[Paper Review] Cold CH radicals for laser cooling and trapping

Joseph Schnaubelt, Jamie Shaw+1 more|arXiv (Cornell University)|Sep 8, 2021

Cold Atom Physics and Bose-Einstein Condensates54 references4 citations

TL;DR

This paper proposes laser cooling and trapping of ultracold CH radicals by calculating vibrational and rotational branching ratios with ground state mixing, enabling optical cycling via the $X^{2}\Pi\leftarrow A^{2}\Delta$ and $X^{2}\Pi\leftarrow B^{2}\Sigma^{-}$ transitions. As a key step, the authors demonstrate effective buffer gas cooling, producing $5\times10^{10}$ CH molecules per pulse at 7(2) K translational and 2(1) K rotational temperatures, with a measured CH–He collisional cross section of $2.4(8)\times10^{-14}$ cm$^2$, establishing a foundation for future laser cooling experiments.

ABSTRACT

Ultracold CH radicals promise a fruitful testbed for probing quantum-state controllable organic chemistry. In this work, we calculate CH vibrational branching ratios (VBRs) and rotational branching ratios (RBRs) with ground state mixing. We subsequently use these values to inform optical cycling proposals and consider two possible radiative cooling schemes using the $X^{2}Π\leftarrow A^{2}Δ$ and $X^{2}Π\leftarrow B^{2}Σ^{-}$ transitions. As a first step towards laser cooled CH, we characterize the effective buffer gas cooling of this species and produce $\sim5 imes10^{10}$ CH molecules per pulse with a rotational temperature of 2(1) K and a translational temperature of 7(2) K. We also determine the CH-helium collisional cross section to be $2.4(8) imes10^{-14}$ cm$^{2}$. This value is crucial to correctly account for collisional broadening and accurately extract the in-cell CH density. These cold CH molecules mark an ideal starting point for future laser cooling and trapping experiments and tests of cold organic chemistry.

Motivation & Objective

• To enable laser cooling and trapping of CH radicals, a chemically relevant but experimentally challenging diatomic molecule with a complex rovibrational structure.
• To calculate vibrational and rotational branching ratios (VBRs and RBRs) including ground state mixing to assess feasibility of optical cycling.
• To demonstrate effective buffer gas cooling of CH radicals as a critical first step toward laser cooling.
• To measure the CH–helium collisional cross section to accurately account for collisional broadening and extract in-cell molecular density.
• To establish a cold CH source with well-characterized temperature and density for future experiments in ultracold organic chemistry and precision measurements.

Proposed method

• Calculated vibrational branching ratios (VBRs) and rotational branching ratios (RBRs) for the $X^{2}\Pi\leftarrow A^{2}\Delta$ and $X^{2}\Pi\leftarrow B^{2}\Sigma^{-}$ electronic transitions, incorporating ground state mixing effects.
• Proposed two optical cycling schemes based on the $X^{2}\Pi\leftarrow A^{2}\Delta$ and $X^{2}\Pi\leftarrow B^{2}\Sigma^{-}$ transitions, suitable for 3D magneto-optical trapping (MOT) with complexity comparable to existing experiments.
• Used buffer gas cooling in a cryogenic cell at 2.35(1) K to produce a cold CH radical beam, with density measured via absorption spectroscopy.
• Measured the rotational temperature by probing the $\Lambda$-doublet splitting between $J^{\prime\prime}=1/2^+$ and $J^{\prime\prime}=3/2^+$ states in the $v^{\prime\prime}=0, N^{\prime\prime}=1$ level.
• Determined the CH–helium collisional cross section as $2.4(8)\times10^{-14}$ cm$^2$ to correct for collisional broadening in in-cell density measurements.
• Validated rotational temperature measurements at elevated cell temperature (50(5) K), observing a 25% population in higher $J^{\prime\prime}$ states, consistent with a 42(5) K rotational temperature.

Experimental results

Research questions

• RQ1 Can the $X^{2}\Pi\leftarrow A^{2}\Delta$ and $X^{2}\Pi\leftarrow B^{2}\Sigma^{-}$ transitions support efficient optical cycling for laser cooling of CH radicals despite high rotational branching?
• RQ2 What is the extent of ground state mixing in CH's rovibrational structure, and how does it affect the feasibility of radiative cooling?
• RQ3 To what extent can buffer gas cooling produce cold, dense CH radicals suitable for subsequent laser cooling and trapping?
• RQ4 What is the CH–helium collisional cross section, and how does it impact collisional broadening and in-cell density calibration?
• RQ5 Can the rotational and translational temperatures of cold CH radicals be accurately measured and controlled in a cryogenic buffer gas source?

Key findings

• The authors achieved effective buffer gas cooling of CH radicals, producing a peak molecular density of $3.5\times10^9$ cm$^{-3}$, corresponding to $5\times10^{10}$ molecules per pulse.
• The translational temperature of the cold CH beam was measured to be 7(2) K, and the rotational temperature was 2(1) K, in excellent agreement with the cell temperature of 2.35(1) K.
• The CH–helium collisional cross section was determined to be $2.4(8)\times10^{-14}$ cm$^2$, a critical parameter for modeling collisional broadening and accurate in-cell density extraction.
• Rotational temperature measurements confirmed the low-temperature population of the $J^{\prime\prime}=1/2^+$ state, with a 25% population in the $J^{\prime\prime}=3/2^+$ state at 50(5) K, yielding a rotational temperature of 42(5) K.
• The calculated vibrational and rotational branching ratios support the feasibility of optical cycling using the $X^{2}\Pi\leftarrow A^{2}\Delta$ and $X^{2}\Pi\leftarrow B^{2}\Sigma^{-}$ transitions, enabling future laser cooling and trapping.
• The measured in-cell lifetime of ~1 ms suggests potential for probing cold organic chemistry, with implications for ion spectrometry and precision measurements in ultracold molecular systems.