Few-electron highly charged muonic Ar atoms verified by electronic $K$ x rays

TL;DR

This study observed and identified muonic argon atoms with few electrons through high-precision electronic K x-ray spectra, revealing new states and charge transfer processes in muonic atoms.

Contribution

It provides the first clear experimental evidence of muonic argon atoms with few electrons, using advanced microcalorimeter spectroscopy and theoretical analysis.

Findings

Detection of one-, two-, and three-electron muonic Ar atoms.

Identification of hyper-satellite K x rays associated with charge transfer.

Consistency of observed x-ray energies with theoretical predictions for highly charged ions.

Abstract

Electronic $K$ x rays emitted by muonic Ar atoms in the gas phase were observed using a superconducting transition-edge-sensor microcalorimeter. The high-precision energy spectra provided a clear signature of the presence of muonic atoms accompanied by a few electrons, which have never been observed before. One-, two-, and three-electron bound, i.e., H-like, He-like, and Li-like, muonic Ar atoms were identified from electronic $K$ x rays and hyper-satellite $K$ x rays. These $K$ x rays are emitted after the charge transfer process by the collisions with surrounding Ar atoms. With the aid of theoretical calculations, we confirmed that the peak positions are consistent with the x-ray energies from highly charged Cl ions, and the intensities reflecting deexcitation dynamics were successfully understood by taking into account the interaction between the muon and bound electrons.