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X-ray Polarization

In the absence of strong external electric or magnetic fields, atomic systems of different magnetic sublevels (but otherwise having the same principal and angular momentum quantum numbers) are degenerate in energy. Because of this degeneracy the measurement of the energies of atomic transitions is not enough to get a complete characterization of the quantum state of the system--the magnetic quantum numbers are also needed. The magnetic quantum numbers are related to the spatial orientation of the atom or ion. In the case of pronounced external symmetry, spatial effects can be important. For example excitation in cylindrically symmetric situations can lead to oriented or aligned systems where the magnetic substates with different angular momentum projections have the same energy but different populations. If such a system undergoes a transition which results in a photon or electron emission the emitted radiation will show anisotropic and polarized behavior.

The above situation is relevant in several natural and laboratory environments. Generally cylindrical symmetry applies for cases when atomic excitation takes place in the interaction by a directed beam of charged particles. An astrophysical example is the atomic excitation occurring in solar flares where a plasma made of charged particles (ions and electrons) moves along strongly directed magnetic field lines. Excitation due to directed flow of particles can be observed also in supernova shock waves. The same situation occurs in many of the laboratory experiments where electron or ion beams excite or ionize atoms or ions.

The EBIT capabilities for measuring electron impact ionization, excitation and recombination cross sections have already been demonstrated in several cases. Transitions in highly charged ions usually involve the emission of photons in the X-ray region. This fact offers an obvious choice of the use of X-ray analyzers (usually solid state detectors and crystal spectrometers) for measuring the above mentioned cross sections. In the EBIT device the ions interact with a narrow (about 0.06 mm diameter) beam of electrons. This very well collimated electron beam acts as a quantization axis making the cylindrical symmetry a natural case for electron-ion interactions inside an EBIT machine. Because of this fact care has to be taken in interpreting X-ray line intensities when they are used for obtaining electron-ion interaction cross sections. They can be strongly affected by anisotropic and polarized emission. The latter is most important in the case of crystal spectrometers where the energy dispersion is polarization selective. On the other hand the measurement of the polarization or the angular distribution of the X-ray emission can give information about the magnetic sublevels involved in the electron-ion collision [7]. This information remains hidden in a simple energy dispersive measurement because of the degeneracy of the magnetic sublevels.

Visible and UV Spectroscopy

We have successfully extended EBIT spectroscopy of highly charged ions into the range of the spectrum by observing forbidden (M1) transitions within the ground term of titanium-like barium and xenon using a scanning grating monochromator [6]. These transitions are of particular interest for their use in the remote monitoring of ion temperatures by Doppler broadening in large future tokamak fusion machines. Because light detected in these experiments arises from transitions between very close lying levels (fine structure), the wavelength is sensitive to small Zeeman shifts which are usually negligible in the X-ray spectra. The availability of advanced reflective and refractive optics for visible light allows one to achieve both high efficiency and high resolution simultaneously. We have recently applied Fabry-Perot interferometry to achieve resolution sufficient to see Doppler-blurred Zeeman broadening from ions in the trap. The present accuracy of our measurement of the wavelength of the visible light is sufficient to reveal large disagreements (~4%) with ab initio Dirac-Fock calculations.

Our UV spectroscopy capabilities are currently being extended into the VUV in order to join up smoothly with our x-ray capabilities and provide continuous coverage from the visible to the x-ray regime.

Electron-Ion Collisions

Electron Impact Ionization


e- + A(q) → e- + e- + A(q+1).




Radiative and Dielectronic Recombination


e- + A(q+) → A(q-1) + photon


e- + A(q+) → A(q-1)** → A(q-1) + photon




Trapped Ion Dynamics

Ion Orbits


Charge state simulation


Atomic Lifetimes




Ion-Surface Interactions

