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Physical Review Letters

Electron Spin Pumping of Rb Atoms on He Nanodroplets via Nondestructive Optical Excitation

Gerald Auböck, Johann Nagl, Carlo Callegari, Wolfgang Ernst

Published Jul 14, 2008• 55 citations
Publisher

Abstract

We measured laser-induced-fluorescence (LIF) and beam-depletion (BD) spectra of rubidium atoms (5S-5P transition) on the surface of superfluid helium nanodroplets (M-He_{N} with M=Rb). It is known that when M is a lighter alkali atom electronic excitation always leads to detachment of the excited atom (M;{*}). The dissociation energy, few tens cm;{-1}, comes either as photon excess energy or from the barrierless formation of a M;{*}-He exciplex. We observe that this picture does not hold when M=Rb and the photon excess energy is small: we are able to excite atoms without detaching them from the droplet, thanks to a barrier preventing formation of the exciplex. This system is ideally suited for optical spin pumping in a He nanodroplet, whose achievement we explicitly demonstrate in a pump-probe magnetic circular dichroism experiment.

DOI
10.1103/physrevlett.101.035301

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We measured laser-induced-fluorescence (LIF) and beam-depletion (BD) spectra of rubidium atoms (5S-5P transition) on the surface of superfluid helium nanodroplets (M-He_{N} with M=Rb). It is known that when M is a lighter alkali atom electronic excitation always leads to detachment of the excited atom (M;{}). The dissociation energy, few tens cm;{-1}, comes either as photon excess energy or from the barrierless formation of a M;{}-He exciplex. We observe that this picture does not hold when M=Rb and the photon excess energy is small: we are able to excite atoms without detaching them from the droplet, thanks to a barrier preventing formation of the exciplex. This system is ideally suited for optical spin pumping in a He nanodroplet, whose achievement we explicitly demonstrate in a pump-probe magnetic circular dichroism experiment.
The population ratio of Zeeman sublevels of atoms on the surface of superfluid helium droplets (T=0.37 K) has been measured. Laser induced fluorescence spectra of K atoms are measured in the presence of a moderately strong magnetic field (2.9 kG). The relative difference between the two states of circular polarization of the exciting laser is used to determine the electron spin polarization of the ensemble. Equal fluorescence levels indicate that the two spin sublevels of the ground-state K atom are equipopulated, within 1%. Thermalization to 0.37 K would give a population ratio of 0.35. We deduce that the rate of spin relaxation induced by the droplet must be <520/s. For the K2 triplet dimer we find instead full thermalization of the spin.
We report the observation of emission spectra from Rb∗Hen{\mathrm{Rb}}^{*}{\mathrm{He}}_{n}Rb∗Hen​ exciplexes in solid 4He^{4}\mathrm{He}4He. Two different excitation channels were experimentally identified, viz., exciplex formation via laser excitation to the atomic 5P3∕25{P}_{3∕2}5P3∕2​ and to the 5P1∕25{P}_{1∕2}5P1∕2​ levels. While the former channel was observed before in liquid helium, on helium nanodroplets, and in helium gas by different groups, the latter creation mechanism occurs only in solid helium or in gaseous helium above 10K10\phantom{\rule{0.3em}{0ex}}\mathrm{K}10K. The experimental results are compared to theoretical predictions based on the extension of a model, used earlier by us for the description of Cs∗Hen{\mathrm{Cs}}^{*}{\mathrm{He}}_{n}Cs∗Hen​ exciplexes. We also report the observation of fluorescence from atomic rubidium in solid helium, and discuss striking differences between the spectroscopic features of $\mathrm{Rb}\text{\ensuremath{-}}\mathrm{He}$ and $\mathrm{Cs}\text{\ensuremath{-}}\mathrm{He}$ systems.
We present a combined experimental and theoretical study of the RbSr molecule. The experimental approach is based on the formation of RbSr molecules on helium nanodroplets. Utilizing two-photon ionization spectroscopy, an excitation spectrum ranging from 11,600 up to 23,000 cm(-1) was recorded. High level ab initio calculations of potential energy curves and transition dipole moments accompany the experiment and facilitate an assignment of transitions. We show that RbSr molecules desorb from the helium droplets upon excitation, which enables dispersed fluorescence spectroscopy of free RbSr. These spectra elucidate X(2)Σ(+) ground and excited state properties. Emission spectra originating from states corresponding to the Rb(5s(2)S) + Sr(5s5p(3)P) asymptote were recorded; spin-orbit coupling was included for the simulation. The results should provide a good basis for achieving the formation of this molecule in cold collisions, thus offering intriguing prospects for ultracold molecular physics.
The desorption of excited rubidium atoms off the surface of helium nanodroplets is studied in detail using femtosecond time-resolved photoion and photoelectron imaging spectroscopy in combination with quantum wave-packet simulations. The good agreement of the measured time-dependent velocity distributions with the simulation when exciting the Rb dopant atoms into the 6p6p6p state supports the pseudodiatomic model for the Rb-He droplet interaction, even on the level of quantum wave-packet dynamics. Time-resolved photoelectron spectra reveal the partitioning of excitation energy into the dopant and the droplet degrees of freedom.
The study of small clusters is intended to fill the knowledge gap between single atoms and bulk material. He nanodroplets are an ideal matrix for preparing and investigating clusters in a superfluid environment. Alkali-metal atoms are only bound very weakly to their surface by van der Waals forces. Due to the formation process, high-spin states of alkali-metal clusters on He nanodroplets are favorably observed, which is in contrast to the abundance in other preparation processes. Until now, the prevailing opinion was that stable clusters of the heavy alkali-metal atoms, rubidium (Rb) and cesium (Cs) on He nanodroplets, are limited to 5 and 3 atoms, respectively (Schulz et al., Phys. Rev. Lett. 2004, 92, 13401). Here, we present stable complexes of Rb(n)⁺ and Cs(n)⁺ consisting of up to n = 30 atoms, with the detection of large alkali-metal clusters being strongly enhanced by one-photon ionization. Our results also suggest that we monitored both high-spin and low-spin state clusters created on nanodroplets. The van der Waals bound high-spin alkali-metal clusters should show strong magnetic behavior, while low-spin states are predicted to exhibit metallic characteristics. Alkali-metal clusters prepared in these two configurations appear to be ideal candidates for investigating nanosized particles with ferromagnetic or metallic properties.
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.