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The Journal of Chemical Physics

A full dimensional investigation of infrared spectroscopy of the RbCs dimer using the multi-configuration time-dependent Hartree method

Huihui Wang, Yonggang Yang, Liantuan Xiao, Suotang Jia

Published Dec 28, 2013• 5 citations
Publisher

Abstract

The geometry and infrared absorption spectrum of (RbCs)2 have been studied by full dimensional quantum dynamics simulations. For this purpose, the potential energy and dipole moment surfaces are generated by means of a cluster expansion with all two and three mode correlations, and fitted to analytical expressions with negligible deviations. Accordingly, the ground state (RbCs)2 has a diamond geometry with D(2h) symmetry. The infrared spectrum with frequencies up to 120 cm(-1), exhibits rich details of the fundamentals, overtones, and combination bands; the highest fundamental frequency of (RbCs)2 is only 40.26 cm(-1). The present study unravels important details of the interactions between the widely investigated ultracold RbCs molecules.

DOI
10.1063/1.4856095

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The geometry and infrared absorption spectrum of (RbCs)2 have been studied by full dimensional quantum dynamics simulations. For this purpose, the potential energy and dipole moment surfaces are generated by means of a cluster expansion with all two and three mode correlations, and fitted to analytical expressions with negligible deviations. Accordingly, the ground state (RbCs)2 has a diamond geometry with D(2h) symmetry. The infrared spectrum with frequencies up to 120 cm(-1), exhibits rich details of the fundamentals, overtones, and combination bands; the highest fundamental frequency of (RbCs)2 is only 40.26 cm(-1). The present study unravels important details of the interactions between the widely investigated ultracold RbCs molecules.
We apply the relativistic configuration-interaction valence-bond method to calculate various characteristics of the alkali-metal RbCs dimer. These include the electronic potentials and transition dipole moments between the ground and first excited states and permanent dipole moments of the X 1sigma+ and a 3sigma+ states of the ground configuration. In addition, we estimate the lifetime of the rovibrational levels of the X state due to blackbody radiation. These data can help experimentalists to optimize photoassociative formation of ultracold RbCs molecules and their longevity in a trap or in an optical lattice. Extended basis sets, constructed from Dirac-Fock and Sturm's orbitals, have been used to ensure convergence of our calculations. We compare our data with other theoretical and experimental results when they were available.
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.
Ultracold polar RbCs molecules are produced via photoassociation in a dual dark-SPOT magneto-optical trap, in which ultracold rubidium and cesium atoms with high densities are formed by laser cooling and trapping. The a3∑+ triplet metastable state ultracold RbCs molecules with 104/s production rate are detected using resonance-enhanced two-photon ionization. The ultra-high resolution rovibrational spectra of (2)0+, (3)0- and (2)0- rovibrational states are observed by scanning the photoassociation laser frequency. The rotational constants of (2)0+, (3)0- and (2)0- vibrational states are 0.00349 cm-1, 0.00649 cm-1 and 0.00372 cm-1 by fitting respectively.
We report high-resolution photoassociation (PA) spectra of RbCs in the 2(0 -) long-range state.Transitions to more than fifty vibrational levels were recorded with the largest binding energy being 507.5 cm -1 .By fitting the experimental transition frequencies to the improved LeRoy-Bernstein formula, the C 6 coefficient for the potential energy curve of the 2(0 -) state was determined to be -1509±97 a.u.. Perturbation-induced energy level shift and state mixing of the longrange 2(0 -) 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 v = 190 vibrational level of the 2(0 -) 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 β 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.
We performed high-resolution Fourier-transform spectroscopy measurements and direct-potential-fit of the shelflike RbCs $(4){\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ state. Overall, 2317 rovibronic ${v}^{\ensuremath{'}},{J}^{\ensuremath{'}}$ term values with ${v}^{\ensuremath{'}}\ensuremath{\in}[3,88]$, ${J}^{\ensuremath{'}}\ensuremath{\in}[7,276]$ were obtained with accuracy 0.01 cm${}^{\ensuremath{-}1}$ from the $(4){\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}\ensuremath{\rightarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+},a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ laser-induced fluorescence (LIF) spectra. The mass-invariant potential energy curve constructed by means of the Chebyshev polynomial expansion reproduced the experimental term values of both 85{}^{85}85Rb133{\phantom{\rule{0.16em}{0ex}}}^{133}133Cs and 87{}^{87}87Rb133{\phantom{\rule{0.16em}{0ex}}}^{133}133Cs isotopologues with a standard deviation of 0.005 cm${}^{\ensuremath{-}1}$. We simulated the optical cycle $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+},a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}({v}^{\ensuremath{'}\ensuremath{'}},{J}^{\ensuremath{'}\ensuremath{'}}=0)\ensuremath{\rightarrow}(4){\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}({v}^{\ensuremath{'}},{J}^{\ensuremath{'}}=1)\ensuremath{\rightarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}({v}^{\ensuremath{'}\ensuremath{'}}=0,{J}^{\ensuremath{'}\ensuremath{'}}=0)$ appropriate to transfer translationally ultracold RbCs molecules into their absolute ground state from the weakly bound rovibronic levels of both singlet $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ and triplet $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ ground states. The optimal transfer via the intermediate $(4){\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ state with ${v}^{\ensuremath{'}}=58,60$, and 61 for 85{}^{85}85Rb133{\phantom{\rule{0.16em}{0ex}}}^{133}133Cs as well as ${v}^{\ensuremath{'}}=59$ and 61 for 87{}^{87}87Rb133{\phantom{\rule{0.16em}{0ex}}}^{133}133Cs was predicted. The required spin-allowed $(4){\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}\ensuremath{-}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ and spin-forbidden $(4){\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}\ensuremath{-}a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ dipole transition moments were obtained within the framework of ab initio quasirelativistic calculations. The predicted $(4){\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}\ensuremath{\rightarrow}X{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+},a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ transition probabilities were consistent with the experimental relative intensity distributions for several $(4){\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}({v}^{\ensuremath{'}})\ensuremath{\rightarrow}X,a({v}^{\ensuremath{'}\ensuremath{'}})$ LIF progressions.
We have investigated photoassociative formation of RbCs molecules in the ${(2)}^{\phantom{\rule{0.16em}{0ex}}3}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Pi}$ excited state correlated to v=8v=8v=8, (5)0+(5){0}^{+}(5)0+ vibrational level in detail. The metastable ground-state RbCs molecules formed by spontaneous decay are ionized by pulsed dye laser through resonance-enhanced two-photon ionization. A rate equation of the photoionization process is introduced to explain the dependence of RbCs+{}^{+}+ molecular ion intensity on the ionization laser intensity. The saturation effect of molecular ion intensity appears as the photoassociation laser intensity increases. The rotational constant and centrifugal distortion constant are derived to be 0.01304 cm${}^{\ensuremath{-}1}$ and 0.000015 cm${}^{\ensuremath{-}1}$ from the photoassociation spectrum with a high sensitivity, respectively. The measured electric dipole moment of the observed $(2){\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Pi}$ state RbCs molecules is 4.7(6) D by Stark effect in static electric field.