Excited Electronic States of Sr2: Ab Initio Predictions and Experimental Observation of the 21Σu+ State.

Despite its apparently simple nature with four valence electrons, the strontium dimer constitutes a challenge for modern electronic structure theory. Here we focus on excited electronic states of Sr2, which we investigate theoretically up to 25000 cm-1 above the ground state, to guide and explain new spectroscopic measurements. In particular, we focus on potential energy curves for the 11Σu+, 21Σu+, 11Πu, 21Πu, and 11Δu states computed using several variants of ab initio coupled-cluster and configuration-interaction methods to benchmark them. In addition, a new experimental study of the excited 21Σu+ state using polarization labeling spectroscopy is presented, which extends knowledge of this state to high vibrational levels, where perturbation by higher electronic states is observed. The available experimental observations are compared with the theoretical predictions and help to assess the accuracy and limitations of employed theoretical models. The present results pave the way for future more accurate theoretical and experimental spectroscopic studies.

The C(2)1Πu state in Rb2. Observation and deperturbation.

We report a systematic study of the C(2)1Πuelectronic state in rubidium dimer, observed in polarization labelling spectroscopy experiment through the C ← X1Σg+ transitions recorded under rotational resolution in two isotopologues 85Rb2 and 85Rb87Rb. Regularity of the vibrational progressions was distorted by numerous interactions with the surrounding 21Σu+, 23Πuand 33Σu+states. Deperturbation was performed by coupled-channels analysis taking into account spin-orbit and rotational interactions. Potential energy curves and parameters describing the off-diagonal matrix elements were determined as functions of internuclear distance. About 3000 line frequencies from the present study are reproduced with a standard deviation of 0.07 cm-1, in agreement with the experimental accuracy. Along with this, the model is consistent with all high resolution experimental data on the involved electronic states, available in the literature.

On the 31Πu state in caesium dimer.

Polarisation labelling spectroscopy technique was employed to study the 31Πu state of Cs2 molecule. The main equlibrium constants are Te=20684.56cm-1,ωe=30.62 cm-1 and Re=5.27 Å. Vibrational levels v=4-35 of the 31Πu state were found to be subject to strong perturbations by the neighbouring electronic states. Energies of 3352 rovibronic levels of the perturbed complex were determined.

Determination of the C(3)1Σ+ state potential energy curve in KCs molecule based on polarisation labelling spectroscopy data.

The C(3)1Σ+ state of KCs molecule was studied experimentally employing the polarisation labelling spectroscopy (PLS) technique. The experiment extended the prior knowledge of the state adding more than 40 vibrational levels to the previously existing data. A pointwise potential energy curve was constructed for the investigated state with the inverted perturbation approach (IPA) method, basing on experimentally determined energies of over 1300 rovibrational levels.

The RbSr 2Σ+ ground state investigated via spectroscopy of hot and ultracold molecules.

We report on spectroscopic studies of hot and ultracold RbSr molecules, and combine the results in an analysis that allows us to fit a potential energy curve (PEC) for the X(1)2Σ+ ground state bridging the short-to-long-range domains. The ultracold RbSr molecules are created in a µK sample of Rb and Sr atoms and probed by two-colour photoassociation spectroscopy. The data yield the long-range dispersion coefficients C6 and C8, along with the total number of supported bound levels. The hot RbSr molecules are created in a 1000 K gas mixture of Rb and Sr in a heat-pipe oven and probed by thermoluminescence and laser-induced fluorescence spectroscopy. We compare the hot molecule data with spectra we simulated using previously published PECs determined by three different ab initio theoretical methods. We identify several band heads corresponding to radiative decay from the B(2)2Σ+ state to the deepest bound levels of X(1)2Σ+. We determine a mass-scaled high-precision model for X(1)2Σ+ by fitting all data using a single fit procedure. The corresponding PEC is consistent with all data, thus spanning short-to-long internuclear distances and bridging an energy gap of about 75% of the potential well depth, still uncharted by any experiment. We benchmark previous ab initio PECs against our results, and give the PEC fit parameters for both X(1)2Σ+ and B(2)2Σ+ states. As first outcomes of our analysis, we calculate the s-wave scattering properties for all stable isotopic combinations and corroborate the locations of Fano-Feshbach resonances between alkali Rb and closed-shell Sr atoms recently observed [V. Barbéet al., Nat. Phys., 2018, 14, 881]. These results and more generally our strategy should greatly contribute to the generation of ultracold alkali-alkaline-earth dimers, whose applications range from quantum simulation to state-controlled quantum chemistry.

High-lying electronic states of the rubidium dimer-Ab initio predictions and experimental observation of the 5(1)Σu(+) and 5(1)Πu states of Rb2 by polarization labelling spectroscopy.

Two-colour polarization labelling experiments have been used to explore the excitation spectrum of the rubidium dimer in the region 25,500-27,000 cm(-1), probing two mutually interacting states, identified from ab initio calculations as the 5(1)Σu(+) and 5(1)Πu states whose atomic dissociation products are Rb(5s) + Rb(5d). Treating the rather irregular progressions observed in the excitation spectra as transitions to single states with (numerous) local perturbations, we propose spectroscopic parameters and potential energy curves to describe the investigated levels. Observations cover more than 20 vibrational levels in the inner minima of both the 5(1)Πu and 5(1)Σu(+) states. Analysis was guided by ab initio calculations performed to describe the (1,3)Λg,u electronic states of Rb2 up to the Rb(5s) + Rb(5f) atomic asymptote. The theoretical potential energy curves are given in ASCII format in an electronic supplement to this paper.