Two-photon photoemission spectroscopy of electronic states at metal surfaces

In two-photon photoemission a photon from a pulsed laser excites an electron from a state below the Fermi level to an unoccupied intermediate state below the vacuum level. A second photon of the same pulse ionizes the intermediate state. The energy distribution of the photoelectrons yields information on the position and the lifetime of both the initial and, in particular, the intermediate state. Most of the work in two-photon photoemission spectroscopy has been devoted to the investigation of image potential states (image states). They are bound states of electrons in front of metal surfaces in the potential well built by the image potential and the surface. The situation is analogous to the hydrogen atom, with a binding energy of the lowest state of 850 meV in first approximation. Up to four members of the Rydberg series are detected. Experimental results on the binding energy of image states in (111) and (100) surfaces of Ag, Ay Cu, and Pd agree with calculations by a one-dimensional scattering model. The results for Co, Ni, and Fe are not yet understood. The effective mass is close to unity except for Ag(111). Lifetimes between 5 and 170 fs are measured. They can be explained by decay via electron-hole pair creation in the bulk. Image states on metal overlayers yield information on film growth and surface morphology. Dielectric overlayers are been investigated. Adsorbate states of atoms and molecules adsorbed on metal substrates can also be studied by two-photon photoemission spectroscopy. Results for Na, O, CO, and NO on Cu(111) are reported.