The secondary electron yield is defined as the average number of electrons ejected per incident atom. The secondary electron yield was calculated from probe current measurements. Ideally this yield would be measured using neutral atoms as primary beam.
Unfortunately the ion source in ILENA only provides molecular ions. This means that all the different dissociation and ionization possibilities at the surface have to be considered separately. From impinging on MgO (see Chapter 5.5) and BaZrO  the model shown in Figure 3.1 was derived.
Each particle impinging on the surface ejects secondary electrons with a yield of for atoms and for molecules. Any dependence on the charge state of the reflected particle is neglected, i.e., a finally negatively charged particle is assumed to produce as many secondary electrons as a finally neutral particle. Equations 3.2 to 3.6 show the transfered charge for each possibility considered in Figure 3.1. For the following calculation it is assumed that is very close to one . The total current induced on the sample is then given by:
We measured the secondary electron yield always at primary beam incidence
angles of 30 and 60 degrees, respectively, as measured to the surface
normal. At such steep angles the ionization efficiency, i.e, the fraction
of negatively charged particles, drops considerably compared to angles
used for ionization efficiency measurements (82 degrees). Considering
a value for of 0.02 to 0.3 measured at grazing incidence
and a rapid decrease towards steeper angles the influence of
was neglected. The above equation thus simplifies to:
To determine the secondary electron yield a measurement of the primary beam intensity has to be performed. This is done by biasing the sample to = 18V with reference to ground. As the energy distribution for the secondary electrons peaks at a few eV virtually no secondary electrons escape from the sample and the measured current is only caused by the incident primary beam (for the ionization efficiency the same simplifications as used above apply):
The secondary electron yield for atoms can then be calculated using Equation 3.8 and 3.9:
Originally the secondary electron yield was determined by taking 6 measurements both for and . From these measurements an average value for was calculated. But this procedure was very time consuming and not reproducible very well mainly because an operator had to be present in front of the instrument. Any movement of the operator disturbed the current measurements in an unpredictable manner. Using an IEEE488 interface on the computer normally used to gather data for scattering measurements an automated measurement procedure was implemented as described below.
March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland