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Transpector SPS Operating Manual
5.3.1.1 Ionization Process
When a sufficiently energetic electron strikes a gas molecule, there are many
processes that can occur, some of which are summarized in
.
In all cases, the reactants are a high energy electron (e
-
) and a gas molecule
(XYZ). The products of the first reaction are the molecule with a single electron
removed (the parent ion) and two low energy electrons. In the second reaction, two
electrons are removed from the gas molecule, resulting a doubly charged ion.
Triply (or even more highly) charged ions are possible, provided the incident
electron has enough energy.
Reactions 3 through 8 are examples where the original molecule is broken into
fragments, at least one of which is positively charged (negative ions can also be
produced in this manner). Only the positive ion fragments are observed; the neutral
(uncharged) fragments are not detected. The mass spectrum obtained when the
parent molecule breaks apart under electron impact is commonly referred to as the
fragmentation pattern
(or
cracking pattern
). The fragmentation pattern for nitrogen
at an electron energy of 70 eV is displayed in
Table 5-1 Electron Impact Ionization Processes
XYZ + e
-
XYZ
+
+ 2e
-
(1)
XYZ
2+
+ 3e
-
(2)
XY + Z
+
+ 2e
-
(3)
XY
+
+ Z + 2e
-
(4)
X
+
+ YZ + 2e
-
(5)
X + YZ
+
+ 2e
-
(6)
XZ + Y
+
+ 2e
-
(7)
XZ
+
+ Y + 2e
-
(8)
