Observation of Electric Octupole Emission Lines Strongly Enhanced by an Anomalous Behavior of Cascading Contribution

TL;DR

This paper reports the first observation of spontaneous electric octupole emission lines in Ag-like tungsten ions, revealing a strong enhancement due to an anomalous cascading effect at atomic number 74.

Contribution

It provides the first experimental detection of electric octupole emission lines and explains their enhancement through a novel cascading contribution mechanism.

Findings

First observation of electric octupole emission lines in Ag-like W$^{27+}$.

Emission intensity is strongly enhanced at atomic number 74.

Cascading contribution behavior causes the enhancement.

Abstract

We present extreme ultraviolet spectra of Ag-like W$^{27+}$ observed with an electron beam ion trap. In the spectra, the $4f_{7/2, 5/2}$ -- $5s$ transitions are identified as the first observation of spontaneous electric octupole emission. Our theoretical investigation shows that the emission line intensity is strongly and specifically enhanced at the atomic number 74 by an anomalous behavior of cascading contribution to $5s$ via $5p\leftarrow5d$.