Laser-induced forced evaporative cooling of molecular anions below 4 Kelvin

TL;DR

This paper demonstrates a laser-based forced evaporative cooling method that significantly lowers the temperature of molecular anions, reaching below 4 Kelvin, surpassing traditional buffer gas cooling limits.

Contribution

It introduces a novel laser-induced evaporative cooling technique for molecular anions, enabling cooling below 4 Kelvin and increasing phase space density substantially.

Findings

Achieved cooling of OH− ions from 370 K to 2.2 K

Reached near-strong Coulomb coupling regime

Provided a full thermodynamic model of the cooling process

Abstract

The study of cold and controlled molecular ions is pivotal for fundamental research in modern physics and chemistry. Investigations into cooling molecular anions, in particular, have proven to be of key consequence for the production of cold antihydrogen, the creation, and study of anionic Coulomb crystals as well as in atmospheric research and astrochemistry. The commonly used anion cooling technique via collisions with a buffer gas is limited by the temperature of the used cryogenic cooling medium. Here, we demonstrate forced evaporative cooling of anions via a laser beam with photon energies far above the photodetachment threshold of the anion. We reach runaway evaporative cooling of an anionic OH$^{-}$ ensemble from an initial temperature of 370(12) K down to 2.2(8) K. This corresponds to three orders of magnitude increase in the ions' phase space density approaching the near-strong Coulomb coupling regime. A quantitative analysis of the experimental results, via a full thermodynamic model including the role of intrinsic collisional heating, represents the anion cooling dynamics without any fitting parameters. This technique can be used to cool, in principle, any anionic species below liquid helium temperature, providing a novel tool to push the frontiers of anion cooling to much lower than the state-of-the-art temperature regimes.