Laser-induced Coulomb explosion of heteronuclear alkali-metal dimers on helium nanodroplets

A sample mixture of alkali-metal homonuclear dimers, Ak(2) and Ak(2)', and heteronuclear dimers, AkAk ', residing on the surface of helium nanodroplets are Coulomb exploded into pairs of atomic alkali-metal cations, (Ak(+), Ak(+)), (Ak '(+), Ak '(+)), and (Ak(+), Ak '(+)), following double ionization induced by an intense 50-fs laser pulse. The measured kinetic-energy distribution P(Ekin) of both the Ak(+) and Ak '(+) fragment ions contains overlapping peaks due to contributions from Coulomb explosion of the homonuclear and heteronuclear dimers. From P(E-kin), we determine the distribution of internuclear distances P(R) via the Coulomb-explosion imaging principle. Using coincident filtering based on momentum division between the two fragment ions, we demonstrate that the individual P(E-kin) distributions pertaining to ions from either hetero- or homonuclear dimers can be retrieved. This filtering method works through the concurrent detection of two-dimensional velocity images of the Ak(+) and the Ak '(+) ions implemented through the combination of a velocity-map-imaging spectrometer and a TPX3CAM detector. The key finding is that P(R) can be measured for any specific dimer in the unavoidably mixed sample of hetero- and homonuclear alkali-metal dimers. We report results for LiK and NaK, which were previously unmeasurable by the Coulomb-explosion technique. Our method should also work for other heteronuclear dimers and for differentiating between different isotopologues of any of the dimers.