Spectroscopy of Lithium Atoms and Molecules on Helium Nanodroplets

Abstract

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 molecules on the droplets. We use this property of helium nanodroplets for the preparation 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 in a broadening of the transitions with a characteristic asymmetric form. The broadening extends to the blue side of each vibronic level, which is caused by the simultaneous excitation of the molecule and vibrations of the droplet (phonons). The two isotopes of Li form 6Li2 and 7Li2 as well as isotope mixed 6Li7Li molecules on the droplet surface. By using REMPI-TOF mass spectroscopy, isotope-dependent effects could be studied.

Photoionisaton of pure and doped helium nanodroplets

“…Besides, the zero-phonon line which is observed in the spectra of singlet dimers is absent in the spectra of triplet dimers [51]. Nevertheless, the low-energy edges of the vibrational lines were found to match the expected line positions within a precision of a few cm −1 [85,87,88]. The first non-alkali REMPI spectra were measured for embedded silver (Ag) clusters [94].…”

Metastable Aluminum Atoms Floating on the Surface of Helium Nanodroplets

“…There appears to be general agreement that alkali metal atoms reside on the surfaces of droplets [5,6], as do several of the alkaline earth metals [7][8][9][10][11][12][13]. Almost all such studies have concentrated on metal atoms with either no or one unpaired electron, and these have been added to the droplets in their electronic ground state; the only higher spin systems that have been investigated are chromium atoms, weakly bound alkali dimers and trimers, and the silver dimer [14][15][16][17][18].…”

Helium-induced electronic transitions in photo-excited Ba+–Hen exciplexes

“…In particular, experimental investigations on photoexcitation of helium nanodroplets doped with an alkali-like atom have shown that exciplexes are formed with the excited atom and one or a few helium attached to it. [17][18][19]23,[30][31][32][53][54][55][56] The corresponding exciplex for Ba + would be characterized by one of the resonances identified in this work, hence its lifetime could be determined.…”