Small molecule photoelectron spectroscopy: Recoil effects, stoichiometric surprises, and double-core-hole ionization

► The effects of recoil momentum on a photoelectron spectrum are illustrated. ► Rotational motion within a molecule leads to Doppler broadening in the XPS spectrum. ► Intensities in an XPS spectrum may not quantitatively reflect molecule composition. ► Measurement of double-core-hole-ionization energies may lead to new insights.