Precision spectroscopic measurements of few-electron atomic systems in extreme ultraviolet region

Abstract

Precision spectroscopic measurements on the few-electron atomic systems have attracted much attention because they shed light on important topics such as the “proton radius puzzle” and testing quantum electrodynamics (QED), etc. However, many important transitions of few-electron atomic systems are located in the vacuum/extreme ultraviolet region. Lacking of a suitable narrow linewidth light source is one of the main reasons hindering the further improvement of the spectroscopic resolution.<br>Recently, narrow linewidth extreme ultraviolet (XUV) light sources based on high harmonic processes in rare gases have opened up new opportunities for precision measurements of these transitions. The XUV comb, achieved recently with a shortest wavelength of about 12 nm, a maximum power of milliwatts, and a linewidth of about 0.3 MHz, is an ideal tool for precision measurements in the XUV band. At the same time, the Ramsey comb in the XUV band can achieve a spectral resolution of the order of kHz, and has the potential to operate in the entire XUV band.<br>With these useful tools, direct frequency spectroscopy and Ramsey comb spectroscopy in the XUV region are developed, and precision spectroscopic measurements on few-electron atomic systems with these methods are becoming a hot topic in cutting-edge science. In this paper, we provide an overview of the current status and the progresses achieved in related researches, both experimentally and theoretically, and expand on the discussion of opportunities for other important transitions in the extreme ultraviolet band.