UPS

Ultraviolet Photoelectron Spectroscopy (UPS)

Useful to study bonding surface orbitals and to map electronic band structure at surface (angle-resolved UPS or ARUPS). In first approximation, the density of states of the solid is shifted by the photon energy, hn. That means that the maximum energy of the photoelectron (total energy) inside the solid is hn, with respect to the Fermi level. The maximum kinetic energy outside the solid is hn - f. The work function f is that of the sample when the energies are measured just outside the sample surface (rarely the case), or that of the electron spectrometer (the usual case). The energy distribution of emitted electrons is distorted by the energy-dependent transmission probability over the surface barrier, that suppresses electrons with small energies (below a few eV) above the vacuum level of the sample. The electron kinetic energy (KE) scale is often converted into a binding energy (BE) scale by making the substitution:

BE = KE - f - hn

Note that KE - f is the energy of the electron with respect to the Fermi level, this is the reference of choice in photoelectron spectroscopy, not the vacuum level.

The approximation of a shifted density of states is good to derive the position of structure in the valence band, e.g., that due to electrons in specific bonding levels. Any detailed study needs to consider that the intensity of each structure depends on the probability of photon absorption, and the angle at which electrons are detected. The probability of photon absorption is approximately proportional to A^.p where A is the electric field vector of the incoming photon and p is the (vector) electron momentum.

Updated, October 5, 2000