A good introduction about the topic of (physical) cross sections can be found on http://en.wikipedia.org/wiki/Cross_section_(physics). If extending or using the cross section database of PIC-MC some terminology issues should be taken into account. Here are the most common terms and definitions:
Previously we used a linear sampling for the energy in the cross section array, i.e. E(i) = E0 · i / N, whereas E0 is the maximum energy in eV and N is the array resolution. However, using a linear sampling scheme we had to distinguish two simulation cases: (i) DSMC gas flow simulation and (ii) PIC-MC plasma simulation. This was because a higher energy resolution in the low energy regime is needed to correctly compute gas flow dynamics and neutral collisions, respectively. To get rid of this limitation we could either increase the array resolution or use a higher order sampling scheme. The latter case was more pratical. Thus, we chose a second order sampling, i.e. E(i) = E0(i / N)², which gives a better resolution in the low energy regime. The crucial range hereby is the thermal energy around 25 meV, which is sufficiantly resolved using the second order sampling as seen in the figure below.
Figure 1: First order sampling for dsmc and pic-mc simulation (black and red curve) and second order sampling (blue curve) to combine dsmc and pic-mc simulation.