Highly charged ions with E1, M1, and E2 transitions within laser range

TL;DR

This paper explores highly charged ions with optical-range transitions near level crossings, proposing their potential for high-precision atomic clocks and tests of fundamental constants due to their unique spectral properties.

Contribution

It provides a comprehensive analysis of level crossings in HCIs, presents scaling laws for energies and amplitudes, and discusses their advantages for atomic clocks and fundamental physics tests.

Findings

Optical E1, M1, and E2 transitions are accessible in HCIs near level crossings.

HCIs can reduce systematic effects like thermal shifts in atomic clocks.

Transitions in HCIs have high sensitivity to variations in the fine-structure constant.

Abstract

Level crossings in the ground state of ions occur when the nuclear charge Z and ion charge Z_ion are varied along an isoelectronic sequence until the two outermost shells are nearly degenerate. We examine all available level crossings in the periodic table for both near neutral ions and highly charged ions (HCIs). Normal E1 transitions in HCIs are in X-ray range, however level crossings allow for optical electromagnetic transitions that could form the reference transition for high accuracy atomic clocks. Optical E1 (due to configuration mixing), M1 and E2 transitions are available in HCIs near level crossings. We present scaling laws for energies and amplitudes that allow us to make simple estimates of systematic effects of relevance to atomic clocks. HCI clocks could have some advantages over existing optical clocks because certain systematic effects are reduced, for example they can have much smaller thermal shifts. Other effects such as fine-structure and hyperfine splitting are much larger in HCIs, which can allow for richer spectra. HCIs are excellent candidates for probing variations in the fine-structure constant, alpha, in atomic systems as there are transitions with the highest sensitivity to alpha-variation.