Multiply charged uranium monoxide as a versatile probe of fundamental physics

TL;DR

This work introduces a practical approach to generate multiply charged actinide oxide ions by high-fluence laser ablation of uranium foils and confirms their identities with high-precision MR-TOF measurements. Relativistic density functional theory then characterizes the electronic structure, stability, and bonding of the observed ions, revealing $UO^{3+}$ as a particularly promising candidate for precision tests of $P$ and $T$-violation, and $UO^{4+}$ as a metastable extreme near Coulomb instability. The combination of experimental accessibility and strong theoretical underpinning positions $UO^{3+}$ as a powerful platform for exploring hadronic CP violation and related nuclear moments in an octupole-deformed uranium context, with potential extension to other actinides and isotopes. Overall, the study broadens the frontier of relativistic actinide chemistry and provides a versatile toolkit for fundamental-physics investigations using multiply charged molecular ions.

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.

Figures (6)

• Figure 1: Image of the ablated uranium foils, each 0.1 mm in thickness, on the Greifswald sample holder. Top: depleted uranium (478 mg; 7.17 kBq); middle: depleted uranium pretreated with hydrogen peroxide (478 mg; 7.17 kBq); bottom: natural uranium (717 mg; 17.90 kBq).
• Figure 2: Comparison of low-mass contaminants from laser ablation of depleted, H2O2-treated, and natural uranium foils.
• Figure 3: Mass spectra of uranium-based species. The upper spectrum in the mass range of compounds containing one uranium atom is recorded at 10 revolutions in the MR-TOF analyzer with 1.0-mJ ablation pulse energy. The lower spectrum for compounds with two uranium atoms is recorded at 22 revolutions at the same pulse energy.
• Figure 4: Spectra of atomic and monoxide uranium ions in charge states 2+ (green), 3+ (red) and 4+ (orange). Laser fluence was varied in the range from 340 to 470 J$\cdot$cm$^{-2}$ and five recorded spectra were summed. The laser fluences for given species (U^z+ and UO_0-1^z+ with $z = 2,3, 4$) are listed in Table \ref{['tab:Charge']}.
• Figure 5: Relevant excerpts of the potential energy curves of UO^3+ and UO^4+ for estimating dissociation energies at the level of 2c-ZORA-cGKS-PBE0/d-aug-dyall.cv3z. Dissociation energies computed from separate atoms are shown as gray horizontal lines. The crossing of the triplet and singlet curves of UO^4+ happens at about 1.9 Å. Lines are shown to guide the eye. Because of an admixture of quartet states the potential curve of the ${}^2\Phi_{5/2}$ in UO^3+ lies between 2.2 Å and 2.6 Å above the dissociation limit.