Angle Resolved Photoemission (ARPES)

Photoemission spectroscopy is a powerful and direct probe of the occupied electronic structure, bonding, and chemical nature of a material.  The introduction of high energy and angle resolution to the technique in the past few years has moved the technique into the forefront of physics and materials research, since the most crucial low-energy excitations near the Fermi surface may be directly probed.  The k-space resolution of the angle-resolved version of the technique is a unique aspect which gives information which typically can not be obtained by any other method.

A schematic of the photoemission process is shown below.  Monochromatic photons of known energy hn impinge upon the sample, exciting an electron with initial energy Ei to a final energy Ef. If this energy Ef is above the vacuum level, it can escape from the solid becoming what we call a photoelectron. The process is governed by energy conservation, and if there are no screening, relaxation, or inter-electron interaction effects, the initial state energy is simply  Ei = hn -Ek-f, where hn is the photon energy, f is the work function of the electron energy analyzer, and Ek is the measured kinetic energy of the photoelectron. In the simplest approximation then, the photoelectron spectrum essentially tells us the occupied density of states of the system, with the only requirement being that the background contribution from inelastically scattered electrons is removed and that the variations in photoionization cross-sections are properly taken into account.  In angle-resolved photoemission spectroscopy the emission angle of the photoelectrons from the surface normal is also constrained, and so we obtain the electron's momentum information in addition to its energy.  This allows for the direct experimental measurement of E vs. k dispersion relations, as shown in  panel c.

The very recent (~ 1999) introduction of new Scienta analyzers for ARPES experiments of correlated electron systems has made a revolution in the field.  These analyzers give a near order-of-magnitude improvement in both energy and angular resolution, with additional large increases in counting efficiency. All of our ARPES experiments are currently being performed on these analyzers.