Electron Scattering Cross Sections Introduction: : 1.4 Historical Background
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1.4 Historical BackgroundTransmission of electrons through a gaseous medium was one of the first phenomena studied after the discovery of elementary particles. The study of electron-atom collisions played a major role in the understanding of quantum mechanics and many- body systems. Electron-atom collisions stimulated largely by the pioneering efforts of Ramsauer and Townsend. Ramsauer8 and his co-workers studied these effects from 1921 at the Radiological Institute at Heidelburg through 1930 at the research Institute of the AEG, Berlin. At this time, vacuum systems were not highly developed; so they took different measurements to avoid difficulties with background gas impurities. However only a few minutes were available after gas filling before the background impurities began to grow to serious proportions. At this time, sufficient measurements were made on essentially all atomic and simple molecular systems. Ramsauer’s first apparatus did not have the ability to apply acceleration potentials to the electron beam, since only energies of up to 1.1 eV were produced by using different arc sources for the photoelectrons. Brode7, at California Institute of Technology, first constructed a modified Ramsauer apparatus in the early 1920’s. Berkeley later constructed a similar apparatus. At this time, these apparatuses had the unique capability to operate in elevated temperatures which allowed them to improve vacuum techniques. Recently, several new Ramsauer type apparatuses have been constructed with improved vacuum and gas-handling techniques. Thus, this method did not change dramatically for over almost a century. Electron-atomic beam techniques played a major role in collision studies because they permit the preparation of the target into specific ionic groups with specific excited states. Uses for this atomic collision data were recognized early, and several important beam-gas experiments were performed in the 1930’s and 1940’s. Fite and coworkers6 performed a series of electron scattering experiments on atomic hydrogen, and demonstrated the power of their cross beam method in 1958 and 1959. These earlier cross beam experiments had many errors in their results. The electron gun used to produce an electron beam was typically a bare resistively heated filament of a refractory metal, such as tungsten or rhenium. A disadvantage of this directly heated filament type of source is that there is a larger range of electron energies in the electron beam as a result of the potential gradient along the length of the heated filament. Also this directly heated filament electron source is the continuum of blackbody radiation emitted by the hot filament, which have to operate at 1800 – 2000 K in order to get sufficient beam current. At this time period, current technology didn’t have ultrahigh vacuum pumps to reduce the background gases to a very low level before the chamber is backfilled with the target gas. The Transmission Beam Technique was first introduced by Blaauw9. Their experimental setup can be viewed as a linearisation of the Ramsauer technique, without the use of any magnetic field. During the past 20 years this technique has improved by the introduction of high resolution electrostatic analyzers, accurate electrometers, longer gas cells and better vacuum systems. As a results of these improvements, electron scattering cross sections can be measured with errors of 5% or less. Further, these improvements have allowed cross section measurements to be extended up to intermediate and high ( 500 – 5000 eV) electron energies. Summarized in Table 1.1 are the gases and energy ranges of available electron scattering cross sections in the literature prior to 199210. As can be seen from this table, the majority of the measurements were taken at a lower energy region 0 – 400 eV. In Table 1.2 are more recent cross section measurements reported in the literature. It is evident from the information in Table 1.2 that there is increased interest in measuring electron scattering cross sections. Major reasons behind this increase are the disagreement of the cross sections produced by different laboratories and the limited availability of intermediate energy cross sections. These two reasons hinder the development of accurate theoretical models to predict many scattering cross sections. Table 1.1 Gases and energy ranges of available total electron scattering cross sections prior to 1992 as summarize by Benderson, B., Walther, H., “Advances in Atomic Molecular, and Optical Physics”, Volume 33, Page 67-69 (1992)10.
Table 1.2 Recent total cross section measurements
Table 1.2 (Continued)
Table 1.2 (Continued)
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