Nonadiabatic laser-induced alignment dynamics of alkali-metal dimers on the surface of a helium droplet

Alkali dimers, Ak2, located on the surface of a helium nanodroplet, are set into rotation through the polarizability interaction with a nonresonant 1-ps-long laser pulse. The time-dependent degree of alignment is recorded using femtosecond-probe-pulse-induced Coulomb explosion into a pair of Ak+ fragment ions. The results, obtained for Na2, K2, and Rb2 in both the ground state 11Σ+g and the lowest-lying triplet state 13Σ+u, exhibit distinct, periodic revivals with a gradually decreasing amplitude. The dynamics differ from that expected for dimers had they behaved as free rotors. Numerically, we solve the time-dependent rotational Schrödinger equation, including an effective mean-field potential to describe the interaction between the dimer and the droplet. The experimental and simulated alignment dynamics agree well and their comparison enables us to determine the effective rotational constants of the alkali dimers with the exception of Rb2(13Σ+u) that only exhibits a prompt alignment peak but no subsequent revivals. For Na2(13Σ+u), K2(11Σ+g), K2(13Σ+u) and Rb2(11Σ+g), the alignment dynamics are well-described by a 2D rotor model. We ascribe this to a significant confinement of the internuclear axis of these dimers, induced by the orientation-dependent droplet-dimer interaction, to the tangential plane of their residence point on the droplet.