Prospects for laser cooling of polyatomic molecules with increasing complexity

Abstract

Optical cycling transitions and direct laser cooling have recently been demonstrated for a number of alkalineearth 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 microkelvin 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 ≡ 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 ≡ 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.

Establishing a nearly closed cycling transition in a polyatomic molecule

“…While the measurements presented here are unique to CaOH, the experimental methods and calculations outline a general framework that could be used to predict and confirm VBRs in other molecular candidates for direct laser cooling. The insights gained here for CaOH can be generalized to support recent proposals extending laser cooling to symmetric and asymmetric top molecules [20,24,25] and even molecules with multiple cycling centers [54][55][56][57] .…”

Large molasses-like cooling forces for molecules using polychromatic optical fields: A theoretical description

“…Figure 14 demonstrates energy correlation diagrams for molecular radicals of different structural symmetries. In the presented scheme, the B 2 Σ + electronic state (second excited electronic state arising from the mixing between the (n − 1) dσ and npσ orbitals), which exists in all diatomic and polyatomic molecules that have been considered for optical cycling applications thus far [14,[54][55][56][57], is chosen as the excited state. Angular momentum selection rules dictate that only the included hyperfine and rotational states take part in the cycle, although there are some basic criteria for this scheme to work.…”

Full-dimensional quantum scattering calculations on ultracold atom-molecule collisions in magnetic fields: The role of molecular vibrations

“…Cold twobody collisions and three-body recombination of large molecules such as naphthalene, stilbene, and benzonitrile may be affected by their vibrational DOF [33][34][35][36]. Most recently, the prospects for laser cooling of complex polyatomic molecules with numerous vibrational modes have been explored [37,38]. Understanding the role of vibrational DOF in ultracold collisions of such molecules is a prerequisite to efficient sympathetic cooling.…”