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Towards rotational control of molecules in helium nanodroplets Fordyce, Jordan A. M.


The feasibility of using rotating molecules as “nanoprobes” for testing the superfluidity of helium nanodroplets is explored in this thesis. Helium nanodroplets have an internal temperature of 0.37 K and are below the superfluid transition temperature in bulk helium of 2.17 K. The onset of superfluidity in this microscopic environment will be explored by rotationally exciting molecules using a tool called an optical centrifuge. This tool affords a high degree of precision in the final rotational frequency that the molecule will reach and makes it useful in probing the coupling between the rotor and helium. A unique helium nanodroplet vacuum chamber system was characterized for the range of operation possible, especially with focus on the signal to background detection conditions. Two techniques were explored to characterize the dynamical rotational behaviour of the molecules in these conditions: direct measurement of the molecular orientation and direct measurement of the angular momentum state. The molecular orientation of a molecule is characterized by it’s confinement to the rotational plane using ⟨cos² θ2D⟩ as the metric. A ⟨cos² θ2D⟩ measurement of ≈ 0.7 was successfully recovered from background for carbon disulfide doped helium droplets using an alignment probe pulse, however, with the centrifuge it was ultimately unclear if the molecule was rotating or simply aligning to the plane of rotation. The angular momenta of a molecule was characterized via its ion signal from a Resonance Enhanced Multiphoton Ionization (REMPI) scheme. The feasibility of measuring a transition in oxygen at the low signal to background densities was studied and would be promising to use with oxygen doped helium droplets. In order to continue the research, improvements need to be made to the set up and the two techniques should be used in tandem so that rotation can be better detected and characterized.

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