Behavior of Free Electrons toward Gas Molecules

Formation of Negative Ions in Gases.---The theories advanced to account for the appearance of abnormal negative mobilities at low pressures are two: it is assumed that in order to form an ion an electron must either have more than a certain amount of energy (Wellisch) or must have made, on the average, n impacts (J. J. Thomson). To test the Wellisch theory, the effects of increasing the energy of the electrons was determined. The fact that the number of ions was not increased is taken to favor the Thomson theory. Assuming this theory correct the Thomson constant n was determined for CO, NH₃, C₂{H}₄$, ${C}₂H₂, C₂{H}₆$, ${C}₂H₅Cl and Cl₂, using carefully purified gases under varying conditions of pressure and frequency of alternating potential. The results depend on the values assumed for the mobility of the electron in the various gases, but they indicate that for gases in the order given, n decreases progressively from about 10⁸ for CO to 10³ or less for Cl₂.Conclusions.---CO behaves like N₂; electronegative gases attach electrons more readily than do electropositive gases; the degree of saturation of hydrocarbons does not make much difference.Mobility of negative ions in chlorine, ethane and ethylene was found by the Rutherford A.C. method to be, respectively, 0.73, 1.30 and 0.91 cm./sec./volt/cm. at atmospheric pressure.