Surface segregation inMo0.75Re0.25(001) studied by low-energy alkali-ion scattering

${\mathrm{Mo}}_{0.75}$ ${\mathrm{Re}}_{0.25}$(001) has been studied by low-energy alkali-ion scattering. Using a method based upon analysis of ion-scattering intensity ratios, the Re atom fractions in the first and second layers of this surface have been determined to be 0.040±0.005 and 0.52±0.05, respectively. From analysis of the critical edges the first interlayer spacing is found to be contracted by about 9%. These results are in excellent agreement with recent low-energy-electron-diffraction experiments. The results are described in terms of various theories of surface segregation, and are found to be in quite good agreement with the simplest models if the surface coordination is adjusted to that associated with a smoother, higher coordination surface.