A cold beam of BaOH molecules using a water-vapour seeded neon gas

TL;DR

This work demonstrates a cold BaOH molecular beam produced by a cryogenic buffer-gas beam source seeded with water vapor in neon. By comparing metal Ba targets to salt targets, the authors show metal targets yield more stable BaOH production, and they achieve an ~11-fold enhancement by resonantly exciting Ba on the $^1\mathrm{S}_0-^3\mathrm{P}_1$ transition. The BaOH beam exhibits forward velocities around 180 m/s and rotational temperatures near 5 K, with fluxes on the order of a billion molecules per pulse in the $N=1$ state, comparable to BaF beams. This establishes BaOH CBGBs as a viable platform for precision tests of fundamental physics and outlines steps toward bending-mode spectroscopy and laser cooling for electron EDM measurements.

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 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 $^1\mathrm S_0 - ^3\mathrm P_1$ 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 $\approx180$ m/s and an intensity of $\sim 10^{9}$ molecules s$^{-1}$ in $N=1$, which is comparable to that of cryogenic BaF beams.

Figures (8)

• Figure 1: Schematic of the buffer gas source and the absorption and fluorescence detection zones. BaOH molecules are produced by laser ablating a rotating barium target in the presence of cold neon buffer gas and water-vapour seeded neon gas. The neon line is cooled by thermal contact to the cryocooled surfaces, the reactant mixture comes from a bubbler filled with water through a heated fill line. Molecular yield is enhanced by sending in laser light through an optical access window orthogonal to the cell.
• Figure 2: (a) Transitions used to characterise the yield of BaOH, denoted by $^{\Delta N}\Delta J(N_\text{ground})$. The $^SR$ transition was generally used for characterising the molecular yield while the $^QQ$ transitions were used for estimating the rotational temperature of the beam. (b) Relevant energy levels in neutral $^{138}$Ba. In green is the pumping scheme associated with driving the transition $^1\mathrm S_0 - ^1\mathrm P_1$ to enhance the production of BaOH, as reported in Davis.Mestdagh.1993. The effect of this pumping scheme is to transfer the $^1\mathrm S$ population into the metastable $^1\mathrm D$ population. In red is the pumping scheme associated with driving the transition $^1\mathrm S_0 - ^3\mathrm P_1$, which is described in the text. Here, the barium population primarily decays to the metastable $^3\mathrm D_2$ state. Also shown are the transitions used to probe the metastable D state populations. The grey dashed lines indicate collisional transfer between the metastable $^3\mathrm D_J$ states Ehrlacher.Huennekens.1994.
• Figure 3: Increased yield of BaOH as a result of the excitation of atomic barium. (a) Time-of-flight curves seen from double pass absorption when absorption laser is locked to the $^SR_{21}(1)$ transition between $\tilde{A}^2\Pi_{3/2} - \tilde{X}^2\Sigma_{1/2}$ in the vibrational ground state, with (in red) and without (in black) enhancement laser close to the $^1\mathrm S_0 - ^3\mathrm P_1$ resonance. (b) The enhancement factor (ratio of the OD integrated over the length of the pulse) as a function of the detuning from the barium $^1\mathrm S_0-^3\mathrm P_1$ transition. The enhancement factor saturates to a value around 10 when we are within 200 MHz of the resonance.
• Figure 4: Absorption time of flight profiles showing the depletion of metastable D states of barium due to the presence of water vapour in the cell. Solid lines indicate time-of-flights without the reactant gas mixture sent in, and dashed lines indicate those with the reactant gas mixture sent in.
• Figure 5: Molecular yield of BaOH as a function of bubbler head pressure.