Nanoscale photoelectron mapping and spectroscopy with an atomic force microscope.

The tip of an atomic force microscope is used as a local probe for photoelectrons excited by laser illumination. The tip-sample distance is precisely controlled by the van der Waals force and the pure photoemission current is measured without tunneling current contribution. The nanoscale photoelectron mapping with high current contrast is obtained on a cesium covered Au(111) surface. By sweeping the laser photon energy, the local photoelectron spectra are measured on Cs islands and terraces. The results reveal distinct electronic states and photoemission thresholds for different Cs coverage, providing the photoemission current contrast mechanism. The contrast in photoelectron mapping can be further tuned by the incident laser polarization exploiting the symmetry selection rules in the optical excitation.