Session P09: Atomic and Molecular Structure and Spectroscopy I

Towards a buffer-gas-loaded, multi-species optical trap for molecules

Despite much interest in studying cold molecules, access to cold, trapped molecules has been limited to only a few select species. We here present progress towards trapping a variety of small, chemically stable molecules, such as N2, CO, O2, and HCl [1]. The molecules will be trapped at cryogenic temperatures by buffer-gas loading a deep optical dipole trap. The ~10 K trap depth is produced by a tightly-focused, 1064-nm cavity capable of reaching intensities of hundreds of GW/cm². Molecules will be directly buffer-gas loaded into the trap using a helium buffer gas at 1.5 K. Both buffer-gas cooling and the very far-off-resonant, quasi-electrostatic trapping mechanism are insensitive to a molecule’s energy level structure and dipole moments, allowing for co-trapping of multiple species. Our trap would open new possibilities in molecular spectroscopy, studies of cold chemical reactions, and precision measurement, amongst other fields of physics.

We are currently constructing the first demonstration of such a trap. Here, we report on our progress and present first experimental results.

[1] A. Singh et al. “Dynamics of a Buffer-Gas-Loaded, Deep Optical Trap for Molecules.” Physical Review Research 5, 033008 (2023).   

Condensed-phase Ytterbium Coordination Complexes for Field Sensing

Crystals composed of molecules containing a single rare-earth ion (REI) featuring narrow f-f transitions may be utilized as sensors analogous to atomic vapor cells but with much higher spin density. However, few REI crystals are able to sustain their advantages at temperatures above several kelvins. Our previous work has highlighted the potential application of large Ytterbium-centered molecules in the solution phase for magnetic sensing even at room temperature. Here, we report on advancements in (thiolfan)YbCl(2-MeTHF) and (thiolfan)^*YbCl(2-MeTHF)₂, revealing narrow hole-burning linewidths in the tens of kilohertz at 77 K. Remarkably, these optical absorption spectra remain stable across temperatures from 20 – 90 K. By harnessing the transition’s power and polarization dependence, we demonstrate frequency-selective optical depletion of a spectral subset of electronic spins. The systematic study shows large REI molecules not only increase the densities of manipulable electronic spins on the order of Avogadro’s number (10²³) but also preserve narrow transitions at higher temperatures, offering an alternative approach to pushing the boundaries of gas-phase atomic systems.   

High resolution spectroscopy and characterization of matrix-isolated thulium

Narrow linewidths of ground state transitions in thulium defects embedded in argon crystal make it a competitive candidate for solid-state applications of quantum information processing and sensing. High resolution absorption spectroscopy reveals that the ²F_7/2 - ²F_5/2 (1140 nm) transition is split into multiple components with linewidths as small as 200 MHz. We propose that the splitting is due to crystal field effects at multiple trapping sites and hyperfine interactions, and we characterize a family of lines belonging to a single trapping site.   

Measurement of the unresolved 9Be+ 2P3/2 hyperfine splittings using quantum-interference-enhanced state-selective repump spectroscopy, and progress toward 7,9,10Be+ splitting isotope shift measurements using single trapped ions

Hyperfine splittings of the ²P_3/2 manifold in ⁹Be⁺ were measured directly using a single laser-cooled ion stored in a radiofrequency Paul trap. As the hyperfine structure is unresolved beneath the natural linewidth of the transition, manipulation of the initial state, polarization, and final-state populations was used to preferentially detect scattering events through specific intermediate excited states. While quantum interference effects typically complicate the modeling of unresolved measurement lineshapes, in this work quantum interference helped to suppress extraneous scattering components. The hyperfine splittings between the |²P_3/2, F=3> state and the |²P_3/2, F=2> and |²P_3/2, F=1> states were measured to be ν₃₂ = 0.801(56) MHz and ν₃₁ = 5.050(83) MHz, respectively. Following recent absolute frequency measurements of the D-lines and fine structure splitting in ⁹Be⁺, progress toward trapping and laser spectroscopy on rare isotopes ^7,10Be⁺ is presented.  In addition, we present a demonstration of direct ablation loading of ⁹Be⁺ from small BeCl₂ deposits.   

Laser Spectroscopy of Short-Lived Radium-224 Ions

Ra-224 is an intriguing candidate for a simple and robust optical clock with a low charge to mass ratio and transitions that are far from the blue compared to most other ions. We measured the energies of the four lowest excited states in trapped and laser-cooled Radium-224 ions (3.6 day half-life), the ²P_1/2 ,²P_3/2 ,²D_3/2 , and ²D_5/2 states. To do this we performed spectroscopy on the the ²S_1/2 → ²P_1/2 cooling transition (468 nm), the ²S_1/2 →²D_5/2 clock transition (728 nm), the ²D_5/2 →²P_3/2 cleanout transition (802 nm) as well as the ²D_3/2 →²P_3/2 transition (708 nm). We determined the transition frequencies by scanning over nearby molecular reference lines in tellurium or iodine for each transition. The ions are produced by a custom-built molecular beam epitaxy (MBE) oven which is loaded with thorium-228. Thorium-228 has a half-life of 1.9 years, and its daughter isotope Ra-224 has a vapor pressure over one trillion times greater than thorium. The newly decayed radium atoms can be heated out of the oven and the thorium is left behind. Combining this source with photoionization, we are able to realize trapped radium ions within minutes. The system has been sealed at low pressure for over a year without any noticeable reduction in ion loading rate.   

Vibrational Branching Ratios of Low-Symmetry Strontium-Containing Molecules

Recent advances in direct laser cooling of linear polyatomic molecules promise advances in quantum information, cold chemistry, and precision measurement experiments [1-3]. Larger and less-symmetric molecules have several additional features. However, the pathway to laser cooling low-symmetry species, such as asymmetric tops, is not yet fully known. Although 1-D laser cooling of symmetric tops works [4], and proposals for laser cooling of asymmetric tops have been published [3], the effect of rotational perturbations and vibrational branching on full laser cooling of low symmetry species is not well understood. To study quantitatively how symmetry affects laser cooling, we present here vibrational branching ratio measurements of strontium-containing molecules from three different symmetry classes. By measuring the vibrational branching ratios of these molecules to the 0.01-0.1% level, we assess one aspect of the viability of laser cooling these species in the future. Additionally, we compare our results to isostructural calcium-containing species to illuminate how the combination of mass and structural complexity can affect laser cooling of low-symmetry molecules more generally.

[1] Albert et al, Phys. Rev. X 10 2020

[2] Kozyryev, Hutzler, Phys. Rev. Lett. 119 2017

[3] Augenbraun et al, Phys. Rev. X 10 2020

[4] Mitra et al, Science 369 2020   

High Precision Theory for the High-n Rydberg States of Helium

Variational calculations readily produce high precision energies and wave functions for the ground state, but typically the accuracy rapidly deteriorates with increasing principal quantum number n. The current limit is n = 10 [1,2]. We will report the results of new variational calculations based on the use of triple basis sets in Hylleraas coordinates. The basis sets are "tripled" in that each combination of powers i,j,k in basis functions of the form r₁ⁱr₂^jr₁₂^k exp(-αr₁ -βr₂) is repeated three times with different nonlinear parameters α and β that are separately optimized to

define different distance scales. Results will be reported for the S- and P-states up to n = 24, including a comparison with high precision measurements for the singlet P state n =24.

 [1] G. W. F. Drake and Z.-C. Yan, Phys. FRev. A 46, 2378  (1992).

 [2] D. T. Aznabaev, A. K. Bekbaev, and V. I.  Korobov, Phys. Rev. A,  98, 012510 (2018).

 [3] ] G. Clausen et. al, Phys. Rev. Lett. 127, 093001 (2021).   

Atomic structure calculations of U and Pu

Actinide atoms with many valence electrons, strong valence-core correlations, and substantial relativistic effects are very challenging for atomic calculations. Using starting potentials that partially include screening by valence electrons, we find that accuracy can be improved and the size of configuration space in the framework of the configuration-interaction (CI) approach can be reduced. In fact, the starting potential with one or two valence electrons removed lead to the dominance of principal configurations, so the perturbation theory can be applied to take into account smaller interactions of these configurations with the others. Thus, the configuration interaction perturbation theory (CI-PT) method can be applied to actinide atoms, such as 8-valence electron Pu. First, it can use an optimal starting potential, second, it can treat the CI effects via computationally much more efficient perturbation theory, and third, the basis is relativistic. Additional fine tuning can be added to improve the accuracy for example by adjusting the energies of configurations. The program was modified to make the parameter adjustments automatic. The results of calculations for various atoms and ions, like U I, U II, Pu II will be presented. Relatively high accuracy has been achieved, and further improvements are possible.   

Two-photon excitation and absorption spectroscopy of gaseous and supercritical xenon

Constructing lasers in the vacuum-ultraviolet spectral range (VUV; 100 – 200 nm) is difficult, as excited state lifetimes scale as 1/ω³, necessitating high pump powers to achieve population inversion. Alternatively, a coherent source in this wavelength regime could be based on a Bose-Einstein condensate of photons, as first realized in our group in 2010 for visible-spectral-range photons. In the used experimental scheme, a photon gas is confined to a wavelength-size optical microcavity, which is filled with a liquid dye solution. The dye molecules exhibit a thermalized internal level structure and in repeated absorption and emission cycles the two-dimensional photon gas is thermally equilibrated. We here propose the adaption of this scheme for the condensation of VUV photons, using dense xenon gas as a thermalization mediator for the short-wavelength photons. Aiming at the identification of possible excitation channels for such a vacuum-ultraviolet photon condensate, two-photon transitions from the 5p⁶ to the 5p⁵6p and 5p⁵6p’ states are probed, using two photons of identical wavelengths in the UV. Further, absorption measurements of the 5p⁶ → 5p⁵6p transitions are presented, involving one light field in the VUV and an auxiliary one near 500 nm wavelength. This nondegenerate scheme aims at increasing the reabsorption probability for photons emitted on the second excimer continuum near 172 nm wavelength.   

Long lifetimes of biophotons through diffusion among subradiant states: experiment and theory

Ultra-weak radiation biophotons have been detected and studied in various biological systems, including cells, tissues, and organisms. They are thought to arise from a variety of biochemical and biophysical processes within living organisms, such as metabolic reactions, oxidative processes, and cellular communication.

In diverse living tissues, prolonged biophoton lifetimes have been experimentally observed. In our presentation, we will introduce relaxation models incorporating radiant and subradiant states of molecules under conditions of intense collisions. We have demonstrated that collisions foster diffusion among subradiant collective states. The lifetimes derived from these models elucidate the exceptionally slow relaxation times observed experimentally in ultra-low radiation, commonly known as biophotons.

The study of relaxation of the ultra-weak radiation brings together biology, physics, and biophotonics, and offers insights into the fundamental processes underlying living organisms and their interactions with light.