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The Journal of Chemical Physics

Alkaline earth metals (Ca, Sr) attached to liquid helium droplets: Inside or out?

Frank Stienkemeier, F. Meier, Hans O. Lutz

Published Dec 22, 1997• 78 citations
Publisher

Abstract

Superfluid helium droplets HeN at 0.4 K are doped with single alkaline earth atoms (Ca, Sr). The absorption spectra for excitation of the lowest singlet transitions display broad, blue-shifted peaks if compared to the atomic lines. The shift increases with droplet size increasing from N≈1000 up to N≈10000 but converges to only about one third of the bulk value.

DOI
10.1063/1.474198

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Superfluid helium droplets HeN at 0.4 K are doped with single alkaline earth atoms (Ca, Sr). The absorption spectra for excitation of the lowest singlet transitions display broad, blue-shifted peaks if compared to the atomic lines. The shift increases with droplet size increasing from N≈1000 up to N≈10000 but converges to only about one third of the bulk value.
Experimental and theoretical results for mixed helium droplets doped with one calcium atom are reported. The absorption spectrum around the $4s4p\ensuremath{\leftarrow}4{s}^{2}$ transition is calculated as a function of the composition of the droplet and compared to the experiment. Our study reveals the distinct feature that for specific 3He^{3}\text{H}\text{e}3He concentrations, Ca atoms sit at the 3He-4He^{3}\text{H}\text{e-}^{4}\text{H}\text{e}3He-4He interface. This particular property of Ca, likely not shared with any other atomic or molecular species but the heavier alkaline-earth atoms, might offer the possibility of using it to probe the 3He-4He^{3}\text{H}\text{e-}^{4}\text{H}\text{e}3He-4He interface of the droplet.
Metal atoms have proved to be sensitive probes of the properties of superfluid helium nanodroplets. To date, all experiments on the doping of helium droplets have concentrated on the attachment of metal atoms in their ground electronic states. Here we report the first examples of metal atoms in excited states becoming attached to helium nanodroplets. The atoms in question are aluminum, and they have been generated by laser ablation in a metastable quartet state, which attaches to and remains on the surface of helium droplets. Evidence for a surface location comes from electronic spectra, which consist of very narrow absorption profiles that show very small spectral shifts. Supporting ab initio calculations show there to be an energy incentive for a metastable Al atom to remain on the surface of a helium droplet rather than move to the interior. The results suggest that helium droplets may provide a method for the capture and transport of metastable excited atomic and molecular species.
The excitation and emission spectra of alkali-metal atoms (Cs and Rb) in liquid helium are studied. The helium pressure is changed from the saturated vapor pressure to about 25 atm at 1.6 K, under which conditions the liquid helium is in a superfluid state. The pressure shift and broadening of the observed spectra corresponding to the D lines are explained qualitatively by a theoretical calculation based on the spherical atomic bubble model. The Cs D2{\mathit{D}}_{2}D2​ excitation spectra are found to have double peaks, indicating the existence of anisotropic oscillations of the bubble surface. The disappearance of the D2{\mathit{D}}_{2}D2​ emission lines and the pressure-dependent quenching of the Rb D1{\mathit{D}}_{1}D1​ emission lines are also discussed.
Lineshapes associated with the absorption and emission spectra of neutral atoms in liquid helium and cold helium gas are discussed. A neutral atom in liquid helium forms a microcavity due to a strong short-range repulsive force between the helium and impurity atoms, producing very broad and largely blueshifted absorption bands and relatively narrow emission lines with much smaller shifts. A broad and weak emission band observed by exciting the D2 line of Ag atoms in liquid helium has confirmed the hypothesis that quenching of D2 emission lines for heavy group 1a and 1b atoms are due to the formation of exciplex. In cold helium gas, sudden decrease of the emission lifetimes below 5.2 K by the excitation of the D2 line of Ag atoms is attributed to the condensation of He atoms on the metastable 2D5/2 state. Very narrow absorption lines of Eu innershell electron transitions have revealed that superfluid helium behaves as a static and highly homogeneous low temperature matrix.
The infrared spectrum of single oxygen carbon sulfide (OCS) molecules was measured inside large superfluid pure helium-4 droplets and nonsuperfluid pure helium-3 droplets, both consisting of about 10(4) atoms. In the helium-4 droplets, sharp rotational lines were observed, whereas in helium-3 only a broad peak was found. This difference is interpreted as evidence that the narrow rotational lines, which imply free rotations, are a microscopic manifestation of superfluidity. Upon addition of 60 helium-4 atoms to the pure helium-3 droplets, the same sharp rotational lines were found; it appears that 60 is the minimum number needed for superfluidity.