Electric Octupole (E3) Extreme Ultraviolet Photon Emission from Heavy Ag-like Ions*

The lowest order of multipole transitions between the 5s and 4f levels of Ag-like heavy ions is an electric octupole (E3) one. The 5s level in high atomic number Z systems decays via an extreme ultraviolet photon emission with high angular momentum. We have investigated the spatial characteristics of the emission of these photons, particularly considering the consequences of the upper level being the spherically symmetric 5s state. 

Ag-like ions are quasi-one-electron systems with a single electron outside the closed 4d shell in their ground state. In neutral Ag and light Ag-like ions, the ground state electron is in the 5s orbit. As the nuclear charge Z increases along the isoelectronic sequence, level crossing occurs between the 4f and 5s levels at around Z=60, establishing the natural hydrogen-like ordering of the levels in high Z elements. However, the first excited 5s level is sparsely populated as electric dipole allowed transitions from higher excited levels directly proceed to the 4f ground state. There is a narrow range of heavy ions between Z=70-80 where allowed transitions are blocked due to another level crossing, resulting in the increased population of the 5s levels in this region [1,2]. 

We have measured the E3 transitions to the ground state in W and Re, and in this work, we investigate the spatial and angular momentum characteristics of the emitted photons. 

[1] H. Sakaue, D. Kato, I. Murakami et al. Phys. Rev. A 100 (2019) 473

[2] E. Takacs, Dipti, D. La Mantia et al. ATOMS 11 (2023) 478