Research Index / Materials Science / Ion-Energy Analyzer
Laser ablation creates a plasma plume from the target material which consists of electrons, ions (charged atoms), and neutrals (neutral atoms). Laser ablation with ultrashort laser pulses can produce highly-charged ions which can react with each other and with the electrons and neutrals for some interesting chemistry. In addition, ultrashort laser pulses can impart a large amount of energy to the plasma plume. The energies and charge-states in the plume can be varied by simply varying parameters of the laser pulses -- in particular: the laser energy, contrast and presence of a pre-pulse (see COLA paper). This ability to pre-tune the state of the ablation plume and to allow highly-reactive chemistry opens up the possibility of producing highly-tailorable films, materials, and devices.
Ion Charge-State & Energy Analyzer: In order to measure the energies and charge-states in the ablation plume, an energy analyzer can be used. We use a spherical-sector, electrostatic energy analyzer (shown to the right). The ions enter the analyzer and pass between two spherical-sector plates (the plates are curved into a portion of a sphere). A voltage up to 5000 volts is placed on each of the plates. An ion passing between the plates will feel a force from this voltage which will act to curve the path of the ion. The magnitude of this force depends on the voltage on the plates and on the charge-state and energy of the ion. Specifically, for a certain voltage difference on the plates, only ions with a certain energy-to-charge ratio will follow the path shown in the photo and reach the detector (the MCPs). All the ions with different energy-to-charge ratios will be curved by the plates, but will hit the plates and not reach the detector.
For the geometry of our particular analyzer, the energy-to-charge ratio (E/q) of the selected ions is equal to 2.254 times the voltage difference (Vdiff) between the spherical-sector plates which gives the equation: (E/q)selected = 2.254 x Vdiff , where E/q is in units of electron volts (eV), and Vdiff is in volts (V). Also the sign of the voltage matters. For positive ions, a negative voltage is placed on the inner plate and a positive voltage on the outer plate. (Remember: opposite charges attract; like charges repel. So, a positive ion will be attracted to the negative charge on the inner plate and curve in the correct direction.) For negative ions and electrons, the opposite is done: a positive voltage is placed on the inner plate and a negative voltage on the outer plate.
Time-of-Flight Analysis: Thus, one voltage difference on the plates gives ions with a particular energy-to-charge ratio. This means that ions with twice the charge-state and twice the energy of other ions will be detected along with them. For example: for a voltage difference of 1000
volts (V), ions with a charge state of +1 and an energy of 2254 electron volts (eV), will be detected along with ions of charge-state +2, energy 4508 eV, and ions of charge-state +3, energy 6762 eV, and so on. However, ions with a different charge-state will not be detected at the same time. An ion with twice the charge-state and twice the energy will be traveling at a faster speed (1.414 times faster or the square-root of 2 times faster speed because energy is related to the square of the speed). What this means is that the higher-energy ion will arrive earlier (for twice the energy, the arrival time will be 0.707 times earlier or 1 divided by the square-root of 2 times earlier since arrival time is related to the inverse of the speed). This technique which uses the arrival time to analyze the detected signal is called time-of-flight analysis.
Energy & Charge-State Distribution: To put together the entire distribution of ions over energy and charge-state, the voltage difference on the sector plates is scanned from 0 to 10,000 volts. Each voltage difference gives a set of charge-states and energies as described above. We use a computer program to pick out the individual charge-states at each voltage difference and put together the ion energy and charge-state distribution.
The distribution of ion energies and charge-states depends on the parameters of the laser pulses. Also, the ion distribution will affect the deposition and growth process of the thin film. By using the energy analyzer, the ion distribution can be measured as a function of laser parameters and can be compared to deposition and thin-film properties.
For more information: Pulse-contrast effects on energy distributions of C1+ to C4+ ions for high-intensity 100-fs laser-ablation plasmas, (P.A. VanRompay, M. Nantel, P.P. Pronko), Applied Surface Science, volume 127-129 (1998), pages 1023-1028.
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