Prospects for laser cooling of polyatomic molecules with increasing complexity

TL;DR

This paper explores the potential for laser cooling of complex polyatomic molecules by engineering molecules with optical cycling centers, using electronic structure calculations to evaluate their suitability for photon scattering and cooling.

Contribution

It proposes chemical substitution methods to design large polyatomic molecules with optical cycling centers suitable for laser cooling, supported by detailed electronic structure analysis.

Findings

Fullerene-based OCCs outperform oxide dimers in certain cases.

Attaching two OCCs may double photon scattering rates.

Electronic structure calculations validate the feasibility of proposed molecules.

Abstract

Optical cycling transitions and direct laser cooling have recently been demonstrated for a number of alkaline-earth dimers and trimer molecules. This is made possible by diagonal Franck-Condon factors between the vibrational modes of the optical transition. Achieving a similar degree of cooling to micro-Kelvin equivalent kinetic energy for larger polyatomic molecules, however, remains challenging. Since polyatomic molecules are characterized by multiple degrees of freedom and have a correspondingly more complex structure, it is far from obvious whether there exist polyatomic molecules that can repeatedly scatter photons. Here, we propose chemical substitution approaches to engineer large polyatomic molecules with optical cycling centers (OCCs) containing alkaline-earth oxide dimers or acetylenic alkaline-earth trimers (i.e. M--C$\equiv$C) connected to CH$_n$ chains or fullerenes. To validate the OCC-character of the selected molecules we performed electronic structure calculations of the equilibrium configuration of both ground and excited potential energy surfaces, evaluated their vibrational and bending modes, and corresponding Franck-Condon factors for all but the most complex molecules. For fullerenes, we have shown that OCCs based on M--C$\equiv$C can perform better than those based on alkaline-earth oxide dimers. In addition, for heavier polyatomic molecules it might be advantageous to attach two OCCs, thereby potentially doubling the photon scattering rate and thus speeding up cooling rates.