Circular dichroism in photoemission from oriented molecules at surfaces

Measurements of a new observable quantity in photoemission from oriented molecules are reported. Using circularly polarized radiation, the photoemission intensities for a special geometry were found to depend on photon helicity. The basic theoretical description of the effect is outlined, showing that it occurs for all spatially fixed molecules in the electric dipole approximation and does not depend on electron-spin effects. Experimental case studies for diatomics (CO, NO), benzene, and the “heavy” molecule CH_3I, all oriented by adsorption on Pd(111) or graphite (0001), reveal huge intensity asymmetries in all cases. Comparisons of these measured asymmetries are also made with the results of model calculations based on oriented CO, NiCO, and NO. The effect provides a sensitive probe of molecular orientation and of photoemission dynamics