Critical potentials of thallium vapor have been measured with the following results—first resonance potential 0.9 volts, first ionization potential 6.04 volts. A higher critical potential detected by the photoelectric effect of the radiation is at 12.4 volts.

Stages in the development of the spectrum were as follows. Below the ionization potential successive lines of the two subordinate series appear with increasing voltage. Above 12 volts another line spectrum appears.

Comparison of the critical potentials with the doublet series system of thallium shows that the first ionization potential is determined by the limit 2p₂ and the first resonance potential by the difference 2p₂-2p₁. Since no radiation can result from this transition the orbit 2p₁ must be meta-stable. Resonance potentials corresponding to frequencies 2p₂-ms and 2p₂-md give rise to the observed low voltage spectra. Corresponding critical potentials are computed for Al, Ga and In.

The 12.4 volt critical potential and the associated second line spectrum are ascribed to a second type of single ionization, the removal of one of the two 6₁ electrons.

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