Intensities in the Hydrogen Spectrum

Variation of the intensity of hydrogen spectrum lines with the energy of the exciting electrons, 29 to 110 volts, pressure and current being maintained constant, was studied quantitatively by the use of a three-electrode tube in which electrons from a long oxide-coated filament were accelerated through a nearby grid with a field-free space between the grid and the plate. The spectra of light from this space, excited with various accelerating potentials, were photographed on the same plate and the densities of certain lines were measured by a microphotometer. Of the series lines, $\mathrm{H}\alpha$ shows practically constant density for the whole range of voltage, while $\mathrm{H}\beta$, $\mathrm{H}\gamma$ and $\mathrm{H}\delta$ increase in density at first rapidly and then more slowly, tending to constant values for the higher electron energies. The density change was greater the higher the term number. Of the lines of the secondary spectrum, $\lambda \lambda 6327$, 6225, 6135, 6122, 6030 and 6018 decrease very rapidly in density as the energy of the electrons is increased, while $\lambda \lambda 5013$, 4934, 4929, 4743, 4632 and 4205 reach maximum density between 30 and 40 volts and then grow weaker but not as fast as the first six lines. Since the series spectrum is associated with the atom and the secondary with the molecule, it is inferred that the ratio of dissociating to non-dissociating collisions increases rapidly with the energy of the bombarding electrons between 29 and 110 volts. The change in relative intensity of the series lines suggests that the higher the energy of electron impact the more likely is the electron within the atom to be displaced to the remoter Bohr orbits when the molecule is dissociated.