Session X08: Ultracold Plasmas and Molecules

Ion thermometry in ultracold neutral plasmas

Disorder-induced heating (DIH) is a well-known feature of ultracold neutral plasmas (UNPs). It occurs as excess electrical potential energy is converted to kinetic energy during a global relaxation of the plasma towards equilibrium. Fluorescence methods are used to measure the ion temperature. In this talk I will present three analysis methods for determining the ion temperature. PMT measurements give spatially-averaged temperatures with excellent time resolution. Except at very early times in the plasma expansion, these 'temperatures' are compromised by the hydrodynamic velocity. Spatially resolved fluorescence images can be analyzed pixel by pixel to produce a temperature map at a particular time. These high-resolution images often suffer from poor signal-to-noise characteristics. We demonstrate that ion temperatures can be obtained from a single image at late times in the plasma expansion. We will discuss ion temperature measurements in both magnetized and un-magnetized plasmas.   

Optimal production of AlCl via laser ablation

Ultracold polar molecules allow for a range of novel studies, including many-body physics of quantum degenerate gases, quantum computing, precision measurements and tests of fundamental symmetries. Laser cooling has been key as a first step towards implementing similar applications with atoms. Applying the same schemes to molecules, however, is challenging due to their additional degrees of freedom, which interrupt the photon cycling process by a decay into dark states. With an estimated Franck-Condon factor of 99.88%, AlCl is an excellent candidate for laser cooling. Starting with a cryogenic buffer-gas beam source, we use pulsed-laser ablation to produce AlCl in the gas phase and a frequency-tripled Ti:Saph laser system to carry out precise spectroscopy on AlCl. To maximize the yield of AlCl, we have systematically studied and compared various precursor targets, including mixtures of KCl:Al, NaCl:Al, CaCl2:Al, MgCl2:Al, and AlCl3. Here, we will give an update on the status of the experiment, our target studies and discuss our progress towards slowing and cooling AlCl.

    

Towards magneto-optical trapping of AlF molecules

Aluminum monofluoride (AlF) is a promising candidate to produce a dense, ultracold gas through laser cooling. We present our recent progress towards implementing a Zeeman slower for the molecules, transverse and rotational cooling, and magento-optical trapping of AlF molecules. We also present a thorough characterisation of a MOT of Cd atoms using the ¹P₁ ← ¹S₀ transition near 229 nm. Cd is an excellent species to test our MOT apparatus as it shares many properties with the more complex case of AlF.   

Optical cycling functionalization of aromatic molecules, towards laser cooling

Laser cooling relies on photon cycling, which can be enabled in polyatomic molecules when an “optical cycling center” (OCC) is attached to an electronegative ligand. It was proposed that molecules with a (metal) alkaline-earth(I)-oxide-radical structure, would have good OCC properties and, thus, would be amenable to laser cooling [1]. More recent theoretical work has indicated that the alkaline-earth(I)-oxide unit attached to a benzene ring would offer a good OCC that would be tunable by substituting the ring hydrogen atoms with more electronegative species [2]. Theory has also indicated that larger rings (such as naphthalene, pyrene, and coronene) can also provide good optical cycling properties [3]. We report the results of dispersed fluorescence measurements on CaO-Ph-X (Ph : phenyl, X = F, CH3, CF3) in a cryogenic buffer gas at 9 K. We find that the vibrational branching ratio (VBR) to the ground vibrational state is >90% for all species and 99% for CaO-Ph-3,4,5F [4]. We also study the napthol-based molecules CaO-Nap and SrO-Nap, and find that the 2-naphthyl positional variant of CaO-Nap also has a highly diagonal VBR, 96% [5]. These results demonstrate that the same principles that have led to laser cooling of di-, tri- and poly-atomic molecules (e.g. SrF, CaOH and CaOCH3) can likely be extended to phenolic and aromatic molecules. 

[1] Kozyryev, et al, ChemPhysChem 2016, 17, 3641

[2] Dickerson, et al, PRL 2021, 126, 123002

[3] Dickerson, et al, JPCL 2021, 12, 3989–3995

[4] Zhu, et al, 2022, manuscript in preparation

[5] Mitra, et al, 2022, arXiv:2202.01685   

The formation of Heavy Magnetic Lanthanide Molecules

The electronic structure of magnetic lanthanide atoms is fascinating from a fundamental perspective.

They have electrons in a submerged open 4f shell lying beneath a filled 6s shell with strong

relativistic correlations leading to a large magnetic moment and large electronic orbital angular

momentum. This large angular momentum leads to strong anisotropies, i. e. orientation

dependencies, in their mutual interactions. The long-ranged molecular anisotropies are crucial for

proposals to use ultracold lanthanide atoms in spin-based quantum computers, the realization of

exotic states in correlated matter, and the simulation of orbitronics found in magnetic

technologies. Short-ranged interactions and bond formation among these atomic species have thus far

not been well characterized. Efficient relativistic computations are required. Here, for the first

time we theoretically determine the electronic and ro-vibrational states of heavy homonuclear

lanthanide Er₂ and Tm₂ molecules by applying state-of-the-art relativistic methods. In spite

of the complexity of their internal structure, we were able to obtain reliable spin-orbit and

correlation-induced splittings between the 91  Er₂ and 36  Tm₂ electronic potentials

dissociating to two ground-state atoms. A tensor analysis allows us to expand the potentials between

the atoms in terms of a sum of seven spin-spin tensor operators simplifying future research.  The

strengths of the  tensor operators as functions of atom separation are presented and relationships

among the strengths, derived from the dispersive long-range interactions, are explained. Finally,

low-lying spectroscopically relevant ro-vibrational energy levels are computed with coupled-channels

calculations and analyzed.   

Optical Pumping of SiO+: How an Intervening Electronic State aids in Ground Rotational State Preparation

State preparation of molecules is a crucial first step in experiments of metrology, fundamental physics, and quantum information. Recently, control over the quantum states of SiO⁺ has been demonstrated^1,2. With an X, A, and B electronic state structure, spectral filtering of light that cycles population between the X and B states can lead to ground or excited rotational state preparation. Here, we present our results modeling the state population dynamics involved. Our results indicate that the electronic A state aids in this process by acting as a pathway for parity flipping. By simulating the rotational population as a function of time, we fit the rates at which rotational state parity is changed and match this to the experiment¹. We find the B-A branching is likely stronger than predicted and in fact beneficial when optical pumping to the ground rotational state. Relevant timescales of other processes that affect state population are modeled and shown, providing a near complete description of optical pumping in SiO⁺.

[1] Stollenwerk, P. R. et al, PRL 125, 113201 (2020)

[2] Antonov, I.O. et al, Nat. Commun. 12, 2201 (2021)   

Strong ortho/para effect in the predissociation spectra of the 35Cl-(H2) complex and its isotopologue 35Cl-(D2) recently measured in an ionic trap at low temperature

The predissociation spectra of the ³⁵Cl^-(H₂) and ³⁵C^l-(D₂) complexes are determined within an accurate quantum approach and compared to those recently measured in an ionic trap at 8 K and 22 K. [1,2,3] The calculations are performed using an existing three-dimensional potential energy surface [4]. A variational approach is used for the accurate quantum calculations of the rovibrational bound states. Several methods are compared for the search and the characterization of the resonant states. A good agreement between the calculated and measured spectra is obtained, despite a slight shift to the red of the calculated spectra. The comparison shows that only the ortho or para contribution is observed in the measured ³⁵Cl^-(H₂) or ³⁵Cl^-(D₂) spectrum, respectively. This result is attributed to the rapid para <-> ortho conversion of the complexes by collision with H₂ or D₂ molecules inside the trap at low temperature. Quantum numbers are assigned to the rovibrational resonant states. It demonstrates that the main features observed in the measured predissociation spectra correspond to a progression in the intermonomer vibrational stretching mode.    

Quantum state tracking and control of a single molecular ion in a thermal environment

Over the past decade, cold molecular ions have emerged as a powerful platform for precision spectroscopy, controlled chemistry, and quantum information, yet the efficient control of their quantum states remains challenging. Recently, the use of quantum-logic spectroscopy (QLS), which maps information between a molecular ion and a motionally-coupled atomic ion, has enabled the state preparation/detection and coherent manipulation of diatomic ions [C.-W. Chou et al., Nature 545, 203 (2017)]. While QLS can project a molecular ion into a pure state, it only does so probabilistically. At room temperature, one must contend with the statistical population of hundreds to thousands of molecular states driven by black-body radiation. Here, we demonstrate a protocol to trap the population of a single CaH⁺ ion in its ground state |X¹Σ, v = 0, J = 0〉against the effects of thermal depopulation. Through repeated detection of population in |J = 1〉, we can catch and reverse the population driven out of |J = 0〉, thus effectively extending the lifetime of the |J = 0〉 state. We also show that the improved state control can lead to substantially higher data rates for precision spectroscopy. The generalizability of this protocol makes it a valuable tool for controlling larger molecular species.