Fine and hyperfine splitting of the low-lying states of $^9$Be

TL;DR

This paper presents highly accurate calculations of energy levels and hyperfine structures of low-lying states in $^9$Be, revealing strong hyperfine mixing and proposing a generalized description applicable to various atomic systems.

Contribution

It introduces a novel approach to describe fine and hyperfine structures in atoms with arbitrary coupling strength, overcoming limitations of standard hyperfine theory.

Findings

Accurate energy level calculations for $^9$Be states.

Identification of strong hyperfine mixing effects.

Development of a generalized hyperfine structure description.

Abstract

We perform accurate calculations of energy levels as well as fine and hyperfine splittings of the lowest $^{1,3}\!P_J$, $^{3}\!S_1$, $^{3}\!P^e_J$, and $^{1,3}\!D_J$ excited states of the $^9$Be atom using explicitly correlated Gaussian functions and report on the breakdown of the standard hyperfine structure theory. Because of the strong hyperfine mixing, which prevents the use of common hyperfine constants, we formulate a description of the fine and hyperfine structure that is valid for an arbitrary coupling strength and may have wide applications in many other atomic systems.