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Schematic setup for a beam experiment

Schematically a beam experiment consists of a source, selection, interaction, analysis and detection.

Source

Considering the source of a beam, the most important factor is the signal noise ratio. Furthermore the beam has to be well characterized, i.e. it must not contain other particles then the intended ones, and the intensity has to be suffuciently high. Chemically speaking a fast neutral beam of the order of 1 - 20 eV is the most interesting. All chemical bonds and the activation energies of most chemical reactions are in that region. Unfortunately this is also the most difficult range to study experimentally.

It is possible to use a nozzle source. The beam material flows through a nozzle out of a chamber under high pressure. In orde to obtain sufficiently high energies it must be used at elevated temperatures or with a seeded beam technique. In a seeded beam, the beam molecules ride along in a bulk flow of light carrier gass, usually helium.

Another possibility is a sputtering source in which a solid target is bombarded with high energy ions. The detached atoms, once collimated and selected, form the beam. This source has been developped by Politiek et al. The main disadvantage of this source is the low intensity of the beam.

For energies higher then 20 eV, charge transfer sources are indicated. In such a source ions are accelerated and then transfer their charge to slow neutral particles of the same species, without losing their energy. this source can only create a low intensity beam.

Energy and state selection

Ideally, the quantum mechnical state of both partners in the collision has to be be determined, concerning the translational, rotational, vibrational and spin quantum numbers. The translational or velocity selection is the most important and also the one which is possible for all systems. Scattering patterns must always be measured as a function of relative velocity: the classical cross sections and the quantum mechnical interference are velocity dependent, the reaction rate is exponentially dependent on the temperature ans therieby on the kinetic energy and the apparatus resolution in dependent on the velocity. The velocity selection can take place mechnically, using rotating discs. The disadvantages are a considerable loss of beam intensity and technical problems for energies larger then a few electron volts. Another way to select the velocity is using time of flight methods. In that case the beam is chopped into pulses, which are synchronized with the detector with a measured time lag. this only works if the detector has a large enough response time. If the atom has a permanent magnetic moment, the magnetic state can also be selected.

Interaction

The interaction takes place in a collision chamber, where the particles from the incoming beam collide with the target particles. There are two possibilities for the setup. Firstly, the target can be a static gas, contained in the collision chamber. This option is preferable for the measurement of the total cross section of the raction. Secondly, we can set up a crossed beam experiment, where the target particles in the second beam have a specified velocity vector as well. Differential cross sections can be measured using this technique.

Analysis

After the interaction the output has to be analyzed in order to distuinguish the intended output from the noise.

Detection and measurement

The detection in molecular beam experiments is based on the fact that the molecular current is tranformed to an electrical current, which can be measured easily. Succesful detection means that there is a high degree of differentiation between the beam and the background molecules. The parameters ruling the detector are:

  1. the sensitivity, the output current resulting from some incident flux,
  2. the noise, random electrical output,
  3. the response time, the time taken for output to rise to 63% of the final value. This response time is a limitation for the variation of the incident beam.
There are different types of detectors, based on different criteria. The most important are ionization detectors. The beam molecules are ionized and the resulting ion current is measured. The initial ionization can be obtained using several techniques:
  1. Hot wire detectors: the molecules collide with a hot wire with a high work function. This method has a high noise and is very inefficient at energies higher then 2 eV.
  2. Surface ionization of fast alkali beams: specifically energetic alkalis can be ionized on a cold filament with a high work function, with very little noise and a small response time. The main problem of this technique of the loss of sensitivity in the range of 3 - 50 eV has been circumvented by politiek et al. (1969).
  3. Electron bombardment detectors: fast electrons intersect the beam, ionizing its molecules. The resulting ions are extracted and measured using mass spectroscopy techniques.
  4. Field ionization detectors. These detectors were not available at the beginning of the research project by Los. In the mid seventies they were considered promising but difficulet to realize in molecular beam experiments. In these detectors the molecules are ionized by quantum tunneling of an electron through a potential distorted by a very intense electric field.
Neutral particles are very difficult to detect. If you need ions in order to facilitate detection, the obvious choice is to use alkali atoms, which have a very low ionization energy (4-6 eV). A Wolfram wire of 1000K ionizes them with an efficiency of 100%. Another type of detectors are bolometer detectors, which are sensitive to the energy of the beam.

To the Bibliographical Information
module 1. Selection Interaction Analysis Detection