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arXiv:2402.10692 · Physical Review X

Understanding Inner-Shell Excitations in Molecules through Spectroscopy of the 4f Hole States of YbF

S. Popa, Sascha Schaller, André Fielicke, Jongseok Lim, Boris G. Sartakov, Michael R. Tarbutt, Gerard Meijer

Published May 30, 2024• 5 citations
arXivHTMLPDF

Abstract

Molecules containing a lanthanide atom have sets of electronic states arising from excitation of an inner-shell electron. These states have received little attention but are thought to play an important role in laser cooling of such molecules and may be a useful resource for testing fundamental physics. We study a series of inner-shell excited states in YbF using resonance-enhanced multiphoton ionization spectroscopy. We investigate the excited states of lowest energy, 8474, 9013, and 9090 cm−1 above the ground state, all corresponding to the configuration 4f136s2 F27/2 of the Yb+ ion. They are metastable, since they have no electric dipole allowed transitions to the ground state. We also characterize a state at 31 050 cm−1 that is easily excited from both the ground and metastable states, which makes it especially useful for this spectroscopic study. Finally, we study two states at 48 720 and 48 729 cm−1 , which are above the ionization limit and feature strong autoionizing resonances that prove useful for efficient detection of the molecules and for identifying the rotational quantum number of each line in the spectrum. We resolve the rotational structures of all these states and find that they can all be described by a very simple model based on Hund’s case (c). Our study provides information necessary for laser slowing and magneto-optical trapping of YbF, which is an important species for testing fundamental physics. We also consider whether the low-lying inner-shell states may themselves be useful as probes of the electron’s electric dipole moment or of varying fundamental constants, since they are long-lived states in a laser-coolable molecule featuring closely spaced levels of opposite parity. Published by the American Physical Society 2024

DOI
10.1103/physrevx.14.021035

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Molecules containing a lanthanide atom have sets of electronic states arising from excitation of an inner-shell electron. These states have received little attention but are thought to play an important role in laser cooling of such molecules and may be a useful resource for testing fundamental physics. We study a series of inner-shell excited states in YbF using resonance-enhanced multiphoton ionization spectroscopy. We investigate the excited states of lowest energy, 8474, 9013, and 9090 cm−1 above the ground state, all corresponding to the configuration 4f136s2 F27/2 of the Yb+ ion. They are metastable, since they have no electric dipole allowed transitions to the ground state. We also characterize a state at 31 050 cm−1 that is easily excited from both the ground and metastable states, which makes it especially useful for this spectroscopic study. Finally, we study two states at 48 720 and 48 729 cm−1 , which are above the ionization limit and feature strong autoionizing resonances that prove useful for efficient detection of the molecules and for identifying the rotational quantum number of each line in the spectrum. We resolve the rotational structures of all these states and find that they can all be described by a very simple model based on Hund’s case (c). Our study provides information necessary for laser slowing and magneto-optical trapping of YbF, which is an important species for testing fundamental physics. We also consider whether the low-lying inner-shell states may themselves be useful as probes of the electron’s electric dipole moment or of varying fundamental constants, since they are long-lived states in a laser-coolable molecule featuring closely spaced levels of opposite parity. Published by the American Physical Society 2024
The YbF molecule is a sensitive system for measuring the electron’s electric dipole moment. The precision of this measurement can be improved by direct laser cooling of the molecules to ultracold temperature. However, low-lying electronic states arising from excitation of a 4f electron may hinder laser cooling. One set of these “4f hole” states lies below the A2Π1/2 excited state used for laser cooling, and radiative decay to these intermediate levels, even with branching ratios as small as 10−5, can be a hindrance. Other 4f hole states lie very close to the A2Π1/2 state, and a perturbation results in states of mixed character that are involved in the laser cooling cycle. This perturbation may enhance the loss of molecules to states outside of the laser cooling cycle. We model the perturbation of the A2Π1/2 state to determine the strength of the coupling between the states, the de-perturbed potential energy curves, and the radiative branching ratios to various vibrational levels of the ground state, X2Σ+. We use electronic structure calculations to characterize the 4f hole states and the strengths of transitions between these states and the A2Π1/2 and X2Σ+ states. We identify a leak out of the cooling cycle with a branching ratio of roughly 5×10−4, dominated by the contribution of the ground state configuration in a 4f hole state. Finally, we assess the impact of these results for laser cooling of YbF and molecules with similar structure.
The open inner-shell electronic structure of lanthanides with large magnetic moments gives rise to a rich spectrum of transitions available for laser cooling, trapping, and coherent control. Despite this, the large number of ultraviolet (UV) transitions below 400nm have so far been rarely utilized in dipolar atom experiments. Here, we investigate multiple UV ground state transitions in dysprosium. Several of these UV excited states have the largest decay strengths to the ultralong-lived, low-lying first excited state which are comparable to the most commonly used strongest transitions found in dipolar atoms. Using two-dimensional shelving spectroscopy which improves detection sensitivity and provides a straightforward way to determine the hyperfine-isotope structure and excited state total angular momentum JJJ, we measure isotope shifts, hyperfine coefficients, and create King plots to determine their electronic nature. Such knowledge of these UV transitions which analogously exist in other magnetic atoms is important for optically populating the first excited state and can be used towards creating an optical clock, high resolution imaging in quantum gas microscopy, and probing lanthanide nuclei with enhanced Schiff moments in search of physics beyond the standard model.
We report radiation pressure slowing of YbF molecules to low velocity. In YbF, laser slowing is hindered by leaks out of the optical cycle attributed to low-lying metastable electronic states arising from inner-shell excitation. We bring this population back into the optical cycle once it has decayed to the electronic ground state using microwaves to couple the relevant rotational levels. We measure the scattering rate and closure of the optical cycle as repumps are added, and study the destabilization of dark states by a magnetic field and by polarization modulation, finding that both are helpful for maximizing the scattering rate. Starting from a beam with a mean speed of 80 m/s, and using frequency-broadened slowing light, we reduce the mean speed of the beam and produce a substantial flux in the low-velocity tail of the distribution. Slowing increases the fraction of molecules below 40 m/s from 0.4(1)% to 7.0(2)%, and the fraction below 30 m/s from zero to 3.2(1)%. The establishment of a nearly closed optical cycle and the production of molecules at low velocity are important steps toward trapping YbF molecules for future measurements of the electron's electric dipole moment.
Full text: It has been shown recently that Yb and Er atoms from the lanthanides group can be efficiently laser-cooled using strong transitions near 400 nm. Degenerate Fermi gases of ytterbium have been also recently demonstrated using laser-cooling based technic. Our goal was to investigate the possibility of laser cooling of Tm atom. Among other lanthanides, thulium possesses relatively simple level structure.Moreover, it has only one stable isotope {sup 169}Tm with a nuclear spin of I = 1/2 which makes possible to use schemes for sub-Doppler cooling. Laser-cooled thulium is a favorable candidate for optical clocks applications, since the forbidden transition between the fine structure sublevels (J{sub g} 5=2) - (J'{sub g} = 7/2) of the ground state 4f{sup 13}6s{sup 2} ({lambda} = 1.14 micron) has a spectral width of approximately 1 Hz. The collisional shift of such kind of transitions in lanthanides is suppressed because of the outer closed 6s{sup 2} shell, which allows for precision spectroscopy in a dense atomic cloud. We studied two candidates for cooling transitions from the ground state 4f{sup 13}6s{sup 2} (J{sub g} 7/2) to the states 4f{sup 12}({sup 3}H{sub 5})5d{sub 3/2}6s{sup 2} (J{sub e} = 9/2) at 410.6 nm and 4f{sup 12}({sup 3}F{sub 4})5d{sub 5/2}6s{sup 2} (J{sub e} = 9/2) at 420.4 nm. By means of saturation absorption spectroscopy, we measured the hyperfine structure and rates of these transitions. We evaluated the life times of appropriate excited levels as 15.9(8) ns and 48(6) ns, respectively. Decay rates from these levels to neighboring opposite-parity levels were evaluated by means of Hartree-Fock calculations. The fraction of atoms which do not return to the ground state is about 10{sup -5} and 5 {center_dot} 10{sup -4} for the 410.6 nm and 420.4 nm transitions respectively. We conclude that the strong transition at 410.6 nm with relative slow leakage rate can be used for the efficient cooling of Tm I. We also measured hyperfine structure of two nearby transitions from the ground state to the states 4f{sup 13}({sup 2}F{sub 7/2})6s6p({sup 1}P{sub 1}) (J{sub e} 5/2) at 409.4 nm and 4f{sup 12}({sup 3}F{sub 4})5d{sub 5/2}6s{sup 2} (J{sub e} = 7/2) at 418.9 nm, which can be used for {lambda}-type excitation of {lambda} = 1.14 {mu}m transition. (author)
The generation of a fountain of laser-cooled ytterbium fluoride, YbF, has been recently proposed\footnote{Tarbutt, M R; Sauer, B E; Hudson, J J; Hinds E A, New J. Phys 15, 053034, 2013.} as a method for long coherent observation times, thereby improving the electron electric dipole moment (eEDM) measurement. Understanding the properties of the excited electronic states of YbF is essential for the development of such a scheme for laser cooling. Here we report on the measurement of the radiative lifetimes,τ\tauτ, permanent electric dipole moments, μ\muμel_{el}el​, and magnetic g-factors for the [18.6] and [18.7] excited states of YbF. The results are compared with the previously determined values for [18.1] state\footnote{Zhuang,X; Le,A.;Steimle, T C; Bulleid, N E; Smallman, I J; Hendricks, R J; Skoff, S M ; M R; Hudson, J J; Sauer, B E; Hinds, Tarbutt, M R, PCCP, 13 19103, 2011}\footnote{Condylis,P C; Hudson, J J; Tarbutt, M R; Sauer, B E; Hinds E A, J. Chem. Phys. 123, 231101, 2005}. The [18.1] state is the Ω\OmegaΩ=1/2 spin-orbit component of the A 2^{2}2Π\PiΠ(v=0) electronic state arising from the Yb+^{+}+(4f$^{14}6p6p6p\pi)F)F)F^{-}(2p(2p(2p^{14}$) configuration. The experimentally determined μ\muμel_{el}el​, and g-factors will be used to unravel the nature of the [18.6] and [18.7] states, which are known to be admixtures A 2^{2}2Π\PiΠ and an additional Ω\OmegaΩ=1/2 state of unknown electronic configuration.
Long-lived metastable states in the 4f$^{13}5d6sconfigurationofYb5d6s configuration of Yb5d6sconfigurationofYb^+$
We study the occurrence of long-lived metastable states in the 4f$^{13}5d6selectronconfigurationofYb5d6s electron configuration of Yb5d6selectronconfigurationofYb^+$. By optical pumping of a single trapped ion on the 2F7/2o→(7/2,0)7/2^2F^\text{o}_{7/2}\rightarrow (7/2,0)_{7/2}2F7/2o​→(7/2,0)7/2​ transition at 377.5 nm, we prepare a wide range of metastable electronic states. We use a co-trapped control ion to sympathetically cool the spectroscopy ion, allowing us to accurately time its subsequent decay. We record a strong decay signal corresponding to a lifetime of 0.92(8) s, a weaker decay signal with lifetime 9.8(+2.9, -2.0) s, and find evidence for a much longer lifetime, >>> 30 s. We identify the metastable states with these lifetimes qualitatively, and corroborate our results with atomic structure calculations that support the observed lifetimes and decay paths. These long-lived states provide new opportunities in qubit and qudit state detection and optical clocks.