Chemistry Reference
In-Depth Information
10
5
Plasma off
10 min
30 min
50 min
70 min
CF
2
+
CF
4
10
4
CF
3
10
3
CF
2
10
2
10
1
5
10
15
20
25
30
35
Electron energy, eV
FIGURE 6.32
ScanofCF
2
ionsforelectronenergies5-35 eVinCF
4
(plasmaoff)andvarious
plasma operating times. Capacitively coupled rf (13.56 MHz) discharge in CF
4
(20Pa). CF
x
indicates the parent radical/molecule of the CF
2
ion.
are clearly visible. In the interval between 10 and 17 eV the parent molecule is the
radical CF
2
, in the range 17-21.5 eV the observed CF
2
is a fragment ion of CF
3
and
for electron energies higher 21.5 eV a fragment ion of CF
4
.
In this way the evidence for the existence of CF
2
and CF
3
radicals is demonstrated.
The decrease of CF
4
with increasing discharge time by thin film deposition is visible.
A quantitative evaluation of the radical concentration requires the knowledge of the
concerning reaction cross sections; data are compiled by [336].
6.5.3.5 Ion Attachment Mass Spectrometry
The chemical ionization of radicals by the attachment reaction between Li
+
ions
and radicals is applied by Fujii [345], see also [346]. In the ion source Li
+
ions are
produced by heating of a glass bead with lithium oxide in an alumosilicate matrix.
The ion source operates in the 10 Pa region, the ions are analyzed in a quadrupole
mass spectrometer. In a CH
4
microwave discharge radicals like C
n
H
2
n
−
1
(
n
=
2
−
11)
and C
n
H
2
n
+
1
(
n
=
2
−
11) could be measured.
6.5.3.6 Measuring Excess Energy
The application of an energy analyzer between the ion source and the mass analyzer
offers the possibility to identify parent molecule ions and fragment ions. If the
electron impact ionization occurs via a higher excited state the kinetic excess energy
is distributed according to the conversion of momentum between the fragments like
m
1
/
m
2
=
E
2
/
E
1
.
