Detection of the $5p-4f$ orbital crossing and its optical clock transition in Pr$^{9+}$

TL;DR

This paper reports the first experimental observation of an orbital crossing in highly charged ions, specifically in Pr$^{9+}$, and identifies a narrow optical clock transition with high sensitivity to new physics.

Contribution

The study experimentally observes the $5p-4f$ orbital crossing in Pr$^{9+}$ and characterizes a highly sensitive optical clock transition suitable for precision physics tests.

Findings

First observation of the orbital crossing in Pr$^{9+}$

Identification of a narrow clock transition at 452.334 nm

High sensitivity of the clock transition to new physics

Abstract

Recent theoretical works have proposed atomic clocks based on narrow optical transitions in highly charged ions. The most interesting candidates for searches of new physics are those which occur at rare orbital crossings where the shell structure of the periodic table is reordered. There are only three such crossings expected to be accessible in highly charged ions, and hitherto none have been observed as both experiment and theory have proven difficult. In this work we observe an orbital crossing in highly charged ions for the first time, in a system chosen to be tractable from both sides: Pr$^{9+}$. We present electron beam ion trap measurements of its spectra, including the inter-configuration lines that reveal the sought-after crossing. The proposed nHz-wide clock line, found to be at 452.334(1) nm, proceeds through hyperfine admixture of its upper state with an E2-decaying level. With state-of-the-art calculations we show that it has a very high sensitivity to new physics and extremely low sensitivity to external perturbations, making it a unique candidate for proposed precision studies.