Symmetry-related polarization effects in angle-resolved synchrotron photoemission from W(001) and W(001) + H

We report results of synchrotron photoemission experiments from W(001) and W(001) plus a saturated layer of hydrogen. For s polarization, photoemission features are observed whose amplitude depends strongly on the direction of the vector potential →A with respect to symmetry elements (mirror planes) of the substrate crystral. The observations are explained in terms of selection rules governing transitions between states of specific parity with respect to reflection in the mirror plane. The observations are consistent with the known parities of the tungsten bands and are used to infer parities for the hydrogen-induced states. The observed parities for the hydrogen states place constraints on possible chemisorption models, and support an existing model which specifies two hydrogen atoms per unit mesh in an antisymmetric combination of $1s$ orbitals.