Abstract:
Based on the idea that systems may very well exhibit continuous changes in symmetry if one focuses on physical quantities such as transition matrix elements, the concept of soft symmetry selection rules for dipole transitions is developed. The high-coverage, low-temperature (1X2)-3H phase of H/Ni(110) is chosen to illustrate the new concept. Photoemission measurements on this adsorption system only reveal a single hydrogen-derived band in variance to the stoichiometry of the adlayer. A special procedure to quantitatively decompose the eigenstates of (1X2)-3H/Ni(110) according to their behavior under 2a/3 translation along [001] is derived. It is demonstrated that this symmetry transformation, though not a strict symmetry element of the adsorption system, can be drawn upon to rationalize the unexpected experimental observations in the spirit of soft symmetry selection.