Electron velocity analysers

Since pulsed laser ionization produces well defined packets of ions and electrons, TOF analysers (which essentially are magnetically shielded, electric-field-free drift tubes with apertures and an electron multiplier) can readily be used. TOF resolution for slow electrons can approach 3 meV, and throughput is similar to that of electrostatic analysers operating without an extraction field (i.e. a detection efficiency < 1%). The kinetic energy is obtained from the flight time, which is proportional to the reciprocal velocity, (KE) /2, whereas the resolution varies as (KE)/2. Thus, the resolution for 1-5 eV electrons is comparable to that for electrostatic analysers, but degrades seriously for 5-10 eY electrons. 

In order to obtain information about the energy distributions of reaction products, it is necessary to use a detection method that can determine either the internal state populations of the products or their recoil velocities. The methods employed to measure electronic, vibrational or rotational energy distributions are generally based on a form of emission or absorption spectroscopy, although there are other techniques that are sensitive to internal excitation. A variety of methods are used to measure recoil energy distributions these are commonly based on a mass spectrometric detection system used with some form of velocity analyser. 

A time-of-flight (TOF) analyser measures the time t required for a particle to travel a fixed distance d. If applied to electron spectrometry, non-relativistic electrons with kinetic energy kin have a velocity v... 

The recoil technique is capable of yielding angular distributions with velocity-selected targets. In addition it is particularly useful in studying the change of spin states in a scattering process, since it is easier to analyse the spin states of scattered atoms than those of electrons. Such measurements are not discussed here but the reader is referred to the articles by Bederson (1968) and Bederson and Kieffer (1971). 

Catalytic oxidation reactions were carried out in a conventional fwed bed reactor under atmospheric pressure [10]. The flow rate through the reactor was set at 500 cm min and the gas hourly space velocity (GHSV) was set at 15000 h. The residence time based on the packing volume of the catalyst was 0.24 s. Following the reactor, a portion of the effluent stream was delivered and analysed on-line using a Hewlett Packard 5890 Series II gas chromatograph (GC) equipped with an electron capture detector (ECD) and a thermal conductivity detector (TCD), and controlled with HP ChemStation software. The concentration of the chlorinated feeds was determined by the ECD after being separated in a HP-VOC column. 

An example of a similar spectrometer is the one used by Champion et al.9 Primary ions were produced by electron bombardment, accelerated and mass analysed by a 60°, 13-3 cm magnetic mass spectrometer. The ions were then retarded to the desired energy and energy selected by a 127° electrostatic cylindrical velocity selector. The energy resolution was 5 %. The beam half angle is reported to be 18°. The ions then entered a reaction chamber at a pressure of about 10 4torr. The chamber had an exit slit which could be rotated with the detecting system which consisted of another 127° velocity selector, a quadrupole mass spectrometer and an electron multiplier. 

The deficiencies of this procedure have been carefully analysed by Boring and Wood [62] who worked with an approximate treatment of the Dirac equations, due to Cowan and Griffith [63]. In this method the spin-orbit operator is omitted from the one-electron Hamiltonian but the mass-velocity... 

---