Doubly charged CO2 clusters formed by ionization of doped helium nanodroplets

- Doubly charged CO2 clusters are fingerprinted by detecting their isotopologues.
- Contributions from singly charged ions are eliminated by background subtraction.
- Smaller doubly charged ions than reported so far are identified.
- A sequential process that involves Penning ionization is invoked to explain the appearance of doubly charged ions.