Relative Electron Impact Ionization Cross Sections by a Double Crossed-Beam Technique

Accurate values of electron impact ionization cross sections of high-temperature species are of importance in mass spectrometric investigations as well as a variety of other fields. However, the present experimental techniques are subject to many inherent difficulties or are limited in applicability and have yielded relatively few reliable values. Theoretical calculations are not sufficiently accurate. An experimental technique for accurate measurement of relative cross sections is described. The measurement depends only on one experimentally determined quantity, as well as the mass and temperature of the gaseous species, and is applicable to a wide variety of systems. The technique utilizes a mass spectrometer that is fitted with a Knudsen cell and employs electron impact ionization. A second, modulated electron beam intercepts the molecular beam, causing a slight modulation in the intensity of the molecular beam that is reflected in the output signal of the mass spectrometer. The relative attenuations of the various component species of the molecular beam are proportional to the relative total ionization probabilities of these species. The relative cross sections for Ca, Tl, Ag, and Pb have been measured and are compared with literature values obtained by other techniques. Preliminary application has also been made to the molecular species W2O6 and W3O9.