Modeling the electronic structures of the ground and excited states of the ytterbium atom and the ytterbium dimer: A modern quantum chemistry perspective

Abstract

We present a comprehensive theoretical study of the electronic structures of the Yb atom and the Yb 2 molecule, respectively, focusing on their ground and lowest-lying electronically excited states. Our study includes various state-of-the-art quantum chemistry methods such as CCSD, CCSD(T), CASPT2 (including spin-orbit coupling), and EOM-CCSD as well as some recently developed pCCD-based approaches and their extensions to target excited states. Specifically, we scan the lowest-lying potential energy surfaces of the Yb 2 dimer and provide a reliable benchmark set of spectroscopic parameters including optimal bond lengths, vibrational frequencies, potential energy depths, and adiabatic excitation energies. Our in-depth analysis unravels the complex nature of the electronic spectrum of Yb 2 , which is difficult to model accurately by any conventional quantum chemistry method. Finally, we scrutinize the biexcited character of the first 1 Σ + g excited state and its evolution along the potential energy surface. K E Y W O R D S relativistic effects, electron corellation effects, spin-orbit coupling, equation of motion coupled cluster, CASPT2 | INTRODUCTIONThe divalent ytterbium atom has in recent years garnered significant attention thanks to its many uses in cold atom physics. It has a nonmagnetic 1 S 0 ground state and several useful optical transitions: the strong 1 S 0 $ 1 P 1 line can be used for Zeeman slowing, whereas the narrow (181 kHz) intercombination 1 S 0 $ 3 P 1 line can be used to directly laser cool Yb atoms to microkelvin temperatures. [1] Yb has seven stable isotopes: two fermions (171 and 173 with nuclear spins of 1/2 and 5/2, respectively) and five bosons (168, 170, 172, 174, and 176) that lack nuclear spin. The rich isotope structure makes it possible to mass-tune the atomic interactions [2] and facilitates a wide array of possible quantum-degenerate gases. [3][4][5][6] The doubly forbidden 1 S 0 $ 3 P 0 transition lies at the heart of optical atomic clocks [7] that are among the most precise physical instruments known to mankind. For example, an ytterbium clock has recently been demonstrated to enable geopotential measurements with an accuracy below a centimeter. [8] The long-lived 3 P 0 clock states also find use in quantum simulations using Yb atoms. [9] The long-range interactions in the Yb dimer have been probed extensively by high-resolution photoassociation spectroscopy (PAS) [10] near the narrow 1 S 0 $ 3 P 1 intercombination line. The excited 1 S 0 + 3 P 1 (0 + u ) [11,12] state has been probed by single color PAS and provided the van der Waals C 6 coefficient and an improved value of the atomic 3 P 1 lifetime. Two-color PAS of ground state 0 + g vibrational levels [2,13] delivered accurate

Electronic spectra of ytterbium fluoride from relativistic electronic structure calculations

“…There is also some interest in the Yb atom, cation and dimer in connection to ultracold physics. [12][13][14] An example 15 is the parity violation observed in the Yb atom.…”

“…The first set of states was obtained by electron attachment on Yb(4f 14 F + ion, where the HOMO (s 6s,1/2 ) of YbF was initially empty. This computation on the (0h,1p) sector of Fock space yielded states with 4f 14 and a valence electron in the s 6s , d or p orbital. This means that, in the process of obtaining the potentials for the ground and excited states of YbF, we immediately obtain energies of CCSD quality for YbF + , and therefore vertical ionization potentials (IP) at each geometry.…”

“…As we separated the computations of states with 4f 13 and 4f 14 character, these states cannot interact with each other, and states with the same O value cross although they should have an avoided crossing. In order to correct this deficiency we considered a simple adiabatization model, in which we set up and diagonalize the following matrix for each O value:…”

Prospects of Forming High-Spin Polar Molecules from Ultracold Atoms

“…The interaction potential for Sr þ Sr has been studied in detail spectroscopically; the well depth and equilibrium distance are known precisely to be 1081.64 cm −1 and 4.672 Å, respectively, and the potential supports 63 vibrational levels for 88 Sr [54,63]. For Yb þ Yb, there is considerable variation in the well depth between different levels of theory [64], but the dependence of the nearthreshold bound states on the isotopic mass shows that it supports 72 vibrational levels [55,65]. We base our calculations on the depth 739.73 cm −1 obtained by Borkowski et al [55].…”

Free-bound excitation and predissociation of ytterbium dimers near theS01−P11atomic transition

“…Quantum chemical calculations are also difficult for this heavy molecule. There have been theoretical studies of potential curves [15][16][17][18][19] and a breakdown of the Born-Oppenheimer approximation [20].…”