In respect to PhotoElectron Spectroscopy (PES), we are analyzing graphs with "binding energy (MegaJoules)" on the x-axis, and with "relative number of electrons" on the y-axis.
I have also seen the x-axis be labeled as "ionization energy." I am really familiar with what ionization energy is (it's the energy required to strip off the highest-energy electron from an atom), but I'm not really sure what binding energy means. My chemistry instructor says that ionization energy and binding energy are different things, but we were not sure why; since 2 different PES graphs used ionization energy and binding energy interchangeably on the x-axis, are they the same? I've also noticed the trends in ionization and binding energy follow each other-- for instance, lower-energy orbitals (like 1s) have higher binding and ionization energies. Thus, what's the difference between these two terms?