Long-Range Ion-Atom Interactions Studied by Field-Dissociation Spectroscopy of Molecular Ions

A long-range ion-atom potential is for the first time measured directly by electric-field-induced dissociation of selectively laser-excited $\mathrm{O}_2^{}{}_{}{}^{+}$ ions in a fast beam. The laser excitation spectrum reflects the molecular properties at "normal" internuclear distances of 1-2 Å while the threshold field required to dissociate the excited molecule determines the ${\mathrm{O}}^{+}{}_{-}{}^{}{}_{}{}^{}\mathrm{O}$ potential in the range 10-20 Å.