Non-dipole effects in high-energy photoelectron emission; identification and quantification using X-ray standing waves

The dipole approximation is routinely used in photoelectron emission studies of gas phase and condensed matter, and is generally thought to be adequate up to photon energies of tens of kiloelectron-volts. There is, however, now ample evidence from theory and experiment that this approximation routinely breaks down at much lower energies, in some cases as low as hundreds of electron-volts. The breakdown is most significant in influencing the angular dependence of photoelectron emission and was first identified in gas-phase experiments. A particular consequence is the introduction of a forward-backward asymmetry into the angular dependence relative to the direction of photon propagation. This effect also has a proved important in surface structure determination using photoelectron-monitored X-ray standing wavefield absorption, and a variant of this technique has been used to determine the quadrupolar asymmetry parameter values for 1s emission from many low atomic number atoms.