¹²⁹Xe^q+(q=17, 20, 23, 25, 27) highly charged ions with a kinetic energy of 1360 keV are incident on the surface of metal Al and Ti solid targets respectively. The near-infrared spectral lines (800-1700 nm) of excited Xe atoms and low ionized Xe ions, and the spectral lines of excited target atoms and excited by ionization are measured during the interaction between the highly charged ions and the surface to achieve surface electron neutralization. The experimental results show that during the process of high charged ion incident on the metal surface, the potential energy carried by the ion instantly (in the femtosecond range) deposits on the target surface, ionizing and exciting the target atoms. Due to the strong Coulomb potential energy, the target atoms can form a highly ionized state and complex electronic configuration to de-excite the emission spectrum. As the charge state of the incident ion increases, the measured spectral line intensity rises, and the increasing trend is generally consistent with the growing trend of the potential energy of the incident ion, which indicates that the classical over-the-barrier model is valid in the near Bohr velocity energy region. We also hope that our experimental data can provide basic support for related research and provide new methods for spectral measurement.

The experiment is performed at the Heavy Ion Research Facility in Lanzhou (HIRFL) and the experimental platform is shown in Fig. 1. Gaseous ¹²⁹Xe atoms repeatedly collide with electrons in an 18 GHz microwave field in the ECR, gradually peeling off to form highly charged ¹²⁹Xe^q+ ions. They are introduced at the required voltage for the experiment, and the required projectile ions are selected by analytical magnets based on the charge-to-mass ratio. The beam spot is controlled to be less than 5 mm using a beam splitter, quadrupole lens, and aperture, and the beam intensity is recorded via a Faraday tube. The beam enters a metal ultra-high vacuum chamber with magnetic shielding (vacuum degree maintained at 10^-8 Pa). The chemical purity of sample Al or Ti is 99.99%, and the surface has been purified with a target area of 15 mm×15 mm and thickness of 0.1 mm. The infrared optical window and monochromator incident slit are perpendicular to the beam direction and form a 45° angle with the target surface. The experiment employs an infrared spectrometer SP-2357 produced by ARC (Action Reserve Corporation) in the United States, with a grating density of 600 g/mm and a flashing wavelength of 1.6 μm. The InGaA-C detector is selected with an effective range of 800-1700 nm and an integration time of 3000 ms. To improve the signal-to-noise ratio and measurement accuracy, we adopt a phase-locked amplifier (SR830) and a chopper (SR540). Additionally, we have to operate in the darkroom or the dark cover screening to eliminate or reduce the background of spectral measurement.

The near-infrared spectral lines (800-1700 nm) emitted from the interaction between high charge state ¹²⁹Xe^q+ (q=17-27) and metal solid targets are measured (Table 1). These spectral lines can be adopted in the research on the damage of space-charged particles to aerospace devices, high-precision optical clocks, and in the infrared background radiation of the universe in laboratory astrophysics. Xe ion emission near-infrared is an important basis for manipulating Hall thrusters, with space stations performing attitude calibration and other actions in space.

Under the action of highly charged ions, it is possible to ionize and excite transitions between complex configurations of target atoms, and electric dipole forbidden transitions (magnetic dipole and electric quadrupole transitions). Additionally, we measure spectral lines of 842.42 nm and 1525.03 nm for helium like (Al XII) Al ions (i.e. Al¹¹⁺) radiation, and 1251.08 nm for lithium like (Al XI) Al (Al¹⁰⁺) ions, which belong to electric dipole transition radiation. The 989.01 nm spectral line for Ti XVIII (i.e. Ti¹⁷⁺) de-excitation radiation belongs to magnetic dipole transition radiation. To our knowledge, these spectral lines are predicted by theoretical predictions from 1987 and 2013, and there have been no reports of experimental data so far.

The classical over-the-barrier model for the interaction between highly charged ions and metal solid targets in the Bohr velocity energy region has been validated. The trend of single particle fluorescence yield increasing with the potential energy of the incident ion is measured, which is roughly the same as the charge state trend of the incident ion rising with the potential energy (Fig. 3). The classical over-the-barrier model suggests that the charge state of the incident ion plays an important role in the ionization excitation of the target atom and the neutralization process of the target electron captured by the incident ion.

Highly charged ions are incident on a metal solid target surface and deposit the carried energy in the nano-space of the target surface within the femtosecond time scale, which ionizes and excites the target atoms and results in the emission of spectral lines. Some of the spectral lines are transitions between complex electronic configurations, leading to strong electric dipole forbidden transitions. There have been no experimental data reports on the 842.42 nm and 1525.03 nm spectral lines emitted by helium-like Al ions, as well as the 1251.08 nm spectral lines emitted by lithium-like Al ions, and the 989.01 nm spectral lines of Ti XVIII (i.e. Ti¹⁷⁺) ion de-excitation radiation since the theoretical calculation results were published in 1987. The relative intensity of the spectral lines (single ion fluorescence production) we measure increases with the growing charge state of the incident ion, and the increasing trend is generally consistent with the potential energy trend of the incident ion increasing with the charge state. This indicates that the classical over-the-barrier model holds true in the energy region of the incident ion's kinetic energy near the Bohr velocity. The research methods for elastic and inelastic scattering caused by collisions between ions and gas target atoms are different. Due to many novel phenomena generated by highly charged ions incident on solid surfaces, such as the controversy over Auger and ICD processes, the energy shift caused by multiple ionization of target atoms, and the dissipation channels of total energy and gain energy of incident ions, a large quantity of work should be done. Meanwhile, we sincerely hope that our study can provide basic data and support for related research, and propose new methods for spectral measurement.