Chemistry Reference
In-Depth Information
6.5.1.2 Energy Analyzer
The measurement of the energy of the plasma ions is possible by a retarding field
analyzer. Here, the energy distribution is obtained by the first derivative of the I-U
characteristic assuming an ion movement parallel to the retarding electrical field. Ion
optical elements are more convenient which directly provide the energy distribution
and diverts the ions in an off-axis direction. Commercial instruments use electrostatic
sector fields, often 45
◦
, and cylindrical mirror analyzers. The electrical field acts
perpendicular to the flight direction of the ions Figure 6.21. The energy range reaches
up to 1000 eV for positive and negative ions, a scan energy at 0.05 eV increments and
a resolution power of 0.25 eV FWHM is indicated.
6.5.1.3 Mass Analyzer
Static analyzers separate the ions according to their
m
/
z
by a static magnetic sector
field and for improvement of the resolution power
m
m
by an additionally static
electric field (double focussing instruments). The resolution power can be very high
(
/
10
5
). In this case, a determination of the elemental formula of the ion is possible
using the exact masses of the single elements. The transmission is independent of the
ion mass. But these instruments are large due to the necessary long ion path, which
also requires a very good vacuum, heavy and expensive.
Dynamic instruments [286] separate the ions by time-varying electromag-
netic fields, e.g., Fourier transform ion cyclotron resonance mass spectrometers,
omegatrons, time-of-flight and quadrupole instruments.
Quadrupole mass spectrometers are widely used instruments for low-resolution
applications and most important in plasma diagnostics [287]. The ions fly through
an electrical quadrupole field generated by four parallel conducting hyperbolic, in
practice mostly cylindrical rods Figure 6.22. The ions oscillate transversal in direction
to the rods. Depending on the potentials only one kind of ions defined by
m
>
/
z
has a
stable path, the others are deflected to the rods. The resolution power
m
/
m
increases
Repeller
Ion beam
Deflector
Repeller
FIGURE 6.21
Scheme of a cylindrical mirror analyzer. (From Zeuner, M., Corpuscular
diagnosties of plasma polymerization processes, in H. Biedermann, ed.,
Plasma Polymer
Films
, pp. 85-142, Imperial College Press, London, U.K., 2004.)
