Electrostatic velocity analyser

A pioneering work on the simultaneous measurement of the angular and velocity distributions was carried out by Champion et al. [98—101] following the velocity work of Vance and Bailey [94] described above. Their apparatus, a tandem mass spectrometer system, consists essentially of three sections a primary ion gun, a collision chamber, and a product-ion analyser and detector. A mass-analysed, velocity-selected ion beam is directed into the collision chamber containing target gas at low pressure. The product ions are velocity-analysed with a 127° electrostatic velocity selector and mass-analysed in a quadrupole mass filter. The angular distributions of the product ions are obtained by rotating the analyser-detector system about the centre of the collision chamber. 

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. 

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 Kingdon trap, an ion trapping device that consists of an outer barrel-like electrode and a coaxial inner spindle-like electrode that form an electrostatic field with quadro-logarithmic potential distribution. The frequency of harmonic oscillations of the orbitally trapped ions along the axis of the electrostatic field is independent of the ion velocity and is inversely proportional to the square root of m/z so that the trap can be operated as a mass analyser using image current detection and Fourier transformation of the time domain signal. 

---