Experimental access to observing decay from extremely long-lived metastable electronic states via Penning trap spectrometry

Long-lived ionic quantum states known as metastable electronic states in highly charged ions (HCIs) are of great interest in fundamental physics. Especially, it generates transitions with very narrow natural linewidth, which is a promising candidate for use in the next-generation HCI atomic clocks to reach an accuracy below 10-19. A recent experiment [Nature (London) 581, 42 (2020)] used Penning trap mass spectrometry to measure the energy of an extremely long-lived metastable electronic state, thus opening doors to search for HCI clock transitions. Building upon prior research, this study introduces an experimental proposal with the goal of measuring lifetimes of the metastable states beyond seconds. Our approach employs a sequential pulse-and phase measurement scheme, allowing for direct observations of the decay processes from metastable electronic states through single-ion mass spectrometry in a Penning trap. This measurement poses a significant challenge to conventional techniques like fluorescence detection. To demonstrate the effectiveness of this method, we conducted a comprehensive simulation under real experimental conditions, yielding promising results in a specific scenario. Two suitable candidates are proposed for testing this method, and the state-of-the-art MCDHF theory are employed for accurate energy levels and transition rate calculations. Some future prospects in the experimental determinations of a wide range of energy and lifetimes of long-lived metastable electronic states, probing hyperfine and magnetic quenching effects on high-order forbidden transitions and search for high-quality HCI clock transitions are discussed.