The formation of helium molecules

Abstract

The occurrence of band spectra in the chemically monatomic gases leads to the belief that molecules are formed in the gas by the electrons or photons producing the spectra. Arnot and Milligan (1936) and Arnot and M’Ewen (1938) have shown that diatomic mercury molecules are formed by the attachment of excited atoms in P-states to each other and to normal atoms. Although it has been inferred from the band spectra found in helium that molecules are formed by some such attachment process, these have not previously been studied electrically, nor have the details of the attachment process been investigated. No definite evidence has been put forward to show that band spectra occur in the other rare gases. This paper contains the results of an investigation made by the balanced space-charge method of the ionization produced in argon, neon and helium when these gases are subjected to bombardment by electrons whose energy was varied from a few volts below the first excitation potential to a few volts above the atomic ionization potential of the gas. Only atomic ionization has been found to occur in argon and neon, but molecular ions are found to be produced in helium when the energy of the electrons exceeds the resonance potential of the gas.