Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 243limit for omegatron operation, the collector current only decreases by 0.5 % withthe emission control in operation. Without the emission regulator, the sameincrease in oxygen pressure causes the collector current to decrease to 10% of itsoriginal value, making quantitative measurements impossible.(c) TopatronThe'theory of this type of spectrometer was discussed by Bennett266 and byRedhead and Cro~ell*~~. It is a resonance spectrometer operating on the principleof a linear accelerator. The constrdction and operation of the device has beendescribed by Vkradi et ul.268~269t and by Ehlbeck et al.270s271.It has the advantage of measuring total pressure as well as partial pressure andrequiring no magnet. A schematic diagram of the device is shown in Fig. 35. Thetube consists of two compartments. One contains an ion source consisting of athoria-coated tungsten filament and a box shaped anode (A). Ions are producedin the box and are then attracted by the first negatively charged grid (Gl). Aproportion of the ions are collected at this grid and the measured ion current isproportional to the total pressure. Ions not trapped at G1 travel through theanalysing grid system (G2-G12) which has an rf voltage between each alternategrid. If the ions have the correct velocity V to clear a grid distance S during a halfperiod of the rf cycle, they gain energy from the rf field and are able to reach thecollector S. From the relationshipwhere f is the radio frequency in Mc. sec-' and V, is the ion accelerating voltage,eJM can be found.After bake-out at temperatures of up to 450" C, the tube will operate over thepressure range 10-3-10-8 torr. Mass numbers up to 100 can be resolved, or upto 250 at the expense of sensitivity.(d) FarvitronThe farvitron, which is also a resonance spectrometer, was developed by Reichand B a ~ h l efrom ~ work ~ ~ ~ by ~ Tretner274. ~ ~ A schematic diagram of this instrumentis shown in Fig. 36:. Electrons emitted from the filament (K) are acceleratedby the grid W and ionize the gas inside electrode A. The ions formed aret A commercial instrument is marketed by Leybold in Europe and by the Raytheon Co. in theU.S.A.The instrument developed by Reich and Bgchler is available commercially from E. LeyboldNachfolger, Germany.References pp. 270-278