The ionization potentials of the free radicals methyl and ethyl

Abstract

Molecular beams, that is beams of neutral molecules moving with thermal velocities in vacuo, possess two salient characteristics: narrow beams are, to a good approximation, unidirectional; and they are essentially collision free. In 1933 Fraser (1934) suggested that the collision-free character of molecular beams could be utilized in the study of free radicals, such as free methyl and free ethyl, which are the primary products of so many chemical reactions. This suggestion promised to be an immediately practicable one, inas­much as Estermann and Stern (1933) had already found in the space-charge detector not only an extremely sensitive means of measuring the intensity of a molecular beam, but also one which looked to be applicable to the detection of beams of any molecular species. The detector is a diode, the cathode a tungsten wire threaded through the closed ends of the cylindrical anode; the ratio of the diameters of wire and cylinder being so chosen, for a given range of anode potentials, that the electron emission from the filament is governed by its negative space charge. The beam enters the diode through a small aperture, and establishes therein an equilibrium pressure, when as many molecules escape, by the beam aperture and filament clearance holes, as the beam brings in. If the potential difference between the electrodes is greater than the ionization potential of the entering molecules, positive ions will be formed, which partially neutralize the negative space charge and hence increase the electron emission from the filament. It is this increase of electron emission which is a measure of the intensity of the entering beam.