Chemistry in a Cryogenic Buffer Gas Cell

Abstract

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.

Figures (4)

• Figure 1: Horizontal cross section of the cryogenic buffer gas cell. Hot Ca atoms are generated via laser ablation of a solid target and subsequently react with H$_2$ molecules that flow in at $\sim$$150$ K. Additionally, $^4$He flows in at $\sim$$6$ K for efficient buffer gas cooling. The Ca atoms and the product molecules are probed through laser absorption, with optical access $\sim$$2$ cm downstream from the target.
• Figure 2: Chemical reaction in a cryogenic buffer gas cell. Time-averaged CaH densities measured within 0--1 ms after ablation, plotted against calcium density for $10$ SCCM of H$_2$ (red circles) and $20$ SCCM of H$_2$ (blue squares), both under $2.2$ SCCM of He. The lines denote theoretical results at the same conditions at a collision temperature of $2400$ K, showing qualitative agreement. The inset shows the reaction efficiency, defined as the ratio of CaH to calcium densities. A notable decrease in reaction efficiency is observed experimentally at higher calcium densities, whereas the theoretical model does not capture this trend, showing the need for further refinement. Error bars on the experimental data correspond to 1-$\sigma$ statistical uncertainties.
• Figure 3: Time-averaged densities (0--1 ms after ablation) of Ca and CaH (or CaD) under different gas flow configurations. Reactant gases H$_2$, HD, or D$_2$ are supplied at $20$ SCCM, with He (if used) added at $8.8$ SCCM. Ablation energy is $19$ mJ per pulse. Green, yellow, and red bars represent results for H$_2$, HD, and D$_2$, respectively, while black-framed bars show the yields with He added. Dashed lines indicate the detection limits, defined as the lowest density detectable with $\text{signal-to-noise ratio (SNR)}>2$, below which measurements are statistically consistent with zero. Relative errors are larger when signals are close to the detection limit. Here the measured densities of CaH and CaD are those in the $X^2\Sigma^+(\nu=0,\ N=1,\ J=1/2,\ -)$ states.
• Figure 4: (a) A Ca density trace as a function of time after the ablation pulse (average of 30 shots). The blue curve is the original data, the orange line highlights the data used for fitting, and the green dashed line indicates the fitted single exponential decay result. This data is obtained with $8.8$ SCCM He flow. (b) Measured collisional cross sections of He, H$_2$, HD, and D$_2$ with Ca and CaH.