We have theoretically investigated the unconventional electronic structure, spectroscopic properties, and chemical bonding of highly magnetic molecules containing lanthanide (Ln) atoms. We are interested in the integration of different physical components into hybrid quantum Ln systems. This hybrid approach allows us to take advantage of each component’s best properties thereby realizing new tools for quantum information processing, highly-correlated many-body physics, and high-precision measurements. First, we explore a class of homonuclear Ln molecules, where laser-cooled lanthanide atoms are brought and bound together via Feshbach resonances. Second, we theoretically investigate the unconventional chemical bond between magnetic Ln atoms and hydroxide molecules that is dominated by the behavior of its single-valence electron. Finally, we investigate the bonding of Ln atoms embedded into fullerene cages. Accurate information on the relevant potential energy surfaces and magnetic properties of these systems were previously unavailable. We acknowledge funding from the AFOSR, grant No. FA9550-21-1-0153, and the National Science Foundation, grant No. PHY-1908634.

Understanding of lanthanide (Ln) compounds is challenging due to their complex electronic structure, which results mostly from the open 4f (except Lu) shell of the Ln atoms. Here we report on the characterization of DyO and of DyO⁺ in a supersonic molecular beam by applying a variety of spectroscopic approaches using different REMPI and PFI schemes, MATI, and (V)UV single-photon ionization. Isotopologue specific excitation schemes allow to obtain rotationally resolved spectra, and several Rydberg-series converging to the ionization limits of different rotational states of DyO⁺ have been analyzed. The Rydberg series can be clearly assigned starting with the lowest J=7.5 state. Their vibrational autoionization - employing an infrared free electron laser - results in vibrational data for the cation. From the spectroscopic data obtained for the fermionic ¹⁶¹DyO and the bosonic ¹⁶²DyO, the values of IE and D₀ are determined with a high precision. This leads to the conclusion that the chemi-ionization reaction Dy + O → DyO⁺ + e⁻ clearly proceeds exothermic.

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 present a rotationally resolved study of a series of inner-shell excited states in YbF using resonance-enhanced multi-photon ionisation spectroscopy. We investigate the excited states of lowest energy, 8474, 9013 and 9090 cm⁻¹ above the ground state, all corresponding to the configuration 4f¹³6s^2 2F_7/2 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 31050 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 48720 cm⁻¹ and 48729 cm⁻¹, which are above the ionization limit and feature strong auto-ionizing resonances that prove useful for efficient detection of the molecules and for identifying the rotational quantum number of each line in the spectrum. We find that the rotational structure of all of these states can be described by a 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. The metastable states may themselves be important for this application. They are long-lived states in a laser-coolable molecule featuring closely-spaced levels of opposite parity, all of which are desirable properties for tests of fundamental physics.

The ytterbium monofluoride molecule has gained significant attention as a system well-suited for measuring the electric dipole moment of the electron. Here we present resonance enhanced multi-photon ionization (REMPI) spectra with excitation via the A²Π_1/2 state of YbF, containing a multitude of the molecule's Rydberg states (n ≥ 24) in the energy range of 48500−48750 cm⁻¹. The measurements are performed on a pulsed YbF molecular beam produced by laser ablation and subsequently cooled by supersonic expansion. We have assigned the measured Rydberg states to series converging to several rotational levels (N⁺=0−11) of the non-vibrationally excited (ν⁺=0) ground state of the YbF⁺ ion. This assignment leads to accurate values for the ionization energy (IE) of YbF and the rotational constant B⁺ of YbF⁺. In addition, we have performed rotationally resolved measurements over the same energy range via the higher-lying [31.05] excited electronic state, and compared the spectra to the ones obtained via A state excitation. We attribute the observed differences in ionization behavior between the two excitation schemes to the hybrid character of the [31.05] state, i.e. to this state stemming from both the usual Yb⁺[4f¹⁴6s¹] configuration, as well as the inner-shell excited Yb⁺[4f¹³6s²] one.

Ytterbium Monohydroxide (YbOH) is a molecule of interest in the search for the electron Electric Dipole Moment (eEDM) due to its large effective internal electric field, high polarizability, and prospects for laser-cooling and molecular trapping. Most of its highly-excited states have only been explored at low resolution by previous experiments, among them a band at 535 nm that was tentatively identified as à ²Π_3/2 by Melville and Coxon in 2001. We present a preliminary rotational analysis of the YbOH band at 535 nm taken at high-resolution using laser-induced fluorescence. The resulting rotational constant B and centrifugal distortion D differ considerably from those found for the à ²Π_1/2 state, suggesting that its assignment as the Ω = 3/2 spin-orbit component of the A state may be incorrect or that it is heavily mixed with another nearby state.

A recent triumph of physical chemistry has been to trap molecules at ultracold temperatures ( < 0.001 K) in “beakers” made of laser light. It is hoped that this will enable advances in quantum information science, precision measurement, and ultracold chemistry. Currently, the applicability of these techniques is limited to a very small subset of molecules, generally comprising an alkaline-earth atom bonded to a halogen-like ligand. We will discuss efforts at Williams College to identify new, complex molecules that can be cooled and trapped. This includes molecules built from coinage-metal and carbon-group atoms, CuX, AgX, and AuX (X=C, Si, Ge, Sn, and Pb). These molecules offer new handles for control and manipulation that stem from their chemical complexity. Excitingly (and unexpectedly), this work is also relevant to fundamental understanding of organic catalysis due to gold’s utility in activating C-C double/triple bonds. We are thus preparing to perform spectroscopy of AuCC and AuCCH, nature’s simplest models of gold-alkyne interaction.

Searches for non-zero electric dipole moment (EDM) in fundamental particles shed light on discrete symmetries of nature and constrain new physics beyond the Standard Model. The most sensitive electron EDM and many ongoing nuclear EDM searches are performed with molecules, benefiting from the substantial intra-molecular electric field. At the Facility for Rare Isotope Beams (FRIB), we are building a new generation of EDM searches using ultracold radioactive molecules. This project will leverage the unique opportunity to access pear-shaped nuclei (e.g. ²²⁵Ra) at FRIB, and the state-of-the-art technology in precision measurement using polar molecules. The former amplifies the Nuclear Schiff Moment and hence the sensitivity to hadronic CP-violation, thanks to the nuclear octupole deformation. The latter, built upon recent advances in atomic and optical physics, aims to bring the ²²⁵Ra-containing molecules into the ultracold regime, where both high phase-space density and seconds-long spin precession time have been demonstrated. With the nuclear enhancement and the quantum upgrades combined, this new project envisions to enhance the EDM sensitivity by orders of magnitude from the current best effort.

Electric dipole moments (EDMs) are a signature of time-reversal, parity, and charge-parity (CP) violation. New sources of CP violation beyond the Standard Model (BSM) are required to explain the baryon asymmetry of the universe, and a non-zero EDM could indicate new CP-violating interactionsSakharov, A. D. J. Exp. Theor. Phys. 5, 24–27 (1967).Purcell, E. M. and Ramsey, N. F. Phys. Rev. 78, 807 (1950).. Radioactive nuclei with large octupole deformations offer an enhanced EDM that can be used to probe for BSM physics with increased sensitivity by forming polar radioactive molecules. The EDM3 method aims to implant such molecules in a noble gas matrix, offering high statistics and control of systematics for an EDM measurementVutha, A. C., Horbatsch, M. and Hessels, E. A. Phys. Rev. A 98, 032513 (2018). We aim to adapt this technique for a nuclear EDM measurement using the FRIB-EDM3 instrumentBallof, J. et al. NIM-B 541, 224–227 (2023). The instrument will form polar radioactive molecules in cryogenic noble gas solids for molecular spectroscopy. The frontend will form molecular ions and perform mass separation of the ion species. Mass filtered ions will be transmitted to the backend where they will be neutralized and implanted in a noble gas film to perform spectroscopy. We report on the design and construction of the frontend, and initial conclusions on the effectiveness of our mass separation and ion transport techniques from testing the existing instrument. We also report on the designs of our charge exchange cell and film growth chambers that constitute the backend.

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Footnotes:

Sakharov, A. D. J. Exp. Theor. Phys. 5, 24–27 (1967).

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Footnotes:

Vutha, A. C., Horbatsch, M. and Hessels, E. A. Phys. Rev. A 98, 032513 (2018).. Ballof, J. et al. NIM-B 541, 224–227 (2023)..

Nuclear Schiff moments (NSMs) present a powerful probe into new physics through their connection to CP-symmetry violation. Such symmetry violations are needed to explain the observed baryon asymmetry of the Universe. We are investigating the application of molecular matrix methodsA. C. Vutha, M. Horbatsch, and E. A. Hessels, Orientation-dependent hyperfine structure of polar molecules in a rare-gas matrix: A scheme for measuring the electron electric dipole moment, Phys. Rev. A 98, 032513 (2018)o the search for NSMs of pear-shaped nuclei in heavy polar radioactive moleculesG. Arrowsmith-Kron et al, Opportunities for fundamental physics research with radioactive molecules, arXiv:2302.02165 [nucl-ex] Pear-shaped nuclei (i.e. those with both octupole deformations), such as radium-225, are expected to have enhanced NSMsN. Auerbach, V. V. Flambaum, and V. Spevak, Collective t- and p-odd electromagnetic moments in nuclei with octupole deformations, Phys. Rev. Lett. 76, 4316 (1996) These methods involve trapping polar molecules in a noble gas matrix, which is predicted to lock their orientation relative to the matrix lattice vectors. This contribution focuses on the spectroscopy calculations leading to the draft NSM measurement scheme but also touches on the FRIB-EDM3 instrument, which consists of two main parts: the frontend, which will create and mass-separate molecular ions, such as RaFJ. Ballof et al, Progress towards the frib-edm3-frontend: A tool to provide radioactive molecules from isotope harvesting for fundamental symmetry studies, Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 541, 224 (2023) and the backend, which will neutralize the ions, co-deposit them in a noble gas matrix, and perform molecular hyperfine spectroscopy, which will ultimately enable an NSM search. Eventually we aim to carry out a sensitive search for the NSM of radium-225 using, for example, RaF molecules in solid argon. Information will be provided on the calculations relevant to developing an NSM measurement scheme, which have been carried out first on a simpler analog ¹³⁸BaF, replicating the results of ^a, then on ²²⁵RaF.

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Footnotes:

A. C. Vutha, M. Horbatsch, and E. A. Hessels, Orientation-dependent hyperfine structure of polar molecules in a rare-gas matrix: A scheme for measuring the electron electric dipole moment, Phys. Rev. A 98, 032513 (2018)t G. Arrowsmith-Kron et al, Opportunities for fundamental physics research with radioactive molecules, arXiv:2302.02165 [nucl-ex]. N. Auerbach, V. V. Flambaum, and V. Spevak, Collective t- and p-odd electromagnetic moments in nuclei with octupole deformations, Phys. Rev. Lett. 76, 4316 (1996). J. Ballof et al, Progress towards the frib-edm3-frontend: A tool to provide radioactive molecules from isotope harvesting for fundamental symmetry studies, Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 541, 224 (2023),