Coulomb explosion imaging identifies a K-CsI complex formed at the surface of helium nanodroplets

Helium nanodroplets are co-doped with a K atom and a CsI molecule, forming a complex of previously unknown structure and location within the droplet. The complex is identified and characterized by femtosecond laser-induced Coulomb explosion and subsequent recording of the momentum distributions of the resulting K+, Cs+, Cs+Hen, I+, and I+Hen fragment ions. The kinetic energy distribution of the K+ ions contains two distinct peaks at 2.2 and 5.5 eV. These values match the outcome of classical molecular dynamics simulations of the Coulomb explosion of the gas-phase K- CsI complex, starting from its ab-initio-calculated equilibrium structure, into (K+,Cs+,I) or (K+,Cs+,I+) fragments, respectively. Covariance maps of the momentum distributions for K+ ions with other fragment ions corroborate the occurrence of these two Coulomb explosion channels, resulting from double and triple ionization of K- CsI. Overall, we conclude that K- CsI is formed at the surface of the droplets with a structure very close to the theoretically determined triangular shape of the gas-phase complex.