Massenspektrometrie am negativen Glimmlicht einer zylindrischen Hohlkathode / Mass Spectrometric Studies of the Negative Glow of a Cylindrical Hollow Cathode Discharge

Abstract

Positive ions were extracted from the negative glow (NG) of a cylindrical hollow cathode discharge (h.c.d.) through a plane sampling probe (diameter 1 mm, orifice dia. 50 [xm, Molyb-denum) and analyzed in a 60° mass spectrometer. The discharge was operated in pure Argon with an admixture of about 0.1% of H20. The principal ions were Ar⁺, Ar⁺⁺,r₂ ⁺, ArH⁺ , H₂0⁺ and H₃O⁺ . The individual ion currents were measured as functions of the probe potential U\>, at dif-ferent discharge currents (2 - 7 mA) and of the gas pressure (0,05 - 0,5 torr). The reaction scheme for the production and conversion of these ions in the NG could be established. In general, the ions were extracted from the axis of the h.c.d. When the sampling probe was mounted excentrically (3 mm out of the axis), the ratio of secondary to primary ions was higher in the axis. At probe potentials above +15 v referred to anode no ions from the NG reached the probe, but electrons were accelerated so that ionization of the neutral gas could arise in and behind the orifice. The Ar⁺⁺ current as a function of U_b together with other results suggested the presence of very fast electrons (energy > 100 v) in the sampling orifice. They could be identified as electrons from the cathode fall region which were scattered into the orifice. Studies of the ion currents from Ar-H₂ mixture proved the sampling probe suitable as an electron impact ion source.