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MacPhail-Bartley, Ian David

University of British Columbia

This work outlines the control of molecular rotation in superfluid helium using nonresonant laser fields. Experiments within bulk superfluid helium-4 demonstrate control over the rotational frequency and direction of rotation of electronically excited helium dimers (excimers), which are created in nanometre-scale bubbles in the fluid. The excimers rotate for thousands of rotational periods, indicating relatively weak but nonzero coupling to the surrounding helium. Controlling the rotation of molecules therefore serves as a probe of superfluid helium, and its coupling to impurities. The weak coupling is attributed to the fact that helium dimers rotate with rotational energy well above that of the expected excitations of the surrounding helium. By studying other molecules embedded in helium nanodroplets, we are able to explore the rotation of molecules below, near, and above this energy scale. The influence of strong coupling to the helium becomes extreme when the energies are comparable, severely distorting observed rotational spectra. Results presented here demonstrate that the rotation of molecules in helium nanodroplets may be controlled in the same manner as molecules in the gas phase. Experiments using an optical centrifuge to attempt to control molecular rotation in helium nanodroplets, and analysis regarding the results, are presented, as well as the first experiments studying rotationally excited nitrogen in helium nanodroplets. Experimental results on rotationally exciting nitric oxide dimers in helium nanodroplets show that (NO)₂ may be a suitable candidate as a molecule for further study. Alignment of the molecule offers insights to its anisotropic polarizability, and upon excitation of long-lasting rotation, exhibits exhibits a stronger observable than previously-studied molecules whose rotational energy may be controlled within the desired range.

Text

2024-06-17

Attribution-NonCommercial-ShareAlike 4.0 International

http://hdl.handle.net/2429/88470

Physics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-sa/4.0/