Recoil Effects #
While not something we would typically need to concern ourselves with in standard XPS, when we start to use higher energy sources we begin to observe noticeable effects due to recoil.[1] The high kinetic energies of the outgoing electrons – particularly for light elements – results in a recoil energy (Er):
Er = ( Me / Ma ) . (hv – B.E.)
Where:
Me = electron mass
Ma = atom mass
B.E. = binding energy
The recoil energy is subtracted from the kinetic energy of the outgoing photoelectron, and as such will move the peak by a specified amount (see below).
We have a similar conservation of momentum principle in REELS measurements, when we reach an electron beam energy of enough strength. We have an electron of momentum k, scattering over an angle θ from an atom. The momentum (q) is absorbed by a single atom –
q = 2k . sin(1/2 θ )
And this energy is given to the atom, of mass m
Recoil energy is then:
The effect of this is a broadening of the elastic peak, and loss spectrum.[2]
1
Artyushkova, Kateryna, Stuart R. Leadley, and Alexander G. Shard. “Introduction to reproducible laboratory hard x-ray photoelectron spectroscopy.” Journal of Vacuum Science & Technology A 42.5 (2024).
2
Vos, Maarten, Sean W. King, and Benjamin L. French. “Measurement of the band gap by reflection electron energy loss spectroscopy.” Journal of Electron Spectroscopy and Related Phenomena 212 (2016): 74-80.
