Page 45 - Inorganic Mass Spectrometry : Fundamentals and Applications

Glow Discharge Mass Spectrometry 35

with 10-7 A (region D) marking the beginning of those discharges that typically
show luminosity [27]. At the point where the discharge is carrying approximately
A of current, the realm of the glow discharge is reached. As current continues
to increase, a transition occurs from the corona discharge (region E) to the
subnormal discharge (region F) to the nomal discharge (region G). While operat-
ing in the nomal discharge mode, an increase in current will produce no change in
voltage (ie., current density is constant) [24,25]. Physically, the luminous region
is observed to increase its coverage over the cathode. When the entire cathode is
covered, the abnormal region is reached (ca. A ) (region H), and any increase
in current produces an increase in the discharge potential [25]. Beyond the
abnormal region is the transition to high current arc discharges (region I).
The most common gas discharge is the low-pressure (typically 0.10-10
ton) glow discharge. Typified by the familiar neon light, the appearance of glow
discharges varies with the gas (type, pressure, purity, etc.), the dimensions of the
vessel, and the geometry of the electrodes. Figure 2.2 is a common depiction of
the luminous and nonluminous regions of a glow discharge; this figure is often
termed the “architecture of the glow” [25]. Below the diagram are plots that show
the status of luminosity, potential, field, charge density, and current density with
respect to the specific discharge regions.
Moving left to right away from the cathode, the first region encountered is
the Aston dark space. This region has a net negative space charge resulting from
secondary electrons released during the sputtering event [24]. Beyond the Aston
dark space and up to the negative glow, positive ions are acting as the primary
charge carriers. Interaction of these positive ions with slow electrons creates
energetic neutrals that radiatively relax to produce luminosity and the cathode
layer (see Fig. 2.2) [24]. Electrons that pass through the cathode layer without
undergoing collisions acquire energies up to that of the cathode fall potential. This
increase in energy results in a lower probability for collision between electrons
and gas atoms, and the creation of a low-emission intensity region called the
cathode dark space (also temed the cat~ode~all region). Throughout the cathode
dark space, a net positive space charge exists. This positive space charge produces
a large enough poten gradient that the majority of the discharge voltage is
dropped across this region. Because of this potential gradient, electrons are
accelerated to a sufficient energy to ionize the discharge gas on collision. Conse-
quences of these ionizing collisions are the multiplication in the number of
electrons [23] and the reduction in their energy. As the energy of the electrons is
reduced, the cross section for excitation of atoms increases. Radiative relaxation
of excited atoms leads to visible radiation and the formation of the negative glow
region. As more collisions occur, electrons are slowed further, decreasing the
collisional cross section until excitation of atoms is no longer favorable. This
results in the fomation of the Faraday dark space. Continuing toward the anode,
the potential gradient accelerates the electrons to the point where excitation and