The Energy of Formation of Negative Ions inO2

This paper reports the results of an attempt to evaluate the energy of attachment of elements to molecules in ${\mathrm{O}}_2$, by determining at what energy the electron is detached from the ${\mathrm{O}}_2$ ion in impact with molecules. A value of 0.34 volt is found. In this investigation the complicated phenomena reported by Cravath and Bradbury as a function of the potentials of the high frequency fields employed in the Loeb electron filter, are investigated and explained. At the proper frequencies and with uniform fields electrons can effectively be filtered out without material ion capture. At higher fields the negative ions break up leading to an increased capture of carriers. If many free electrons are present at about the first radiating potential in ${\mathrm{O}}_2$ (8 volts of energy) at high frequencies photoelectrons liberated at the wires and escaping capture may cause an increase in the current through the filter, especially at higher pressures ("hump effect" of Cravath). Finally at very high fields and especially at higher pressures the electrons ionize by impact producing an increase in current followed by a rapid decline due to capture. Shortly thereafter an arc breaks. At the highest pressures studied (20-60 mm) reattachment of electrons masks all the phenomena and a spark breaks very shortly after a rise of current is observed. The ions have been shown to break up in an $\frac{X}{p}$ (field strength in volts/cm, pressure in mm of Hg) of 90. This corresponds to an ion energy of between 0.136 and 0.68 volt with the higher value the more likely. Of this only about half is available for electron detachment so that one can set 0.34 volt as the probable upper limit of the attachment energy which might correspond to a wavelength of 36,000A.