| He+ | |
|---|---|
| Name | Singly ionized helium |
| Caption | Schematic of a helium ion (He+) |
| State | Ion |
He+
He+ (singly ionized helium) is an atomic ion consisting of a helium nucleus and a single bound electron. It is a fundamental one-electron system that provides a bridge between hydrogenic theory and multi-electron atoms, playing a central role in precision tests of Quantum electrodynamics and atomic structure. Studies of He+ inform models in astrophysics, plasma physics, and metrology.
He+ is produced when one electron is removed from a neutral Helium atom, leaving a nucleus of two protons (and typically two neutrons in the common isotope) with a solitary electron in bound states similar to Hydrogen atom levels but modified by the higher nuclear charge. Because He+ is a one-electron ion, it can be described to high accuracy with solutions to the Schrödinger equation for a Coulomb potential plus relativistic and radiative corrections from Quantum electrodynamics (QED). Its simple structure makes it a benchmark for comparison with experiments from groups such as those at National Institute of Standards and Technology (NIST), Max Planck Institute for Quantum Optics, and university laboratories.
The bound states of He+ follow the hydrogenic energy formula to leading order, E_n = -Z^2 R_\infty / n^2, with nuclear charge Z = 2 and the Rydberg constant R_\infty. Corrections include the Fine structure (spin–orbit and relativistic terms), the Lamb shift from QED, and finite nuclear mass/recoil effects. Precision calculations incorporate contributions from Bethe logarithm terms and higher-order radiative corrections developed in theoretical work by researchers such as Hans Bethe and later QED theorists. Isotopic differences between Helium-3 and Helium-4 introduce measurable isotope shifts used to probe nuclear charge radii, complementing results from muonic-atom experiments and electron scattering.
He+ exhibits spectral lines analogous to the Lyman series (transitions to n=1) and Balmer-like series (transitions to n=2) shifted by Z^2. Prominent UV and extreme-UV lines arise from transitions such as 2p→1s and higher-n cascades, which are observed in astrophysical spectra and laboratory plasmas. High-resolution spectroscopy of He+ transitions provides tests of QED and contributes to determinations of fundamental constants, pursued by collaborations involving institutions like European Southern Observatory (for space/astronomical observations) and precision spectroscopy groups at Harvard University and University of Oxford. Spectroscopic line shapes are influenced by Doppler broadening, Stark broadening, and collisional effects in dense plasmas.
He+ forms via photoionization by ultraviolet or X-ray photons, collisional ionization in energetic environments, or charge-transfer reactions in mixed-species plasmas. Photoionization cross sections are governed by dipole matrix elements computed with methods from Quantum scattering theory and measured in synchrotron facilities such as European Synchrotron Radiation Facility (ESRF). Recombination proceeds radiatively or via three-body processes, with rate coefficients tabulated for plasma modeling by groups affiliated with Princeton Plasma Physics Laboratory and fusion research centers. Dielectronic recombination and electron-impact ionization rates are essential inputs for collisional-radiative models used in modeling tokamak and astrophysical plasmas.
He+ lines are diagnostic of ionization and temperature in stellar atmospheres, H II regions, planetary nebulae, and the interstellar medium. Observations from instruments on the Hubble Space Telescope and space telescopes such as Far Ultraviolet Spectroscopic Explorer (FUSE) detect He+ emission and absorption, constraining models of stellar winds, photoionization by O-type star spectra, and the ionization state of the intergalactic medium. In laboratory plasmas and magnetic confinement experiments at institutions like ITER-related facilities and Lawrence Livermore National Laboratory, He+ behavior affects impurity transport, radiative cooling, and diagnostic interpretations using helium line ratios.
Laboratory investigations of He+ employ trapped-ion techniques, beam-foil spectroscopy, electron-beam ion traps (EBITs), and radio-frequency ion traps. Penning trap and Paul trap experiments, often coordinated with metrology institutes such as National Physical Laboratory (United Kingdom) and NIST, enable precise determination of transition frequencies and lifetimes. Synchrotron radiation and free-electron laser facilities provide tunable extreme-UV sources for photoionization experiments. Laser spectroscopy experiments exploit narrow-linewidth lasers and frequency combs to measure transitions with high accuracy, linking to the work on optical frequency standards and atomic clocks.
The one-electron nature of He+ permits highly accurate theoretical modeling using nonrelativistic quantum mechanics supplemented by relativistic (Dirac equation) and QED perturbative corrections. Computational approaches include variational methods, explicitly correlated wavefunctions, and modern perturbation expansions. Software tools and numerical packages used in atomic physics calculations are developed in academic groups and national laboratories; results are compared with experimental values to refine QED contributions and fundamental constants. Theoretical efforts often reference foundational texts and papers in atomic theory and QED by authors such as Bethe and Salpeter and later reviews in journals like Physical Review A and Reviews of Modern Physics.
Category:Helium Category:Atomic physics Category:Quantum electrodynamics