Page 52 - Inorganic Mass Spectrometry - Fundamentals and Applications

42 ~arshick

Although the average energy of glow discharge electrons (reported to be on
or
the order of 2-4 eV) is not high enough to ionize many elements the discharge
gas [23,47,48], the Boltzmann distribution of energies provides a fraction of
electrons with enough energy to ionize all elements. Chapman [23] has performed
calculations using the Maxwell-~oltzmann distribution to determine the fraction
of electrons with enough energy to cause ionization of the noble gases, For a
thorough account of these calculations, the reader is referred to the book Glow
Discharge Processes [23].
The second major ionization mechanism in an abnormal glow discharge
is
named for F. M. Penning, who discovered the effect in 1925 [49]. Penning
ionization involves the transfer of energy from a metastable state of the discharge
gas to ap atom or molecule. If the ionization potential of the atom or molecule is
lower than the energy of the metastable state, ionization can occur. This process
may be thought of as nonselective since different elements generally exhibit
similar ionization cross sections for Penning ionization. Metastable states are
of
created through the excitation discharge gas atoms into an electronic level from
which radiative decay is forbidden by transition selection rules. For argon these
are the 3P, and 3P, states at 11 S eV and 1 1.7 eV, respectively. Metastable species
are long lived, existing for several milliseconds under typical discharge conditions
for
[50]. In addition, metastable species have a reported cross section ionization an
order of magnitude or more higher than electrons. For these reasons, metastable
atoms play a disproportionate role in ionization in some discharges [ 12,5 1-53].
Table 2.1 lists the metastable states for the rare gases most often used in CDMS.
Other mechanisms besides electron and Penning ionization have been
shown to contribute to the ionization of sputtered neutral species, but their roles
are believed to be less sig~~cant. Table 2.2 is a partial list of these mechanisms.
These are reviewed in detail elsewhere 154-571. Since factors such as discharge
pressure, type of discharge gas, electron energy and number density, and cathode
geometry all influence the ionization process, no one mechanism has surfaced as
dominant in all glow discharge devices.

Low-Lying Metastable Levels of Rare Gas Atoms
Gas Metastable energy (eV) Ionization potential (eV) Spectroscopic notation
He 19.8, 20.7 24.58 23s, 21s
Ne 16.6, 16.7 21.56 3P2, 3P0
Ar 11.5, 11.7 15.76 3p2, 3P0
Kr 9.9, 10.5 14.00 3P2, 3P0
Xe 8.3, 9.4 12.13 3P2, 3P0
Source: Ref. 25.