Laser cooling Rydberg molecules -- a detailed study of the helium dimer

TL;DR

This study explores the feasibility of laser cooling helium dimer molecules in a metastable state, analyzing their energy levels, transition properties, and potential for ultracold experiments and precision measurements.

Contribution

It provides a detailed theoretical analysis of laser cooling transitions, energy structures, and trapping schemes for helium dimer molecules, advancing the prospects for ultracold molecular physics.

Findings

Identified three laser cooling transitions for He₂* molecules.

Simulated a laser slowing scheme for magneto-optical trapping.

Found that loss mechanisms do not significantly hinder laser cooling.

Abstract

The helium dimer in its metastable triplet state is a promising candidate to be the first laser-cooled homonuclear molecule. An ultracold gas of He$_2^*$ would enable a new generation of precision measurements to test quantum electrodynamics for three- and four-electron molecules through Rydberg spectroscopy. Nearly diagonal Franck-Condon factors are obtained because the electron employed for optical cycling occupies a Rydberg orbital that does not take part in the chemical bond. Three possible laser cooling transitions are identified and the spin-rovibronic energy-level structure of the relevant states as well as electronic transition moments, linestrengths, and lifetimes are determined. The production of He$_2^*$ molecules in a supersonic beam is discussed, and a laser slowing scheme to load a magneto-optical trap under such conditions is simulated using a rate equation approach. Various repumping schemes involving one or two upper electronic states are compared to maximize the radiative force. Loss mechanisms such as spin-forbidden transitions, predissociation, and ionization processes are studied and found to not introduce significant challenges for laser cooling and trapping He$_2^*$. The sensitivity of the vibrational levels of He$_2^+$ with respect to the static polarizability of atomic helium is determined and its implications for a new quantum pressure standard are discussed.