The goal of this experiment is to develop techniques to bring polyatomic molecules into the ultracold regime (T<1mK) using direct 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 diverse degenerate quantum gases [1,2], creation of novel frequency standards , and precision measurements of fundamental constants [4,5]. 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 [9,10], and chemistry . Study of parity violation in biomolecular chirality —which plays a fundamental role in molecular biology —necessarily requires polyatomic molecules.
Our approach starts with buffer gas cooling [14-16], a technique that dramatically reduces the number of populated internal rotational and vibrational states by thermalizing a sample of molecules with He gas at ~1K. This initial cooling step is critical for working with molecules to limit the number of quantum states that have significant population. We are now working to adapt the laser cooling techniques that were so successful with atoms to work on molecular samples. While atomic species have selection rules that limit the number of states populated by spontaneous decay, molecules have selection rules for electronic and rotational degrees of freedom but not vibrational degrees of freedom. Therefore, the major complication with molecules is branching to higher vibrational states outside of the cycling transition.
Motivated by the recent success in laser cooling and magneto-optical trapping for diatomic molecules [16-26] and by insights gained in efforts underway in our own lab we have successfully extended Doppler and sub-Doppler cooling techniques to polyatomic molecules.
We are currently working to extend laser cooling to larger molecules. We have demonstrated that magnetically induced Sisyphus cooling works for SrOH, but we would like to examine how to generalize this technique for larger, more complex species.
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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