Chemisorption-Induced Surface Umklapp Processes in Angle-Resolved Synchrotron Photoemission from W(001)

Angle-resolved photoemission, using synchrotron radiation, from the chemisorption system W(001) + H shows a very large doublet of peaks at initial energies -1.3 and -0.6 eV for quarter-monolayer coverage which disappear at higher coverage. The emission lobes are strongly peaked in the $〈110〉$ azimuths and are attributed to essentially pure bulk W transitions. The electrons are emitted in the new directions through the influence of "extra" surface reciprocal lattice vectors associated with the hydrogen c(2×2) superstructure.