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Physical Review Letters

Helium-Droplet-Assisted Preparation of Cold RbSr Molecules

Florian Lackner, Günter Krois, Thomas Buchsteiner, Johann Valentin Pototschnig, Wolfgang Ernst

Published Oct 6, 2014• 25 citations
Publisher

Abstract

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.

DOI
10.1103/physrevlett.113.153001

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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.
This paper deals with the electronic structure of RbSr, a molecule possessing both a magnetic dipole moment and an electric dipole moment in its own frame, allowing its manipulation with external fields. Two complementary ab initio approaches are used for the ground and lowest excited states: first, an approach relying on optimized effective core potentials with core polarization potentials based on a full configuration interaction involving three valence electrons and second, an approach using a small-size effective core potential with 19 correlated electrons in the framework of coupled-cluster theory. We have found excellent agreement between these two approaches for the ground-state properties including the permanent dipole moment. We have focused on studies of excited states correlated to the two lowest asymptotes Rb(5p2P)\mathrm{Rb}(5p{\phantom{\rule{0.16em}{0ex}}}^{2}P)Rb(5p2P)+Sr(5s21S)+\mathrm{Sr}(5{s}^{2}{\phantom{\rule{0.16em}{0ex}}}^{1}S)+Sr(5s21S) and Rb(5s2S)\mathrm{Rb}(5s{\phantom{\rule{0.16em}{0ex}}}^{2}S)Rb(5s2S)+Sr(5s5p3P)+\mathrm{Sr}(5s5p{\phantom{\rule{0.16em}{0ex}}}^{3}P)+Sr(5s5p3P) relevant for ongoing experiments on ultracold quantum degenerate gases. We also present approximate potential curves including spin-orbit interaction based on atomic spin-orbit constants. These potential curves are an excellent starting point for experimental studies of molecular structure of RbSr using high-resolution spectroscopy.
Besides the use as cold matrix for spectroscopic studies, superfluid helium droplets have served as a cold environment for the synthesis of molecules and clusters. Since vibrational frequencies of molecules in helium droplets exhibit almost no shift compared to the free molecule values, one could assume the solvated particles move frictionless and undergo a reaction as soon as their paths cross. There have been a few unexplained observations that seemed to indicate cases of two species on one droplet not forming bonds but remaining isolated. In this work, we performed a systematic study of helium droplets doped with one rubidium and one strontium atom showing that besides a reaction to RbSr, there is a probability of finding separated Rb and Sr atoms on one droplet that only react after electronic excitation. Our results further indicate that ground-state Sr atoms can reside at the surface as well as inside the droplet.
Abstract We performed ab initio calculations in order to assess the reactivity of ultracold RbSr ( Σ + 2 ) + RbSr ( Σ + 2 ) collisions occurring on the singlet as well as the triplet potential. At ultracold energies reactions are energetically possible if they release energy, i.e., they are exoergic. The exoergicity of reactions between RbSr molecules producing diatomic molecules are known experimentally. We extend this to reactions producing triatomic molecules by calculating the binding energy of the triatomic reaction products. We find that, in addition to the formation of Rb2 + 2Sr and Rb2 + Sr2 in singlet collisions, also the formation of Sr2Rb + Rb and Rb2Sr + Sr in both singlet and triplet collisions is exoergic. Hence, the formation of these reaction products is energetically possible in ultracold collisions. For all exoergic reactions the exoergicity is larger than 1000 cm−1. We also find barrierless qualitative reaction paths leading to the formation of singlet Rb2 + 2Sr and both singlet and triplet Rb2Sr + Sr and Sr2Rb + Rb reaction products and show that a reaction path with at most a submerged barrier exists for the creation of the singlet Rb2 + Sr2 reaction product. Because of the existence of these reactions we expect ultracold RbSr collisions to result in almost-universal loss even on the triplet potential. Our results can be contrasted with collisions between alkali-diatoms, where the formation of triatomic reaction products is endoergic, and with collisions between ultracold SrF molecules, where during triplet collisions only the spin-forbidden formation of singlet SrF2 is exoergic.
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.
Abstract Molecular ions formed in cold hybrid ion–atom experiments may find interesting applications ranging from precision measurements to controlled chemical reactions. Here, we investigate the electronic structure of the Sr 2 + molecular ion, which may be produced by photoassociation of laser-cooled Sr+ ions immersed in an ultracold gas of Sr atoms or by ionization of ultracold Sr2 molecules. Using ab initio electronic structure methods, such as the coupled cluster and configuration interaction ones with small-core relativistic energy-consistent pseudopotentials and large Gaussian basis sets, we calculate potential energy curves for the ground and 41 excited electronic states, and electric dipole transition moments between them. We show that alkaline-earth molecular ions, despite their apparently simple structure with only three valence electrons, are challenging for state-of-the-art quantum chemistry methods due to their multireference nature and high density of states. Finally, we calculate and analyze Franck–Condon factors governing the photoionization of ground-state Sr2 molecules into Σ u + 2 and Σ g + 2 states of Sr 2 + molecular ions. The present results may be useful for studying and guiding the formation and spectroscopy of cold Sr 2 + molecular ions.
The formation of ultracold metastable RbCs molecules is observed in a double species magneto-optical trap through photoassociation below the 85 Rb(5S 1/2 ) + 133 Cs(6P 3/2 ) dissociation limit followed by spontaneous emission. The molecules are detected by resonance enhanced two-photon ionization. Using accurate quantum chemistry calculations of the potential energy curves and transition dipole moment, we interpret the observed photoassociation process as occurring at short internuclear distance, in contrast with most previous cold atom photoassociation studies. The vibrational levels excited by photoassociation belong to the 5th 0 + or the 4th 0 − electronic states correlated to the Rb(5P 1/2, 3/2 ) + Cs(6S 1/2 ) dissociation limit. The computed vibrational distribution of the produced molecules shows that they are stabilized in deeply bound vibrational states of the lowest triplet state. We also predict that a noticeable fraction of molecules is produced in the lowest level of the electronic ground state.