comes off as ${\mathrm{He}}^{+}$, approximately in a one-to-one ratio with ${\mathrm{W}}^{+++}$ and ${\mathrm{W}}^{++++}$. The dim, high work-function regions around (011) predominantly yield ${\mathrm{WHe}}^{+++}$ molecular ions with the temperature-dependent relative abundance peaking at liquid nitrogen temperature.