Stages in the Excitation of the Spectrum of Indium

The indium metal was vaporized at a temperature of 650°C in an iron cylindrical anode within a quartz tube. Electrons from an oxide-coated Pt filament passed through a helical grid into a force-free space where they collided with the atoms of indium vapor. The spectrum was viewed end-on through a transparent quartz window. At 3.3 volts the lines $2p_1-2s$ and $2p_2-2s$ appeared. At 4.2 volts the additional lines $2p_1-3d$ and $2p_2-3d$ appeared. At seven volts higher members of the series and several unclassified lines were present on the plates. At 13.2 volts the spectrum became very strong and the line 2306A appeared in addition to the recognized series lines. It is possible that the second arc spectrum was excited at this potential. The resonance lines were found to be less intense than the corresponding lines due to the metastable atoms.