Voltage-intensity curves have been obtained for twenty lines of the mercury spectrum, when excited by electron impact. The range of accelerating voltage was from excitation to fifty volts, the discharge tube being designed to eliminate most of the disturbance due to positive ion formation. The components of four of the triplet groups were resolved and investigated separately. No significant change appears in the number of atoms in a given energy state as the ionization potential is passed. The components of each of the $\mathrm{dD}$ groups of energy levels are excited at practically the same voltage but have quite different excitation functions. The excitation functions for $d_1$, $d_3$, and $s$ levels and for the $2p_2$ level appear to be similar in form and to have sharp maxima within a few volts of the excitation voltage. The excitation functions for $d_2$ and $D$ levels also appear similar, increasing uniformly to 30 or 40 volts where they have broad maxima. The voltage-intensity characteristics of lines originating in transitions down from the same initial level agree in type but sometimes show certain differences in shape. It appears that the probability of different transitions down from a given energy state may not be independent of the way in which this state is excited. There is evidence that a large number of excitations of $2p_2$ by transitions down from higher levels are not followed by $1S-2p_2$ transitions.

• Received 12 August 1927

DOI:https://doi.org/10.1103/PhysRev.31.800