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arXiv:1910.11331 · New Journal of Physics

Enhanced molecular yield from a cryogenic buffer gas beam source via excited state chemistry

Arian Jadbabaie, Nickolas Pilgram, Jacek Kłos, Svetlana Kotochigova, Nicholas R. Hutzler

Published Jan 22, 2020• 58 citations
arXivHTMLPDF

Abstract

Abstract We use narrow-band laser excitation of Yb atoms to substantially enhance the brightness of a cold beam of YbOH, a polyatomic molecule with high sensitivity to physics beyond the standard model (BSM). By exciting atomic Yb to the metastable 3P1 state in a cryogenic environment, we significantly increase the chemical reaction cross-section for collisions of Yb with reactants. We characterize the dependence of the enhancement on the properties of the laser light, and study the final state distribution of the YbOH products. The resulting bright, cold YbOH beam can be used to increase the statistical sensitivity in searches for new physics utilizing YbOH, such as electron electric dipole moment and nuclear magnetic quadrupole moment experiments. We also perform new quantum chemical calculations that confirm the enhanced reactivity observed in our experiment and compare reaction pathways of Yb(3P) with the reactants H2O and H2O2. More generally, our work presents a broad approach for improving experiments that use cryogenic molecular beams for laser cooling and precision measurement searches of BSM physics.

DOI
10.1088/1367-2630/ab6eae

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Abstract We use narrow-band laser excitation of Yb atoms to substantially enhance the brightness of a cold beam of YbOH, a polyatomic molecule with high sensitivity to physics beyond the standard model (BSM). By exciting atomic Yb to the metastable 3P1 state in a cryogenic environment, we significantly increase the chemical reaction cross-section for collisions of Yb with reactants. We characterize the dependence of the enhancement on the properties of the laser light, and study the final state distribution of the YbOH products. The resulting bright, cold YbOH beam can be used to increase the statistical sensitivity in searches for new physics utilizing YbOH, such as electron electric dipole moment and nuclear magnetic quadrupole moment experiments. We also perform new quantum chemical calculations that confirm the enhanced reactivity observed in our experiment and compare reaction pathways of Yb(3P) with the reactants H2O and H2O2. More generally, our work presents a broad approach for improving experiments that use cryogenic molecular beams for laser cooling and precision measurement searches of BSM physics.
Cryogenic buffer gas sources are ubiquitous for producing cold, collimated molecular beams for quantum science, chemistry, and precision measurements. The molecules are typically produced by laser ablating a metal target in the presence of a donor gas. The radical of interest emerges due to a barrier-free reaction or under thermal or optical excitation. High-barrier reactions, such as between Ca and H 2, should be precluded. We study chemical reactions between Ca and three hydrogen isotopologues (H 2, D 2, and HD) in a cryogenic cell with helium buffer gas. We observe that H 2 can serve as both a reactant and a buffer gas, outperforming D 2 and HD. We use a reaction network model to describe the chemical dynamics and find that the enhanced molecular yield can be attributed to rapid vibrational excitations of the reactant gas. Our results demonstrate a robust method for generating bright cold beams of alkaline-earth-metal hydrides for laser cooling and trapping.
Abstract Doppler and Sisyphus cooling of 174YbOH are achieved and studied. This polyatomic molecule has high sensitivity to physics beyond the Standard Model and represents a new class of species for future high-precision probes of new T-violating physics. The transverse temperature of the YbOH beam is reduced by nearly two orders of magnitude to < 600 μK and the phase-space density is increased by a factor of > 6 via Sisyphus cooling. We develop a full numerical model of the laser cooling of YbOH and find excellent agreement with the data. We project that laser cooling and magneto-optical trapping of long-lived samples of YbOH molecules are within reach and these will allow a high sensitivity probe of the electric dipole moment of the electron. The approach demonstrated here is easily generalized to other isotopologues of YbOH that have enhanced sensitivity to other symmetry-violating electromagnetic moments.
Abstract In this paper we report on the production and characterization of a cold beam of BaOH molecules using a cryogenic buffer-gas beam source. BaOH is a highly suitable molecule for studies of the violation of fundamental symmetries, such as the search for the electron’s electric dipole moment. BaOH molecules are synthesized inside the cold source through laser ablation of a barium metal target while water vapor is seeded into the neon buffer gas. The BaOH flux is significantly enhanced (∼11 times) when laser-exciting the barium atoms inside the buffer-gas cell on the 1 S 0 – 3 P 1 transition. A similar enhancement has been reported for other alkaline-earth(-like) monohydroxides. For typical source conditions, the molecular beam has an average velocity of ≈ 180 m s −1 and an intensity of ∼ 10 9 molecules s −1 in N = 1, which is comparable to that of cryogenic BaF beams.
Cryogenic buffer gas beams are central to many cold molecule experiments. Here, we use absorption and fluorescence spectroscopy to directly compare molecular beams of AlF, CaF, MgF and YbF molecules, produced by chemical reaction of laser ablated atoms with fluorine rich reagents. The beam brightness for AlF is measured as 2×1012 molecules per steradian per pulse in a single rotational state, comparable to an Al atomic beam produced in the same setup. The CaF, MgF and YbF beams show an order of magnitude lower brightness than AlF and far below the brightness of Ca and Yb beams. The addition of either NF3 or SF6 to the cell extinguishes the Al atomic beam, but has a minimal effect on the Ca and Yb beams. NF3 reacts more efficiently than SF6, as a significantly lower flow rate is required to maximise the molecule production, which is particularly beneficial for long-term stability of the AlF beam. We use NO as a proxy for the reactant gas as it can be optically detected. We demonstrate that a cold, rotationally pure NO beam can be generated by laser desorption, thereby gaining insight into the dynamics of the reactant gas inside the buffer gas cell.
In this paper we report on the production and characterization of a cold beam of BaOH molecules using a cryogenic buffer-gas beam source. BaOH is a highly suitable molecule for studies of the violation of fundamental symmetries, such as the search for the electron's electric dipole moment. BaOH molecules are synthesised inside the cold source through laser ablation of a barium metal target while water vapor is seeded into the neon buffer gas. The BaOH flux is significantly enhanced ($\sim$11 times) when laser-exciting the barium atoms inside the buffer-gas cell on the 1S0−3P1^1\mathrm S_0 - ^3\mathrm P_11S0​−3P1​ transition. A similar enhancement has been reported for other alkaline-earth(-like) monohydroxides. For typical source conditions, the molecular beam has an average velocity of ≈180\approx180≈180 m/s and an intensity of ∼109\sim 10^{9}∼109 molecules s−1^{-1}−1 in N=1N=1N=1, which is comparable to that of cryogenic BaF beams.
Bright beams of cold molecules provide valuable quantum sensors for measuring permanent electric dipole moments that violate fundamental symmetries, which enables these tabletop experiments to probe energy scales exceeding those available with current or near-future particle colliders. The precision of such experiments generally scales with the number of molecules that are detected per unit time, typically a few meters downstream of the place where the molecular beam is created. So, improving the precision -and thereby probing new physics at higher and higher energy scales- requires increasing the downstream flux of molecules. This can be achieved by collimating the molecular beam to maintain a high number density. These molecular experiments can borrow techniques for control and manipulation from the extremely well-developed field of atomic physics. One such technique is laser cooling, which is a powerful method to achieve this boost in brightness, whose implementation however remains challenging due to the complex energy level structure of molecules. In this work, we demonstrate and analyze laser cooling of a focused cryogenic beam of BaF molecules. We conclude that with some follow-up steps this method will increase the beam brightness by a factor 100, a crucial milestone towards a high precision measurement of the electron electric dipole moment (eEDM).