Multiply charged uranium monoxide as a versatile probe of fundamental physics

TL;DR

This paper demonstrates a method to produce and detect multiply charged uranium monoxide ions, which are promising for studying fundamental physics and symmetry violations due to their simple electronic structures and high relativistic effects.

Contribution

The authors develop a systematic approach to generate and analyze multiply charged actinide molecules, enabling new high-precision tests of fundamental symmetries.

Findings

Successfully produced UO$^{3+}$ and UO$^{4+}$ ions.

Relativistic density functional theory supports experimental observations.

UO$^{3+}$ shows high sensitivity to symmetry-violating effects.

Abstract

Multiply charged actinide molecules provide a unique platform to study fundamental physics and the chemical bond under extreme conditions. Beyond the inherently large relativistic effects associated with a high proton number $Z$, an increased molecular charge can further enhance the electronic sensitivity to symmetry-violating nuclear effects, including nuclear Schiff moments. Experimental investigations of multiply charged actinide molecules are challenging because the high charges severely destabilize chemical bonds, leading to spontaneous Coulomb explosion. We demonstrate a method to systematically generate and detect molecular ions at the edge of chemical stability. By applying high-fluence laser ablation to a depleted uranium metal foil, we produce atomic uranium ions U$^{z+}$ and uranium monoxide cations UO$^{z+}$ with $z = 1$--4. Among them, we observe UO$^{3+}$ and UO$^{4+}$, which exhibit comparatively simple electronic structures and are therefore promising for precision spectroscopy. The experiments are supported by relativistic density functional theory calculations of equilibrium bond lengths, charge distributions, and binding energies of all observed molecules. Calculations of symmetry-violating properties suggest a pronounced sensitivity of UO$^{3+}$ to hadronic $CP$ violation. This approach opens a pathway for high-precision investigations of fundamental symmetries and the exploration of relativistic actinide chemistry in previously inaccessible regimes.