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The Journal of Chemical Physics

Satellite bands of the RbCs molecule in the range of highly excited states

Mario Rakić, Robert Beuc, Nadia Bouloufa-Maafa, Olivier Dulieu, Romain Véxiau, Goran Pichler, Hrvoje Skenderović

Published May 28, 2016• 12 citations
Publisher

Abstract

We report on the observation of three RbCs satellite bands in the blue and green ranges of the visible spectrum. Absorption measurements are performed using all-sapphire cell filled with a mixture of Rb and Cs. We compare high resolution absorption spectrum of Rb-Cs vapor mixture with pure Rb and Cs vapor spectra from the literature. After detailed analysis, the new satellite bands of RbCs molecule at 418.3 nm, 468.3, and 527.5 nm are identified. The origin of these bands is discussed by direct comparison with difference potentials derived from quantum chemistry calculations of RbCs potential energy curves. These bands originate from the lower Rydberg states of the RbCs molecule. This study thus provides further insight into photoassociation of lower Rydberg molecular states, approximately between Cs(7s) + Rb(5s) and Cs(6s) + Rb(6p) asymptotes, in ultracold gases.

DOI
10.1063/1.4952758

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Physical Review A•Apr 22, 2019•1 citation
We report on the observation of three RbCs satellite bands in the blue and green ranges of the visible spectrum. Absorption measurements are performed using all-sapphire cell filled with a mixture of Rb and Cs. We compare high resolution absorption spectrum of Rb-Cs vapor mixture with pure Rb and Cs vapor spectra from the literature. After detailed analysis, the new satellite bands of RbCs molecule at 418.3 nm, 468.3, and 527.5 nm are identified. The origin of these bands is discussed by direct comparison with difference potentials derived from quantum chemistry calculations of RbCs potential energy curves. These bands originate from the lower Rydberg states of the RbCs molecule. This study thus provides further insight into photoassociation of lower Rydberg molecular states, approximately between Cs(7s) + Rb(5s) and Cs(6s) + Rb(6p) asymptotes, in ultracold gases.
We present high resolution photoassociation spectroscopy of RbCs molecules in (2)0(+) long-range state below the Rb(5S1/2) + Cs(6P1/2) asymptote and derive the corresponding C6 coefficient, which is used to revise the potential energy curves. The excited state molecules are produced in a dual-species dark spontaneous force optical trap and detected by ionizing ground state molecules after spontaneous decay, using a high sensitive time-of-flight mass spectrum. With the help of resonance-enhanced two-photon ionization technique, we obtain considerable high resolution photoassociation spectrum with rovibrational states, some of which have never been observed before. By applying the LeRoy-Bernstein method, we assign the vibrational quantum numbers and deduce C6 coefficient, which agrees with the theoretical value of A(1)Σ(+) state correlated to Rb(5S1/2) + Cs(6P1/2) asymptote. The obtained C6 coefficient is used to revise the long-range potential energy curve for (2)0(+) state, which possesses unique A - b mixing characteristic and can be a good candidate for the production of absolutely ground state molecule.
The Rb 2 ion-pair satellite band located in the blue wing of the rubidium second resonance doublet line was observed in the absorption measurements of a dense rubidium vapor. The observed satellite band was identified as a long-range ion-pair state, by using the Rb 2 long-range potential curves based on ab initio calculations. In order to perform the semiclassical spectral simulations of the observed satellite band, the relevant Rb 2 long-range potential curves and corresponding molecular dipole oscillator strengths were calculated using the asymptotic method. The importance of the observed satellite band (long-range ion-pair state) is discussed in connection with the formation of Rb 2 ultracold molecules.
We investigate the possibility of forming deeply bound ultracold RbCs molecules by a two-color photoassociation experiment. We compare the results with those for Rb 2 in order to understand the characteristic differences between heteronuclear and homonuclear molecules. The major differences arise from the different long-range potential for excited states. Ultracold 85 Rb and 133 Cs atoms colliding on the X 1 Σ + potential curve are initially photoassociated to form excited RbCs molecules in the region below the Rb(5S)+Cs(6P 1/2 ) asymptote. We explore the nature of the Ω=0 + levels in this region, which have mixed A 1 Σ + and b 3 Π character. We then study the quantum dynamics of RbCs by a time-dependent wavepacket (TDWP) approach. A wavepacket is formed by exciting a few vibronic levels and is allowed to propagate on the coupled electronic potential energy curves. We calculate the time dependence of the overlap between the wavepacket and ground-state vibrational levels. For a detuning of 7.5 cm −1 from the atomic line, the wavepacket for RbCs reaches the short-range region in about 13 ps, which is significantly faster than for the homonuclear Rb 2 system; this is mostly because of the absence of an R −3 long-range tail in the excited-state potential curves for heteronuclear systems. We give a simple semiclassical formula that relates the time taken to the long-range potential parameters. For RbCs, in contrast to Rb 2 , the excited-state wavepacket shows a substantial peak in singlet density near the inner turning point, and this produces a significant probability of de-excitation to form ground-state molecules bound by up to 1500 cm −1 . The short-range peak depends strongly on non-adiabatic coupling and is reduced if the strength of the spin–orbit coupling is increased. Our analysis of the role of spin–orbit coupling concerns the character of the mixed states in general and is important for both photoassociation and stimulated Raman de-excitation.
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.
Atomic line self‐broadening was observed in ultracold cesium. Using resonant ionization detection we were able to photoassociate atoms into excited states of ultracold molecules below the second excited PJ atomic asymptotes Cs(6S1/2) + Cs(7P1/2) and Cs(6S1/2) + Cs(7P3/2). We were able to observe blue detuned satellite band features at 560 MHz and 800 MHz above the 7P1/2 and 7P3/2 atomic limits, respectively, as well as sharp hyperfine split features on the blue and red side of the atomic resonances. Such spectral features can be qualitatively supported by the local maxima in the long range molecular structure beyond 50 Å.
We report high-resolution photoassociation (PA) spectra of RbCs in the $2({0}^{\ensuremath{-}})$ long-range state. Transitions to more than 50 vibrational levels were recorded with the largest binding energy being 507.5 ${\mathrm{cm}}^{\ensuremath{-}1}$. By fitting the experimental transition frequencies to the improved LeRoy-Bernstein formula, the C6{C}_{6}C6​ coefficient for the potential energy curve of the $2({0}^{\ensuremath{-}})$ state was determined to be $\ensuremath{-}1509\ifmmode\pm\else\textpm\fi{}97$ a.u.. Perturbation-induced energy level shift and state mixing of the long-range $2({0}^{\ensuremath{-}})$ and 3(1) states have been analyzed using an effective Hamiltonian that may be applied to mixing between other excited states of RbCs, as well as other heteronuclear diatomic molecules. Experimentally observed PA transitions to the $\ensuremath{\nu}=190$ vibrational level of the $2({0}^{\ensuremath{-}})$ state and a vibrational perturbing level in the 3(1) state have been fit using the effective Hamiltonian, which provides the accurate value of the perturbation coefficient ${\ensuremath{\beta}}_{0}$. The experimentally determined rovibronic structure and the deperturbation analysis provide critical information for the search of new schemes for efficient production of ultracold RbCs molecules in the ground state.