Spectroscopy of Lithium Atoms and Molecules on HeliumNanodroplets

摘要

We report on the spectroscopic investigation of lithium atoms and lithium dimers in their triplet manifold on the surface of helium nanodroplets (HeN). We present the excitation spectrum of the 3p ← 2s and 3d ← 2s two-photon transitions for single Li atoms on HeN. The atoms are excited from the 2S(Σ) ground state into Δ, Π, and Σ pseudodiatomic molecular substates. Excitation spectra are recorded by resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) mass spectroscopy, which allows an investigation of the exciplex (Li*–Hem, m = 1–3) formation process in the Li–HeN system. Electronic states are shifted and broadened with respect to free atom states, which is explained within the pseudodiatomic model. The assignment is assisted by theoretical calculations, which are based on the Orsay–Trento density functional where the interaction between the helium droplet and the lithium atom is introduced by a pairwise additive approach. When a droplet is doped with more than one alkali atom, the fragility of the alkali–HeN systems leads preferably to the formation of high-spin moleculeson the droplets. We use this property of helium nanodroplets for thepreparation of Li dimers in their triplet ground state (1³Σu⁺).The excitation spectrum of the 2³Πg(ν′= 0–11) ← 1³Σu⁺(ν″ = 0) transition is presented.The interaction between the molecule and the droplet manifests ina broadening of the transitions with a characteristic asymmetric form.The broadening extends to the blue side of each vibronic level, whichis caused by the simultaneous excitation of the molecule and vibrationsof the droplet (phonons). The two isotopes of Li form ⁶Li2 and ⁷Li2 as well as isotopemixed ⁶Li⁷Li molecules on the droplet surface.By using REMPI-TOF mass spectroscopy, isotope-dependent effects couldbe studied.