Structure determination of alkali trimers on helium nanodroplets through laser-induced Coulomb explosion

Alkali trimers, Ak 3, located on the surface of He nanodroplets are triply ionized following multiphoton absorption from an intense femtosecond laser pulse, leading to fragmentation into three correlated Ak + ions. Combining the information from threefold covariance analysis of the emission direction of the fragment ions and their kinetic energy distributions P(E kin), we find that Na 3, K 3, and Rb 3 have an equilateral triangular structure, corresponding to that of the lowest lying quartet state A 2 ′ 4 , and determine the equilibrium bond distance R eq(Na 3) = 4.65 ± 0.15 Å, R eq(K 3) = 5.03 ± 0.18 Å, and R eq(Rb 3) = 5.45 ± 0.22 Å. For K 3 and Rb 3, these values agree well with existing theoretical calculations, while for Na 3, the value is 0.2-0.3 Å larger than the existing theoretical results. The discrepancy is ascribed to a minor internuclear motion of Na 3 during the ionization process. In addition, we determine the distribution of internuclear distances P(R) under the assumption of fixed bond angles.