Critical potentials associated with excitation of alkali spark spectra

Various forms of electron tubes used in this work are described, including a new type of tube suitable for spectroscopic study of discharge with very low current density.Experimental results are as follows:Rubidium.-Electricalmeasurements show critical potentials for production of radiation at 16.0 and 21.6 volts and with less certainty at 18.0, 25.2, and 68 volts.There is an increase in ionization at 16 volts.Spark lines are excited near 25 volts with small currents and at 16 volts with high-current density.Caesium.-Spark lines are excited with small currents at 21.5 volts and with large currents near 14 volts.Another spectrum appears above 40 volts.A table of lines and intensities is given.Potassium.-(Publishedresults give radiation potentials at 19 and 23 volts.)There is an increase in ionization at 19 volts, and spark lines are excited at this point with high current and near 28 volts with low currents.Sodium.-Thespark lines appear near 35 and 44 volts, with large and small currents, respectively.Lithium.-There is a critical radiation potential at 54 volts, and a spark line appears near this point.Conclusions.-Thepotentials Li 54, Na 35, K 19, Rb 16, Cs 14 measure the work required to remove a single electron from the rare gas shell of the normal atom.Each of these values is equal within experimental error to the ionization potential of the neighboring rare gas ion.The critical potential for low-current spark excitation gives the work required to remove two electrons by a single collision.The potentials K 23 volts and Rb 21.6 volts apparently measure the work required to remove the second electron from an ion with the valence electron removed.