SF6+: Stabilizing Transient Ions in Helium Nanodroplets.

There are myriad ions that are deemed too short-lived to be experimentally accessible. One of them is SF6+. It has never been observed, although not for lack of trying. We demonstrate that long-lived SF6+ can be formed by doping charged helium nanodroplets (HNDs) with sulfur hexafluoride; excess helium is then gently stripped from the doped HNDs by collisions with helium gas. The ion is identified by high-resolution mass spectrometry (resolution m/Δm = 15000), the close agreement between the expected and observed yield of ions that contain minor sulfur isotopes, and collision-induced dissociation in which mass-selected HenSF6+ ions collide with helium gas. Under optimized conditions, the yield of SF6+ exceeds that of SF5+. The procedure is versatile and suitable for stabilizing many other transient molecular ions.