Atomic Physics Research
Precision spectroscopy of few electron ions
Quantum electrodynamics (QED) is one of the most important foundations of modern physics. The five standard deviation inconsistency between the measurement of a muonic hydrogen transition and theory has led to four years of intensive research by many groups around the world. Leading theorists consider the discrepancy of 0.42 meV to be well outside possible causes within the Standard Model, claimed to have an uncertainty of no more than 0.01 meV This puzzling situation has stimulated much theoretical activity and highlights the current difficulty in low-Z atomic spectroscopy due to complexities of the nucleus.
Spectroscopy of highly charged ions and muonic atoms probe a relatively unexplored regime of physics in which the peak of the radial wavefunction of the lepton is reduced by more than an order of magnitude. Effects associated with QED and the nucleus are greatly enhanced, due to the increased overlap of the nucleus with the wavefunction of the orbiting lepton. In the case of muonic hydrogen, the lepton orbital radius is reduced by the mass of the lepton, while in the case of highly charged ions the lepton orbital radius is decreased by the increased nuclear charge. While hydrogenic (1-electron) atomic systems are exotic and are critical challenges for theory and experiment, helium-like (He-like) atomic systems lie at one of the forefronts of QED research, because they display qualitatively new effects (including the ‘two-electron Lamb shift’) which are not present at any level in one-electron ions.
Atomic processes relavant to hot astrophysicsal and laboratory plasmas
X-ray spectroscopic measurements are used to determine the temperature distribution, density, ionization state, and elemental composition of hot plasmas. Knowledge of these basic parameters provides an understanding of physical processes in the hot universe such as atmospheric heating, transport, shock waves, and accretion. The determination of the physical parameters that define the plasma relies on complex models of the continuum and line emissions.
To achieve the best scientific interpretation of the data from Chandra, XMM, and the upcoming Astro-H observatories, theoretical calculations must be verified or modified by the results obtained from spectroscopic measurements in the laboratory. The electron beam ion trap at Clemson University produces customized, well-characterized, homogeneous plasmas well suited to a wide variety of precision measurements. The manipulation of the plasma conditions in the EBIT can generate impact excitation rates, excited state lifetimes, ionization cross sections, resonant excitation and dielectronic recombination cross sections for comparison with theoretical atomic physics calculations.
Charge exchange of highly charged ions with neutral atoms
In collisions of low energy (few keV/u) highly charged ions with neutral atoms, the electron capture process is far dominant over other processes such as excitation or ionization. It is well established that an electron from a target atom is usually captured into high a Rydberg state of the projectile ion. Even in multi-electron target atoms, single electron capture is generally dominant, though the contribution of double and triple electron capture becomes significant for higher ion charge states.
In the case of multiple electron capture, Auger electron emission becomes an important first step of the relaxation process. In essentially all cases, however, the stabilization ends in a cascade of photon emitting transitions. When the projectiles are highly charged, the photons emitted in these final steps are x-rays. The observation of these x-rays can provide information on both the electron capture and the following cascade. Recently, the unexpected observation of x-rays from comets and other objects of the solar system brought the collisions of highly ionized projectiles with neutral gases into the forefront of research.