Heteronuclear and Homonuclear High-Spin Alkali Trimers on Helium Nanodroplets

Abstract

The electronic excitation spectra of all possible homo- and heteronuclear high-spin (quartet) trimers of K and Rb (KxRb(3-x), x=0...3) assembled on the surface of superfluid helium droplets, are measured in the spectral range from 10,600 to 17,400 cm(-1). A regular series of corresponding bands is observed, reflecting the similar electronic structure of all these trimers. For the assignment and separation of overlapping bands, we determine x directly, with mass-selected beam depletion, and indirectly with a V-type double-resonance scheme. The assignment is confirmed by high-level ab initio calculations of the electronic structure of the bare trimers. The level structure is rationalized in terms of harmonic-oscillator states of the three valence electrons in a quantum-dot-like confining potential. We predict that three should be a magic number for high-spin alkali clusters.

Homo- and heteronuclear alkali metal trimers formed on helium nanodroplets. Part II. Femtosecond spectroscopy and spectra assignments

“…Note that the observed frequency also fits well to a ∆v = 2 overtone beat of the Q x/y -mode of the 1 4 A 2 PES (2 22.7 cm −1 ). This component is observed over a broader band of excitation wavelengths (500 cm −1 ) than in the cw excitation spectrum (300 cm −1 ) of the same system 12 . This supports the interpretation in terms of a Raman transition to the lowest quartet state 1 4 A 2 .…”

“…However, with the advances in computational power and techniques, PES for homo-and heteronuclear alkali trimers involving potassium, rubidium and cesium have recently been determined by several groups. 12,34,36,48,49 Besides, alkali trimers in quartet states are particularly well suited for studying nonadditive three-body interaction terms, which represent significant contributions to the binding energy. 30,50 Recent advances in forming ultracold molecules by photoor magneto-association through Feshbach resonances using trapped samples of ultracold atoms has stimulated increasing demand for accurate PES of three-atomic systems.…”

“…E-mail: wolfgang.ernst@tugraz.at species. 12,[32][33][34] In the case of Na 3 and K 3 , excitation and emission spectroscopy allowed for vibrational resolution. 8,30,31,34 High-spin alkali trimers are particularly interesting model systems for studying nonadiabatic transitions from weakly bound van der Waals aggregates to covalently bound metallic clusters.…”

“…6 Due to their low binding energy, high-spin alkali-metal molecules are hardly accessible in conventional gas-phase experiments but are preferentially formed on helium droplets. [6][7][8] Alkali atoms, molecules and clusters in high-spin states have been extensively studied by cw [9][10][11][12][13][14] and time-resolved spectroscopy. 2,[15][16][17][18][19][20][21][22] Although helium is the least perturbing solvent and helium nanodroplets are often referred to as a nearly ideal spectroscopic matrix, electronic spectra of atoms or molecules in or on He droplets are still strongly broadened.…”

Alkali-metal electron spin density shift induced by a helium nanodroplet

“…We have previously performed a whole series of experiments on the valenceelectron spin of potassium (K) and rubidium (Rb) atoms [27,28], dimers [27], and trimers [29] on He droplets in a strong magnetic field. These have culminated in the successful implementation of optically detected magnetic resonance (ODMR), and we have been able to recently report the first observation of hyperfine-resolved electron spin resonance (ESR) spectra of K and Rb on the surface of He nanodroplets [30,31], with such high resolution that shifts could be followed as a function of droplet size for 85 Rb atoms, despite the fact that they amount to few parts per million only.…”