Collisions of the Second Kind and their Effect on the Field in the Positive Column of a Glow Discharge in Mixtures of the Rare Gases

Abstract

A study was made of the collision processes in the uniform positive column of a glow discharge in mixtures composed of two of the rare gases, helium, neon and argon and a mixture of each of these gases with mercury vapor, by means of measurements of the electric field and by spectroscopic observations. The discharge tube was of cylindrical form, 4.4 cm in diameter, with plane parallel electrodes The electrode distance could be varied from zero to 32 cm by means of an external electromagnet. The phenomena were studied with currents from 20 to 40 m.a. and pressures from 5 to 30 mm of mercury. The mixtures were circulated through the discharge tube and a purifying system during the entire time that observations were being made. It was very important to maintain extreme purity of the gases. The results show that the electrical and spectral characteristics of the uniform positive column in mixtures of monatomic gases can be explained principally in terms of collisions of the second kind between the ions or metastable atoms of one gas and the neutral atoms of the other. The effect of limitation of electron velocities is insufficient to explain the results, and its effect is shown to be negligible as compared with collisions of the second kind when the concentration of one gas is very small. The necessary condition for a large effect to be produced in the electrical and spectral characteristics of the positive column by a small percentage of one gas added to another is that a close resonance exists between the metastable states of the main gas and the ionization potential or excited states of the added gas atom or ion. By the introduction of as little as 0.4 percent neon or argon into helium, a marked increase in the electric field is produced, and the spectrum emitted is changed almost completely from the arc spectrum of helium to that of neon or argon respectively. The addition of mercury vapor at room temperature to neon caused a decrease of 45 percent in the field while the addition of mercury to either argon or helium produced a small increase. In the cases where the excitation and ionization potentials of the added gas were above the metastable states of the main gas, no reactions between the two gases could occur to affect the concentration of metastable atoms. In these cases practically no change in either the spectral or electrical characteristics were observed upon adding a small amount of one gas to another. However, in larger proportions the change produced in the field was proportional to the amounts of the two gases and the difference between the field in each of the pure gases. This result is accounted for mainly by the difference in the energy of the ionization processes in the two gases by electron impact.