Pass energy is a term used to define the resolution of an acquisition.

To understand exactly what it is, we must consider the XPS analyser (figure 1):

So here, we have 3 radii; R₁ – the inner hemisphere, R₂ – the centre of the hemisphere and R₃ – the outer hemisphere. We also have a potential applied (V₀) so that V₁ on the outer hemisphere is negative and V₂ is positive. Finally, we have slit widths – the entry slit, W₁ and the exit slit, W₂.

Note: R₂ = (R₁ + R₃)/2

Electrons are retarded by the column and focused into the entry slit. The analyser is set to an energy, and any electrons which match this energy are refocused into the exit slit and detector, resulting in a signal. This energy is called the pass energy (E_P). For example, if the pass energy is set to 20 eV, an electron travelling with a kinetic energy of 1000 eV would need to be slowed by 980 eV.

Energy is reduced as:

t = T – qV_p

Where T = initial K.E., q = particle charge, V_p = decelerating voltage, t = resultant K.E.

For a given V₀, only electrons of a specific E_P may travel to the detector along R₂.

Our resolution (ΔE) is dependant on pass energy (among other things) via the following relationship:

ΔE = E_P . (W/2R₂ + α²/2)

Where W is the slit width (W₁ = W₂) and α is the angle of acceptance.

Thus, for a higher pass energy, we observe poorer resolution (figure 2), though improved signal intensity.

References

1. Omicron EA 125 Energy Analyser Manual