4 - 6
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.
