Structure of an unusual tilted state of CO on Fe(001) from x-ray photoelectron diffraction.

The highly tilted ${\ensuremath{\alpha}}_{3}$ state of Co on Fe(001) has been studied using combined polar and azimuthal x-ray photoelectron diffraction. A more accurate structural picture is derived, with the molecule being tilted at an angle of 55\ifmmode^\circ\else\textdegree\fi{}\ifmmode\pm\else\textpm\fi{}2\ifmmode^\circ\else\textdegree\fi{} with respect to the surface normal along 〈100〉 azimuths and probably occupying fourfold hollows. An estimate is also made of the vertical C distance above the Fe surface. The utility of combined \ensuremath{\theta}/\ensuremath{\varphi} x-ray photoelectron diffraction measurements for such unusual adsorbate structures is demonstrated.