The goal of this experiment is to bring polyatomic molecules into the ultracold regime using direct laser cooling. The use of laser radiation to control and cool external and internal degrees of freedom has revolutionized atomic, molecular, and optical physics . The powerful techniques of laser cooling and trapping using light scattering forces for atoms led to breakthroughs in both fundamental and applied sciences, including detailed studies of degenerate quantum gases and many-body physics [2,3], creation of novel frequency standards , and precision measurements of fundamental constants [5,6]. Polyatomic molecules are more difficult to manipulate than atoms and diatomic molecules because they possess additional rotational and vibrational degrees of freedom. Partially because of their increased complexity, cold dense samples of molecules with three or more atoms offer unique capabilities for exploring interdisciplinary frontiers in physics, chemistry and even biology. Precise control over polyatomic molecules could lead to applications in astrophysics , quantum simulation  and computation , fundamental physics [10,11], and chemistry . Study of parity violation in biomolecular chirality —which plays a fundamental role in molecular biology —necessarily requires polyatomic molecules.
Motivated by recent successes in the laser cooling and trapping of diatomic molecules [15-19], as well as recent demonstrations in our group of laser cooling of triatomic molecules [20-22], we are currently working to load a magneto-optical trap (MOT) with the linear, triatomic molecule calcium monohydroxide (CaOH). Our approach starts with buffer gas cooling [23-25], a technique that dramatically reduces the number of populated internal rotational and vibrational states by thermalizing a sample of molecules with helium gas at ~1K. This initial cooling step is critical for working with molecules to limit the number of quantum states that have significant population, and our two-stage buffer gas cell design has additionally allowed us to produce a slow beam of CaOH molecules with a peak velocity below 100 m/s. We are now working to adapt successful atomic and diatomic laser cooling techniques to triatomic CaOH. We will begin by transversally compressing the molecular beam using a two-dimensional MOT ; this collimation stage is critical to reducing losses during subsequent laser slowing, which can be accomplished using a white-light slowing scheme . We will then produce a 3D MOT using techniques similar to those demonstrated with diatomic SrF and CaF [15-17]. Unlike electronic and rotational transitions, there are no selection rules governing vibrational degrees of freedom in molecules, forcing us to rely on molecule-dependent vibrational branching ratios to avoid decay out of the cooling cycle. These have been shown to be favorable in CaOH, meaning that direct laser cooling and slowing should be feasible with only three repumping lasers .
We are additionally examining how to extend laser cooling techniques to more complex polyatomic molecules, such as symmetric tops like CaOCH3, and have identified a number of large polyatomics expected to have near-diagonal Franck-Condon factors .
Our results on “Coherent bichromatic force deflection of molecules” have been published in PRL as an Editor’s Suggestion.Our results on “Sisyphus laser cooling of a polyatomic molecule” have been published in PRL as an Editor’s Suggestion and Featured in Physics. See the accompanying viewpoint article “A diatomic molecules is one atom too few”.
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