Ion formation at the boundary between a fast flow glow discharge ion source and a quadrupole mass spectrometer

Abstract

An ion source is described for use in low power glow discharge mass spectrometry, using fast flowing argon as the discharge gas, and attached to a quadrupole analyser. The flow carries the plasma downstream onto an ion exit sampling cone which can be variably biased relative to the anode, enabling the physical and chemical properties of the plasma to be studied independently of the main ionisation region, the cathode fall. The parts of the plasma studied were characterised by fixed probe potential measurements. Cone bias effects are consistent with a plasma which contains large quantities of high n Rydberg atoms of both the discharge gas and the sputtered cathode. The ionisable atom flux passing through the sampling aperture was measured directly, giving densities inside the plasma of 10¹⁰–10¹¹ cm⁻³. The densities of lower n Rydbergs are expected to be very much higher. H₂ is shown to quench these states and promote excitation of the cathode atoms to their Rydberg states. Field-ionisation or Auger deexcitation of the Rydberg gas inside the cell gives a high intensity cathode ion beam, which is relatively uncontaminated by the higher ionisation potential species, such as are formed from the discharge gas.