Electron-Ion Collisions

Electron Impact Ionization

When an electron collides with an atom or ion, there is a small probability that the electron kicks out another electron, leaving the ion in the next highest charge state (charge q increased by +1). This is called electron-impact ionization and is the dominant process by which atoms and ions become more highly charged. The rate equation is given by:

e- + A(q) → e- + e- + A(q+1).

From energy conservation, it is clear that the initial energy of the incident electron must be larger than the ionization potential of the electron being removed.

Theorists have found electron impact ionization cross sections difficult to calculate from first principles, even for relatively simple systems like hydrogen-like (one electron) and helium-like (two electron) ions. There has been recent progress in developing a phenomenological theory by Dr. Yong-Ki Kim here at NIST.

A simple empirical formula for calculating electron impact ionization cross sections was developed by W. Lotz over 25 years ago. It is not very accurate, but it does give experimentalists a useful qualitative picture.

Radiative and Dielectronic Recombination

Radiative recombination is a process which takes place when a positively charged ion captures an electron to one of its bound orbits with a simultaneous emission of a photon:

e- + A(q+) → A(q-1) + photon

In the dielectronic recombination process the energy which becomes available during the capture process is carried away by the promotion of a bound electron to another bound orbit:

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

In the second step of the dielectronic recombination process a photon is emitted characteristic to the doubly excited state (**) of the q-1 times ionized ion. The dielectronic recombination is a resonant process, because of the discrete energy nature of the bound electron orbits.

Both radiative and the dielectronic recombination are important capture processes which play a dominant role in determining the charge state balance of highly ionized astrophysical and laboratory plasmas.

In a recent investigation carried out with our EBIT [8], scandium-like and titanium-like barium ions were created, trapped, and excited. X-ray peaks arising from both radiative recombination and dielectronic recombination were studied simultaneously. In the DR process a 2p electron was promoted to the 3d orbital. One of the M-shell electrons of the recombined ion subsequently decayed radiatively to the 2p vacancy, and emitted an x-ray of energy almost twice the incident kinetic energy of the projectile electron. Comparison with theoretical estimates showed a favorable agreement with the data. The theoretical calculations were carried out by the theory group of the University of Connecticut (McLaughlin and Hahn).

Trapped Ion Dynamics

Ion Orbits

In order to have a better understanding of the motion of the highly charged ions inside the trap a computer simulation was developed by Eric Meyer. The program accounts for the presence of the magnetic field of the superconducting coils, the electron beam, and the neutral background gas atoms. It can also simulate the effect of changing electron beam currents which has importance in time resolved (e.g., lifetime) measurements. Here are a few examples of different orbits.

Charge state simulation

The charge state balance (e.g., the population of different charge states) inside the EBIT is determined by the balance between the different ionization and recombination processes. In order to predict the charge states of the ions inside the trap and to help to determine the EBIT parameters for the optimalization for the production of a certain charge state these processes should be taken into account. To this end a computer program which was originally developed by Penetrante in Livermore and modified by Margolis at Oxford was installed and further modified. The predictions of the simulation can be checked by analyzing the x-rays emitted from the trap. An alternative way to look into the same problem is the extraction and charge to mass ratio analysis of the highly charged ions. Experimental efforts in this direction are under way.

Atomic Lifetimes

The measurement of excited-state lifetimes is complementary to measuring transition wavelengths as a way of studying atomic structure. Although the lifetimes are determined by the same wavefunctions as the energy levels the measurements of the atomic decays carry different information since they are sensitive to the long-range behavior of the wavefunctions. The knowledge of the lifetimes also has important practical applications. They are critical in the density diagnostics of laboratory and astrophysical plasmas.

The principle of measuring lifetimes with an EBIT lies in the periodic fast switching of different voltages in the machine. Since the ions are created and excited with the same beam of electrons, by changing the electron beam energy one can selectively exclude certain levels from being excited. This can simply be done by setting the electron beam energy below the excitation threshold of the level to be excluded. Without further excitation the time dependence of the emitted photon signal carries the information about the lifetime of the level. After a certain period of time (determined by the lifetime of the level) the electron beam energy is set to be above the excitation threshold to repopulate the level and repeat the sequence. An alternative method for measuring lifetimes with an EBIT is to switch off the electron beam completely, take data, and turn the beam back again to re-excite the ions in the trap. While the electron beam is off, the ions remain trapped by the magnetic field. The lifetime range that can be measured with an EBIT is determined by the capabilities for the fast switching of voltages. In principle the 10 ns to 10 ms lifetime range can be addressed by this method. Since this lifetime range is only partially covered by other methods the EBIT is a unique tool for measuring the lifetime of long living metastable levels.

In a recent experiment we have measured the lifetime of a visible light emitting metastable level [18]. The transition takes places within the ground state configuration of titanium-like ions. The measured lifetimes fall into the millisecond range.

Ion-Surface Interactions

The EBIT was originally developed for in situ spectroscopic measurements of highly charged ions. It soon turned out, however, that the machine is easily capable of producing the highest charge states not accessible even for sophisticated ion sources. To use the EBIT as a source for ions an extraction system has to be built which removes the ions from the trap region and transports them outside the machine. The construction of the extraction system on the NIST EBIT was completed in the first half of 1995 [12,20]. When the ions are extracted from the EBIT they pass several ion beam steering and shaping ion-optical elements. A bending magnet is available as a part of the system to charge-to-mass select the ions and direct them into experimental chambers. Recent experiments with the extracted ions involve the study of nanometer scale damages of insulator surfaces caused by the impact of highly charged ions. X-ray [16] and Electron spectroscopic investigations [15] of metal-surface -- highly-charged-ion interactions are under way.

Both the atomic spectroscopy and ion-surface interactions component of our work is proceeding in connection with International SEMATECH.