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

Understand

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$^{9+}$.