Diffuse Continua-Emitting Discharges in the Rare Gases with and without Nitrogen

Abstract

If a current the order of 1 mA is passed through 50–750 torr of a pure rare gas in a 10-cm-diam tube a diffuse discharge results filling the tube and emitting mainly a continuous spectrum, strong in the vacuum uv, weak in the near uv, visible and ir. The discharge is characterized by a very high voltage gradient and a substantially flat volt—ampere characteristic. The ion and electron concentration is 108−109 cm−3 and the electron energies are high. Energy losses are principally elastic and the ions and electrons reach the wall by ambipolar diffusion. The continuous spectra are of molecular origin. Continua from Xe, Kr, and Ar are whitish, that from Ne a light blue. In any of the above discharges if a critical current is exceeded the discharge goes to a filamentary form. The addition of 0.1% N2 to the pure rare gas has profound effects, lowering the gradient in Xe 18-fold and bringing out the arc lines of Xe. New spectral structures are found at 3466 Å, 3650 Å, and at 1.3 μ which are believed to be due to complex molecules of Xe and N2. There is an afterglow lasting up to 10 sec that is spectroscopically much the same as the discharge itself. A high concentration (1013−1014 cm−3) of long-lived energy carriers is indicated and are believed to be mainly N2(3Δu). Collisions of pairs of 3Δu with Xe as a third body and/or of 3Δu with N2(A) produce the higher excited states of Xe and probably Xe2+ and higher excited states of N2 and N atoms. Blue fluorescence of a Pyrex wall indicates strong vacuum uv in Xe. The Vegard—Kaplan (VK) bands are developed with fair intensity indicating a population of N2(A) of the order of 1010 cm−3. The discharges have a positive characteristic indicating that the ions and electrons are disappearing by recombination and that the metastable energy carriers are partially saturated with respect to current. Interruption of the current is followed by an abrupt fall in luminosity to ∼⅓ of the initial intensity, followed by a recovery to ∼¾ initial, and then a slow decay of ∼½ sec. In N2+Kr the first negative system of N2+ is strongly developed perhaps indicating the presence of a high-energy metastable state of N2.