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

Abstract

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.