Prospect for attosecond laser spectra of highly charged ions
Authors: Zhang Da-Cheng , Ge Han-Xing , Ba Yu-Lu , Wen Wei-Qiang , Zhang Yi , Chen Dong-Yang , Wang Han-Bing , Ma Xin-Wen ,   School of Optoelectronic Engineering, Xidian University, Xi’an 710071, China,   Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Published: August 2023

The spectra of highly charged ions (HCIs) are of great significance for astronomical observation, astrophysical model establishment, and test of quantum electrodynamics (QED) theory. However, the transitions of HCI are mostly in the extreme ultraviolet or even X-ray range, the excitation spectra of HCI measured by laser spectroscopy in laboratory are very limited due to lack of the suitable light source. Up to now, only few experiments on the spectra of HCIs performed on synchrotron radiation, free electron laser or heavy-ions storage ring have been reported, which are summarized in this work. With the development of attosecond technology, several attosecond light source facilities have been built, such as extreme light infrastructure attosecond light pulse source (ELI-ALPS) and synergetic extreme condition user facility (SECUF), which have high photon energy and ultra-short pulse duration in the extreme ultraviolet and even soft X-ray range, providing new opportunities for laboratory research on HCI spectra and ultra short energy level lifetimes. Electron beam ion trap (EBIT), electron cyclotron resonance (ECR), and heavy-ion storage ring are usually used to generate ion target. But it is difficult to combine the attosecond laser source with large scale facility of HCI, for none of laboratories has both these two facilities now. Thus, two possible experimental schemes for attosecond spectrum of HCIs are proposed in this work. One scheme is that an EBIT can be designed as a terminal of attosecond laser facility, such as ELI-ALPS and SECUF, which can output different laser beams with high photon energy, ultra-short pulse duration or high flux. Another scheme is that a table-top HHG system pumped by an all-solid-state femtosecond laser or fiber femtosecond laser with high power can be combined with heavy-ion storage ring, such as ESR, CSRe, HIAF, and FAIR. Owing to high energy of ions in storage ring, the measurable energy levels of HCIs can even be extended to keV by the Doppler shift. Three different measurement methods: fluorescence detection, ion detection and attosecond absorption spectroscopy, can be used to obtain the HCI spectrum. Finally, a preliminary experimental setup for attosecond laser spectrum of HCI is proposed. The proposal on combining extreme ultraviolet attosecond light source with HCI target is discussed, and the feasibility of attosecond time-resolved precision spectrum for HCI is analyzed according to the typical parameters of attosecond light source and the known excitation cross-section and detection efficiency, which can provide a new platform for implementing ion level structure calculation, QED theory high-precision test and astronomical spectroscopic observation. It can be used to measure the ultra-short lifetime, low excitation cross-section ionic energy level, and even some transitions with large energy interval. We hope that this work can provide a reference for the experimental measuring of HCI spectrum and ion energy level lifetime in future.