A Search for Isotopes of Hydrogen and Helium

The well-known triatomic hydrogen ion (H¹H¹H¹)⁺ of mass 3 is formed by electron impact at a rate proportional to the square of the pressure if the pressure is very low. The isotopic molecular ion (H¹H²)⁺, also of mass 3, would be expected to vary linearly with pressure. These two facts constitute a method for distinguishing between the two with a mass spectrograph whose resolving power is insufficient to separate them by magnetic analysis. The method becomes successful only when the working pressures are very low. With a mass spectrograph designed for low pressure work evidence was found which amply confirms the existence of an isotope of hydrogen of mass 2 and also gives a fair estimate of the abundance. In a sample of ordinary commercial electrolytic hydrogen the data indicate an abundance ratio H²H¹=1/30000±20%. The same ratio for a sample of hydrogen which had been concentrated by Urey, Brickwedde and Murphy came out to be H²H¹=1/1050±5%. An effort was made to detect a difference in mass between the ions (H¹H¹H¹)⁺ and (H¹H²)⁺ without success. Considering the resolving power of the apparatus this failure was interpreted as meaning that the packing fraction of H² is greater than 4×{}10^-3 i.e., the atomic weight is greater than 2.008. A search was also made for isotopes 3 and 5 of helium but none were found. It is concluded that their abundance must be less than one part in 50,000.