The Electron Beam Ion Trap
CHAPTER 2. THE ELECTRON BEAM ION TRAP
2.3 Interaction processes in the trap
A plethora of interactions takes place between the electrons, atoms and ions in an EBIT. First and foremost is the generation of HCIs with the elec-tron beam, via successive elecelec-tron impact ionization of the precursor species injected into the trap. However, several other inelastic collisions also oc-cur. These result in the emission of radiation which can be measured. The dominant source of this emission is direct electron impact excitation. Fur-ther radiative collisions, which involve the removal of electrons and hence play a role in determining the ionization balance in the trap, are electron-ion recombinatelectron-ion processes and charge exchange between HCIs and residual gas neutrals. Elastic electron-ion and ion-ion collisions are central to energy exchange in the EBIT, with those HCIs gaining sufficient kinetic energy es-caping the trap. As a result the remaining ion ensemble is cooled, which also affects the ionization balance attained.
In this section the process of ionization by the electron beam and the radiative interactions to which the HCIs then succumb are discussed. Elastic collisions are considered in Section 2.4, in the context of the evolution of ions in the trap.
0 1 2 3 4 5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Ar charge state
Ionizationenergy[keV]
Removal fromn= 1 shell
Removal fromn= 2 shell
Removal fromn= 3shell
Figure 2.3: Ionization energies for the formation of Ar+to Ar18+[Dyall et al., 1989].
CHAPTER 2. THE ELECTRON BEAM ION TRAP
2.3.1 Electron impact ionization
Ionization occurs when an incident electron transfers enough kinetic energy to a bound electron for it to be ejected from the atom or ion concerned.
Specifically, the removal can only take place when the beam electron carries an energy into the collision which is greater than or equal to the binding energy of the electron in the target. Successive ionization impacts then result in the generation of HCIs. This is illustrated by Figure 2.3 which shows a plot of the charge states of argon versus the ionization energy for their production, calculated using the GRASP code [Dyall et al., 1989]. Energies range from 16 eV for the creation of Ar+ to 4.4 keV for the creation of Ar18+. The energy gaps between quantum shells give rise to the steps seen in the figure, with progressively greater amounts of energy being needed to remove electrons bound more tightly to the nucleus. In an EBIT the maximum charge state of a particular ion species in the trap is thus determined by the energy selected for the electron beam.
0 10 20 30 40 50
10−21
10−22
10−23
Cross section maximum
Ionization threshold Crosssection[cm2 ]
Electron energy [keV]
Figure 2.4: Electron impact ionization cross sections for the formation of Ar18+ versus electron energy [Lotz, 1968].
The cross sections for electron impact ionization are widely calculated using the semi-empirical Lotz formula [Lotz, 1968] and depend on the electron beam energy, the ionization potential of the electron to be removed and
CHAPTER 2. THE ELECTRON BEAM ION TRAP
the ion’s particular electronic configuration. The formula allows the general trend over a large range of electron energies to be obtained. Figure 2.4 shows the result for the formation of Ar18+ by electron impact ionization of Ar17+ ions. It can be seen that the cross section peaks at an electron energy of∼10 keV, which is approximately twice the ionization energy. This relationship generally holds for all HCIs. As a result, to maximize the yield of a particular ion in an EBIT the electron beam energy is typically set a factor of two higher than the ionization energy required for its creation.
2.3.2 Radiative collisions
As previously mentioned, the dominant radiative process occurring in the trap is direct electron impact excitation of the HCIs. This is related to impact ionization, although rather than an electron being ejected into the continuum it is promoted to a higher bound state. The excited system then decays by the refilling of the electron hole accompanied by the emission of a photon.
Further radiative interactions involve the capture of beam electrons by HCIs. If the excited state does not auto-ionize then a reduction in the charge on the ion results. One scenario is radiative recombination, by which an elec-tron is captured into a certain energy level of the ion and then on relaxation a photon is emitted with an energy equal to the sum of the kinetic energy of the beam electron and the binding energy of the capture state. Essentially this is the time-reversal of the photoelectric effect. The cross sections for radiative recombination are typically one or two orders of magnitude lower than those for electron impact ionization. For example, the ionization cross section for 10 keV electrons impacting on Ar17+ ions is 10−21cm2, as demonstrated in Figure 2.4. The corresponding cross section for radiative recombination is of the order of 10−23cm2 [Kim and Pratt, 1983].
At resonant beam energies an alternative process called dielectronic re-combination can occur. This involves the simultaneous excitation of a second electron giving rise to a doubly excited state which then also decays. The resonance condition is that the energy of the incoming electron must equal the transition energy of the excitation. In this work, however, this resonance was not sought.
In the collisions of HCIs with neutrals, electron capture proceeds into Rydberg states of the HCI, resulting in radiative cascades to the ground state.
This is the process of charge exchange, the key interaction investigated in this
CHAPTER 2. THE ELECTRON BEAM ION TRAP
thesis, and is described in more detail in Chapter 4. The cross sections for charge exchange are relatively large. For example for Ar17+,18+ions they are of the order of 10−14cm2 [Müller and Salzborn, 1977]. Hence when it comes to preserving HCIs in their high charge states the presence of background gas in the trap is an interference. This explains why ultra high vacuum conditions are needed in the EBIT. It has been found, however, that the injection of small amounts of a low mass neutral gas significantly increases the yield of the highest charge states in an EBIT. This is due to evaporative cooling, which is described in the following section.