The Recombination of Argon Ions and Electrons

Abstract

Afterglow spectrum in argon due to recombination.---The arc spectrum of argon is found to persist approximately 0.001 sec. after an arc of 0.4 amp. in pure argon at 0.5 mm pressure is cut off. Lines involving jumps from high energy levels are relatively much stronger in the afterglow than in the arc. When sodium vapor is present the $D$ lines are strong in the arc but absent in the afterglow showing that the electron speeds in the afterglow are too low to excite the spectrum of argon by direct electron impact. The highly excited argon atoms must therefore be produced by the recombination of ions and electrons. The presence of 0.001 mm of hydrogen does not affect the intensity of the afterglow so that the persistence of metastable atoms is not involved in the production of the afterglow.Effect of applied potentials on the afterglow.---Accelerating voltages of 3 to 10 volts quench the afterglow during the period of application of 0.001 sec. Retarding after-voltages up to 90 volts have practically no effect. Measurements with an intermittently connected exploring electrode show that the velocities of the electrons are increased by the applied accelerating voltage but unaffected by the retarding voltage. The quenching is apparently the result of the decreased probability of recombination because of the higher velocities of the electrons. When low accelerating voltages have been applied for 0.001 sec. the intensity of the afterglow and the conductivity of the arc space in the period immediately following this application are both greater than they would be had the voltage not been applied. The intensity of the afterglow is thus directly related to the concentration of positive ions and the quenching of the afterglow shown to be connected with a saving up of ions as it should be if the effect of the after-voltages is to prevent recombination.Ordinary arc spectra are obtained under conditions which are here most unfavorable to the occurrence of recombination. They are presumaly primarily excitation spectra. The use of intermittent discharges for obtaining recombination spectra and strong high series members is suggested.The measurements show positive ion concentrations in the afterglow of the order of ${10}^{12}$ per cc. The mean energy of the electrons is 0.4 volt. From the measured rate of change of the concentration of ions, there results a value of 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ for the coefficient of recombination. This value may be in error by a factor of 5 because of several unavoidable errors.