Inficon Transpector XPR 3+ - Page 66

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Transpector XPR 3+ Operating Manual
4.1.1.3 Electron Energy Effects
As was previously mentioned, the exact fragmentation pattern observed will
depend on the energy of the bombarding electrons. Figure 4-2 (from a paper by W.
Bleakney, Physical Review, 36, p. 1303, published in 1930) graphs the number of
argon ions (of different charge states) produced per incident electron per Torr of
gas pressure as a function of electron energy.
Figure 4-2 Electron energy effects
This graph shows the number of argon ions, N, formed per electron per Torr at
0 °C versus electron energy.
The appearance potential (i.e., the minimum electron energy required to produce
a specific ion) for Ar
+
is 15.7 eV. The number of argon ions produced rises steeply
with energy until a maximum is reached at about 55 eV. As the electron energy
rises above this level, the rate of Ar
+
production slowly decreases.
The appearance potential for Ar
2+
is 43.5 eV, and the ion production rate does not
maximize until the electron energy exceeds 100 eV. The appearance potential for
Ar
3+
is approximately 85 eV, while the appearance potential for Ar
4+
is over 200 eV.
Transpector XPR 3+ normally is set for 40 eV (Low Emission) setting to produce
Ar
+
ions. The low electron energy (40 eV) model of Transpector XPR 3+ operation
suppresses production of
36
Ar
2+
ions at mass 18, resulting in the mass 18 current
being principally a measure of H
2
0.